2021, Volume 28 Issue 1
28 January 2021
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Major challenges and development in Chinese sedimentological research on petroliferous basins
ZHU Xiaomin, DONG Yanlei, LIU Chenglin, YE Lei, ZHANG Meizhou
2021, 28(1): 1-11. 
DOI: 10.13745/j.esf.sf.2020.5.1

Abstract ( 87 )   PDF (1652KB) ( 153 )  
Sedimentological research is highly relevant to human survival and mineral resource exploration and exploitation. In this paper, we briefly analyzed the rearch hotspots in sedimentological researches both at home and abroad by examining the recent 200 years of research history. The International Association of Sedimentology has paid much attention to the IODP, deep-time climate and global sea level changes, source-to-sink system, carbonate sedimentary environment, mixed clastic-carbonate sedimentation, microbialite, shelf edge delta, gravity flow, fine-grained sedimentation, tectono-sedimentology, and volcanic and seismic sedimentology. While Chinese sedimentologists have focused on continental sedimentation, sedimentary models of continental basins, sedimentology of the South China Sea, sedimentary process of different types of basins in China, shallow and shelf edge deltas, source-to-sink systems of different sizes and geological ages, and carbonate deposits in small cratonic basins of southern China. Presently, the major global sedimentological research hotspots include continental sedimentology, onshore and shallow-water deposits, deep-water sedimentation and event deposits, carbonate sedimentation, sedimentary process and new methods. Future areas of sedimentological research development in China shall include theory of sedimentary dynamics of different petroliferous basins, regional characteristic continental sedimentology, multi-disciplinary approach, quantitative sedimentology, and new research methods. Sedimentology as a very important discipline of geology can guide more efficient hydrocarbon prospecting and exploration.

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Middle—Late Triassic basin prototype and tectonic paleographic response in the Ordos Basin
RUAN Zhuang, LUO Zhong, YU Bingsong, LU Yuanzheng, XIE Haochen, YANG Zhihui
2021, 28(1): 12-32. 
DOI: 10.13745/j.esf.sf.2020.5.2

Abstract ( 61 )   PDF (9260KB) ( 94 )  
 The Ordos Basin deposited a large set of high quality sandstone reservoirs during the late Middle to Late Triassic. The sedimentary origin of the sandstone reservoirs and their distribution in the basin are well understood. However, the exact paleoboundary of the Ordos Basin and the tectogenetic mechanism of the paleogeographical evolution of the basin are still unclear. Here, by conducting detailed stratigraphic and sequence analyses of 165 boreholes and 57 outcrops in the Ordos Basin and its surrounding basins, we determined the MiddleLate Triassic boundary of the Ordos Basin, and explored the paleogeographic evolution and tectogenesis of the basin. The basin is constrained by the Dalateqi, Inner Mongolia and Datong, Shanxi boundaries to the north, the NingwuTaiyuanYushe, Shanxi and AnyangHenanKaifengDengfeng, Henan boundaries to the east, the suture zone between the North Qinling Terrance (NQT) and the North China Block (or the ChenheLuonan, Shanxi and Nanzhao, Henan boundaries) to the south, and the Malianggou, Gansu boundary to the west. Its northwestern boundary extends along the western margin of the Helan Mountain fault zone. The MiddleUpper Triassic Yanchang Formation can be divided into four sedimentary cycles (SQ1SQ4), representing the lake evolutionary process from the initial stage (SQ1) to the maximum subsidence stage (SQ2 and SQ3) and then to the later closing stage (SQ4). The sedimentary pattern of the Ordos Basin features a meandering river-delta sedimentary system in the north-northeast and an alluvial fan-braided river-delta sedimentary system in the south-southwest, showing obvious NS spatial differentiation and EW temporal differential evolution of depocenter. Our study show that the depocenter migration and paleogeographic differentiation in the basin were controlled by the tectonic evolutions of the QinlingDabie Orogenic Belt (QDOB) in the south and the XinganlingMongolia Orogenic Belt (XMOB) in the north. The paleogeographic evolution of the southern basin was mainly controlled by the QDOB activities. The activation of the North Qinling Orogenic Belt (NQOB), driven by the closure of the Mianlue Ocean in the MiddleLate Triassic, not only led to the formation of the southern steep slope of the basin and the abrupt paleogeomorphologic change in southeastern basin, but also the development of an underwater low uplift, separating the inner craton basin and the southwestern foreland-like basin in the Late Triassic. The paleogeomorphological evolution in western basin was controlled by complex structural mechanisms. There had been a minor north-south uplift in the western margin during the Middle Triassic, most likely affected by a strong compression occurred remotely in southeastern basin. During the middle Late Triassic, the uplift sink and the Ordos Basin area were connected to the Hexi Corridor Basin, presumably under the flexure subsidence effect of the extrusion stress in the Tethyan tectonic domain to the southwest. These results have present-day significance for the oil and gas exploration in the basin as well as for advancing the theory of basin-mountain coupling.
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Structural characteristics and formation mechanism of the Palaeozoic buried hills of the Zhuanghai area in the Jiyang Depression.
LUO Xia, FANG Xuqing, ZHANG Yunyin, ZHANG Yuntao
2021, 28(1): 33-42. 
DOI: 10.13745/j.esf.sf.2020.5.3

Abstract ( 16 )   PDF (4249KB) ( 61 )  
In the Paleozoic buried hills of the Jiyang Depression, the complex middle-lower section between the positive structural units has become the main exploration focus after the initial prospecting of the residual hill and peripheral buried hills. The eastern part of the Zhuanghai region is close to the Tanlu fault zone and has been profoundly impacted by its multistage tectonic movements. The structural style of the Zhuanghai Paleozoic buried hill is very complex, and deep burial can result in low accuracy in the early seismic data, causing ambiguous structural characterization of the zhuanghai area thus greatly hindering the exploration progress. In this paper, we discussed the complex structural characteristics and formation mechanism of the Paleozoic buried hills of the Zhuanghai area, using high precision 3D seismic and drilling data and regional stress field test results. Our results showed that a set of paleozoic stable marine and transitional sedimentary rock series developed in the Zhuanghai area, where Cambrian, Ordovician, Carboniferous and Permian series developed successively from bottom to top. There are four groups of different types of Paleozoic faults in the Zhuanghai area, grouped as the NW, SN, NE and EW-trending faults. The five NW-trending faults are high angle normal and reverse faults; the three NS-trending faults are strike-slip faults; the NE-trending faults, which cut through the NW-trending fault group, are normal faults; and the EW faults, crossing the NW fault group, are mainly the southern and northern boundary faults of the Zhuanghai buried hills.
The fault development can be divided into four periods: the first period in the Indosinian when reverse faults formed by Paleozoic compression; the second period from the Late Jurassic to Early Cretaceous when normal faults formed by structural reversions through the Mesozoic-Paleozoic era; the third period in the Late Cretaceous when thrust faults formed by Mesozoic-Paleozoic compression; and the fourth period in the Eocene when normal faults formed by strike slip and extension movements through the Paleozoic-Mesozoic-Paleogene-Neogene period. Intersecting and intercutting faults of different trendings formed a “chessboard” type complex structure. The Paleozoic top surface resembled a W-E trending anticline and can be divided from west to east into four rows of buried hills. The complex evolutionary process is responsible for the disparity in the reserved strata and for the structural types of different rows of buried hills. Formation mechanism study showed that the Late Triassic to Eocene Paleozoic Zhuanghai strata underwent four stages of evolution: compression, extension, compression, and strike-slip, under the control of the reciprocating strike-slip stress field of the Tanlu fault. At the end of the Triassic, three NW-trending thrust fold belts formed under the control of the NE-SW-trending compression by the sinistral strike-slip movement of the Tanlu fault in the Zhuanghai area. From the Late Jurassic to Early Cretaceous, the thrust faults reversed unevenly, resulting in the NW-trending alternation of normal and thrust faults. In this period, four rows of NW-trending buried hills were formed. The Zhuanghai area of the Late Cretaceous experienced a 2nd NWW-SEE-trending compression, under a conversional stress field of the Tanlu fault in a sinistral to dextral strike slip movement. From the second to the fourth row of the buried hills, the strata were napped from east to west and the Paleozoic structures were reconstructed again to establish the basic structural pattern of the Paleozoic. In the Eocene, the NS-trending faults, NE-trending strike slip faults and EW-trending adjustment faults were formed by the dextral strike slip movement of the Tanlu fault, intersecting the early formed NW-trending faults and a south to north downward tilting movement occurred in the Zhuanghai area at the same time to form the present-day complex structural styles. The structural evolution controlled the disparities in the buried hill structures as well as reservoir forming conditions in the Paleozoic Zhuanghai area. The stratum gradually thinned from the east to the seriously eroded west. The first row of buried hills only retained Cambrian strata, while Ordovician strata reserved in the three rows to the east gradually thickened. The first row of buried hill developed fault block traps of Cambrian strata; the second row developed anticline traps of Ordovician and Cambrian strata; and the third and fourth rows developed fault block traps of Ordovician and Cambrian strata. The trap types changed from fault block on both sides to fold in the middle, resulting in better reservoirs. The second row of Ordovician thrusting fold hills are most favorable for forming reservoirs.
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Forming conditions and indicators for deep-water evaporite deposits in inland lake basins: A case study of the Dongpu Sag and modern salt lakes
ZHUO Xizhun, ZHENG Xu, CHEN Xiaoshuai, XU Tianwu, CUI Jianjun
2021, 28(1): 43-59. 
DOI: 10.13745/j.esf.sf.2020.5.4

Abstract ( 28 )   PDF (5923KB) ( 59 )  
Eastern China holds many Cenozoic basins where saline giants developed episodically throughout geologic time. Yet, in part, because modern analogous for ancient halitic depositional environments are lacking, in terms of both size and mineralogy, the stratigraphy of evaporite deposition in eastern China, especially whether the depositional environments were “deep basin-deep water” or “deep basin-shallow water”, remains controversial after more than 40 years of research. In order to better understand the factors controlling the formations of ancient evaporite deposits, previous researchers have investigated the hydrology and hydrochemistry of many modern salt lakes with different mineral assemblages, but the forming conditions and indicators for deep-water evaporite deposits are still unclear. In this paper, we compared the salt-forming characteristics of modern salt lakes that possess deep-water evaporite deposits, such as the Dead Sea and Freefight Lake, with that of the Dongpu Sag in eastern China, and discussed the salt-forming conditions, including salinity, water depth, lake level fluctuation and brine stratification. In addition, we analyzed the fabrics in and distributions of the deep-water evaporite sediments and summarized the indicators for deep-water salt deposition. Studies have shown that multistage cyclicity in halite sequence core and log data can be used for salt rock identification. The thickness of the brine profile obviously controls the formation and distribution of evaporites, as thick single layered homogenous salt deposit with lateral stability is more likely to form in the inland “deep basin-deep water” type of salt lakes, such as the Dead Sea and Dongpu Sag. The deep-water evaporite deposits around the depocenter coexist with the sallow-water salt deposits along the lakeshore, and they are all formed by brine concentration as lake level recedes. The rock salt texture is closely related to the sedimentary facies. The coarse grained evaporite minerals mainly precipitate and aggregate at the brine-lakefloor sediment interface in the deep-water region, and are often interbedded with black shales rich in organic matter and pyrite. The shallow-water salt deposits along the lakeshore usually contain more finer grained terrigenous clastics. The evaporite deposits in both modern salt lakes, such as the Dead Sea, and ancient evaporite basins, like the Dongpu Sag, show that declining lake level and thicker brine profile promoted the “deep basin-deep water” type halite deposition. This study revised the current understanding of the mechanisms of salt deposition in inland lake, especially the brine stratification phenomenon in modern salt lakes. It has many implications for other evaporitic events in the geologic record. The indicators for “deep-water evaporite deposition” on the basis of modern salt lakes can provide constraints in building the depositional model of ancient rock salt.
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Signals of depositional response to the deep time paleoclimate in continental depression lakes: insight from the Anjihaihe Formation in the northwestern Junggar Basin
WANG Tong, ZHU Xiaomin, DONG Yanlei, YANG Daoqing, SU Bin, TAN Mingxuan, LIU Yu, WU Wei, ZHANG Yaxiong
2021, 28(1): 60-76. 
DOI: 10.13745/j.esf.sf.2020.5.8

Abstract ( 29 )   PDF (4728KB) ( 62 )  
The deep time paleoclimate study concerns with the pre-Quaternary geological records and aims at predicting climate change in the future. In the northwestern Junggar Basin, the Anjihaihe Formation period coincided with the PaleoceneEocene Thermal Maximum (PETM), an abrupt warming event in the early Paleogene and one of the most pronounced transient climatic events in the geological record. This warming event was likely related to the greenhouse gas emission from seafloor gas-hydrate accumulations. On a millennial timescale, the rate of greenhouse gas emission is, to large extend, comparable to current industrial events, thus providing a unique insight into our uncertain climate future. Therefore, this event has always been a hotspot in Paleogene climate research. During the early Paleogene, typical basin lake deposits were widely distributed in the northwestern Junggar Basin. However, the much smaller scale lacustrine sediments were much more sensitive to climate change than the oceanic system, thus providing more accurate recordings of the paleoclimate evolution that controlled the sedimentary system of the lake basin. In this study, we selected 28 sandstone and 23 mudstone samples from the Anjihaihe Formation for the grain-size analysis and trace element testing, respectively. The results showed that the median diameter of the sandstone grains varied from 7.58 to 393 μm, averaging at 74.44 μm and belonging to fine sand. Furthermore, the enrichments of Ni, Cu and Zn indicated high paleoproductivity during the Anjihaihe Formation period; the Mo, Re, V, Co and Cr contents and Re/Mo, V/Sc, V/Cr and Ni/Co ratios demonstrated the study area was developed in an oxidizing environment; and the Sr/Cu ratio suggested dry and hot climate. We next studied comprehensively the response of continental depression lake to the PETM using core observation and microscopic identification methods, searching response signals from geological archives of both sedimentary and biogeochemical records. Under the conditions of the PETM, the entire study area was in a hot and dry climate. As drought condition intensified, intense evaporation caused lake level to fall. As a result, sediment grain size gradually coarsened, and exposed sediment near the water surface was oxidized to form expansive continental red beds. Meanwhile, due to increasing partial pressure of CO2 in lake water, supersaturated carbonate minerals precipitated in the form of basement carbonate cement. Continuing concentration of lake water also caused rising salinity, creating a continental brackish water sedimentary environment. Oxidability of water increased too, whereas paleoproductivity increased first and then decreased.
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Tectono-sedimentary development of multiphase rift basins: An example of the Lufeng Depression
GE Jiawang, ZHU Xiaomin, LEI Yongchang, YU Fusheng
2021, 28(1): 77-89. 
DOI: 10.13745/j.esf.sf.2020.5.9

Abstract ( 27 )   PDF (4846KB) ( 54 )  
Numerous rift basins worldwide were shown to experience two or more distinct extensional phases, and are referred to as multiphase rift basins. Natural observations and physical results revealed distinctive fault behaviors between the first (RP1) and second (RP2) rift phase in multiphase rifts. The transition between fault growth and reactivation in separate rift phases would inevitably lead to changes in drainage catchments, sediment entrance and sandstones dispersal pathways. The evolution of multiphase rift basins, especially the fault array evolution and displacement patterns, have attracted the attentions from numerous geologists worldwide. Typically, RP1 or single rift cycle involves progressive evolution, featuring isolated faults, fault interaction and linkage, and through-going faulting during the climax stages, followed by a final fault-death stage. In proposed genetic models, the basin is mostly relatively scatted and isolated in the early stage dominated by transverse-sourced systems, then axially-source system becomes facilitated in the fault linkage stage and towards the end of the rifting process. Compared to RP1, the rift-related faulting and basin-infill patterns during RP2 are unclear and currently an international hot topic. Our case study was in the Lufeng Depression located in the northern Pearl River Mouth basin of the South China Sea. The Lufeng Depression is a two-phase rift basin. The directional rotation of the regional tectonic stress field might have induced the differential activities of rift-related faults, resulted in northward migration of the subsidence center. The depositional responses to the tectonic activities during RP2 were in marked contrast to those during RP1. Firstly, the initial stage of RP1 was commonly low-displacement with a shallow lake setting, while the rapidly reactivation of selected master faults resulted in a high tectonic subsidence rate associated with a dominant deep lake setting during the early stage of RP2. Secondly, the initial basins of RP1 were usually small-sized with isolated depocenters, but the depocenters were generally large and uniform during the early stage of RP2. Thirdly, the initial and climax rift stages of RP1 were dominated by transverse drainages, whereas the rapid reactivation of selected faults tended to establish the large axially-sourced drainage catchments during RP2. Consequently, a tectono-sedimentary motif for the multiphase rift basin is proposed. It suggests that those pre-existing faults that are nearly perpendicular to the new extensional direction are highly susceptible to immediate reactivation and can propagate rapidly to their full displacements. The RP2 basins therefore can be considered as 1) sustained axially-sourced depositional systems in the inactive fault controlled areas, or 2) facilitating newly captured axially-dominated systems in the rapidly reactivating fault areas. This study may have significant implications for fault depositional evaluation and sandstone prediction in the Pearl River Mouth basin or other multiphase rifts worldwide.
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Stratigraphic division and correlation of the Ordovician Yingshan and Qrebake Formations in the Shuntogole area, Tarim Basin
ZHANG Zhili, LI Huili, JIAO Cunli, GAO Xiaopeng
2021, 28(1): 90-103. 
DOI: 10.13745/j.esf.sf.2020.5.10

Abstract ( 17 )   PDF (3607KB) ( 35 )  
The Ordovician in the Tarim Basin is an important period of source rock and reservoir formations in the basin. The establishment of an isochronous stratigraphic framework for the Ordovician Yingshan, Yijianfang and Qrebake Formations in the Shuntuoguole area is the basis for geological research and discovery of oil and gas resources. In this contribution, based on the characteristics of petroelectricity, we established the standard of carbon isotope correlation, perfected the conodont zones from Ordovician platform to slope facies, established the division correlation framework of the Ordovician Yingshan-Qrebake Formation and revealed its features and distribution patterns. In perfecting the sequence of conodonts zones, we found that the carbon isotope obviously changed at the boundaries of the three formations, and the stratigraphic sequence was well developed. The sedimentary period of the Yingshan Formation was the peak period of platform development in the Tarim Basin. The sedimentary environment was stable, the water depth basically unchanged, and the carbonate sedimentary rate was relatively high and remained nearly constant. The Yingshan Formation was about 700 m thick in general and its distribution relatively stable. It can be divided into four members, each member was 130-150 m thick. The thickness of the Yijianfang Formation varied from 100-250 m. The Shunnan section was the thickest, with thickness generally over 200 m and possibly reaching 250 m; the Gulong and Shunbei sections came in second, they were generally 130-150 m thick; and the Yuejin section was the thinnest, only 100 m thick. During the sedimentary period of the Yijianfang Formation, the sedimentary environment of the Shuntuoguole area was a carbonate gentle slope environment, and the carbonate sedimentary rate was relatively low. The Yijianfang Formation can be divided into two members: the ~60-70 m thick upper member and the ~70-80 m thick lower member. During the sedimentary period of the Qrebake Formation, the study area was in a deep-water slope environment, with low sedimentary rate, stable sedimentary environment and uniform thinness, generally only about 30-40 m thick, over the whole area.
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Sedimentary differentiation characteristics of and the main factors controlling the Upper Devonian—Lower Carboniferous sediments in the eastern Qaidam Basin
PENG Bo, LIU Chenglin, QI Kening, LIU Dapeng, WANG Jiaqi, LI Zongxing, MA Yinsheng, HU Junjie
2021, 28(1): 104-114. 
DOI: 10.13745/j.esf.sf.2020.5.11

Abstract ( 23 )   PDF (4414KB) ( 38 )  
The DevonianCarboniferous boundary is an extensive unconformity. In the eastern Qaidam Basin, typical sedimentary diversity developed in the Upper DevonianLower Carboniferous sediments, representing a large-scale sedimentary environmental change during the Late DevonianCarboniferous period. To investigate the differences and inherent relations between the Upper Devonian and Lower Carboniferous, we compared the rock assemblage, distribution and lithofacies between the two, using petrologic methods, and described the spatial distribution characteristics of the Late DevonianEarly Carboniferous sediments. By combining the above results with geological background, we discussed the inherent factors for the sedimentary distribution difference. The results demonstrate not only obvious sedimentary differences but also inherent correlation between the Upper Devonian and Lower Carboniferous strata in the eastern Qaidam Basin according to material characteristics and paleogeomorphologic features. Although there is a depositional discontinuity between the two periods, the early-stage Early Carboniferous sediments display inherited paleogeomorphologic features of the Upper Devonian. Under the Late Caledonian NS-trending stress, a NS-differentiated geomorphologic framework started to emerge in the Late Devonian, and the area developed sedimentary systems featuring alluvial fan-volcano. In the early-stage Early Carboniferous paleogeomorphologic framework showing more obvious inheritance, the basic sedimentary composition was similar to that of the Late Devonian period. However, deplanation became dominant as orogeny weakened, and humid climate further altered sedimentary morphology. The rock assemblage evolved from the bottom oxide color upward to gray and grayish-black, and the lithofacies changed from discerning river to mixed tidal flat, accompanied by small-scale interbedding of transgressive and biological marls. The late-stage Early Carboniferous exhibited greater variety than inheritance until deplanation ended. The warming of climate caused glacier melting, and the changing sea level led to global large-scaled transgression. Thus, with the expansion of the Paleo-Asian and Paleo-Tethys oceans, seawater flooded northward across the study area. In addition, due to its low altitude location during that period, the Qaidam Basin experienced marine bloom under warm climate that eventually promoted sedimentary deposition of large-scale epicontinental marine carbonate rocks.
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Sedimentary provenance of the first member of the Shahejie Formation, Lixian Slope, Raoyang Sag
DU Yifan, ZHU Xiaomin, GAO Yuan, LI Linglong, YE Lei, LI Xiaodong, LIU Qianghu, LI Chenghai, ZHAO Tiedong, CHEN Yaqing
2021, 28(1): 115-130. 
DOI: 10.13745/j.esf.sf.2020.5.12

Abstract ( 9 )   PDF (4609KB) ( 22 )  
Source supply not only affected the distribution characteristics of the 1st Member of Shahejie Formation, Lixian Slope, Raoyang Sag, Jizhong Depression, but also controlled the sedimentary type and facies distribution, which in turn controlled the development of favorable hydrocarbon reservoirs. In order to clarify the sedimentary provenance of the Lixian Slope to support further research on the sedimentary system and oilfield exploration, we carried out a comprehensive analysis of sedimentary provenance based on 3D seismic data, petrological characteristics, heavy mineral compositions as well as the ZTR index, detrital zircon geochronology and morphology of the Paleogene Shahejie Formation. The results are summarized below. (1) Progradational seismic reflections were mapped downlapping in the northern, western and southwestern parts of the slope, and the composition maturity and ZTR indices of clastic rocks increased toward southeast, east and northeast, respectively, indicating water systems carrying material sources were injected, into the northern, western and southwestern parts of the slope, respectively. (2) According to the component characteristics of lithics and heavy minerals, the Lixian Slope can be divided into three regions A, B and C. Strata in region A resemble basement uplift or dissected arc in terms of component characteristics, and contain recycled orogenic elements; the components of lithic and heavy mineral assemblages are igneous and sedimentary rocks and garnet-zircon-magnetite-hematite, respectively. In regions B and C, strata resemble basement uplift. In region B, lithification resulted in magmatic and sedimentary rocks with small amount of metamorphic rock, and heavy minerals are garnet and zircon. Region C featured igneous/sedimentary/metamorphic rock combination in lithic rock fragments, and heavy minerals are garnet, zircon and ilmenite. Sedimentary deposition on the slope was controlled by the water systems injected from the north, west, and southwest and converging in the Xiliu area. (3) The morphology and geochronology of detrital zircons showed that the Taihangshang uplift was the parent rock area corresponding to each provenance in the survey area. The rivers carrying the northern, western and sourthwestern provenances mainly flew through the WutaiXinzhouLaiyuan, FanshiLaiyuanFuping and WutaiFupingDingzhou areas, respectively, and the parent rocks they carried were mainly Proterozoic and Paleozoic, Paleozoic and Proterozoic and Paleozoic magmatic rocks, respectively. Compared with the 2nd Member of the Shahejie Formation, the capacity of the western provenance in the 1st Member was significantly enhanced while that of the southwestern provenance relatively decreased. (4) The shallow-water delta deposition systems formed after the water system injection, with the tendency of stronger provenance supply thus better sand-body development closer to the source.
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Sedimentary sequence and prediction of favorable stratigraphic traps in the western slope zone of the southern-central Muglad Basin, Sudan
RAN Huaijiang, FAN Leyuan, KONG Qingdong, XU Xiaojing
2021, 28(1): 131-140. 
DOI: 10.13745/j.esf.sf.2020.5.13

Abstract ( 13 )   PDF (3854KB) ( 38 )  
The Muglad Basin in Sudan is rich in oil and gas resources and currently is under intensive hydrocarbon exploration. The western slope of the southern-central part of the basin in the western slope zone of the Kaikang depression is one of the important exploration areas in the Muglad Basin. At present, the thickness of the remaining strata is about 5000 m as a result of severe denudation during the structural transformation period. Influenced by regional tectonic activities, the evolutionary development of the sedimentary reservoir in the study area is closely related to the tectonic evolution. In the study area, as exploration for tectonic traps entering the middle and late stages, finding new exploration fields and targets has become an urgent task, for which litho-stratigraphic trap exploration becomes an important approach. Currently, the major factor limiting oil and gas exploration is lacking  a systematic understanding of the basic issues, such as the sequence stratigraphic structure of the western slope of the southern-central Muglad Basin. Here, we focused on the evolutionary characteristics of the sedimentary sequence of the western slope zone and their controlling factors through a comprehensive analysis of seismic, well logging and core data using the principle of sequence stratigraphy. We discussed the favorable source-reservoir-cap assemblage and predicted and evaluated the favorable stratigraphic traps in the study area. The main results and achievements are as follows: 1) The basin experienced three large rift movements and three post-rift thermal subsidence depression movements, which resulted in six secondary sequences. In this secondary sequence stratigraphic framework, the target formations, from Abu Gabra to Adok, were divided into thirteen third-order sequences. The filling and evolution of each sequence was controlled by regional tectonic background. 2) Stratigraphic development in the study area experienced three stages of rift-depression while the basin maintained its sedimentary characteristics under the control of structural background in each stage, with obvious difference in sedimentation controls. Based on the analyses of core, logging, earthquake, and paleocurrent in the western slope zone of the southern-central Muglad Basin, it is considered that the lake and braided river delta deposition system mainly developed during the rift period, whereas the braided river and delta mainly developed during the depression period. Due to multistage expansion and contraction of the lake basin, the sedimentary systems exhibited inherited and diverse spatiotemporal distributions. 3) According to the formation sequence of the Cretaceous-Paleogene source rocks, reservoirs and caprocks in the western slope and their spatial assemblage relationships, four sets of source-reservoir-cap assemblages could be identified in the study area featuring two main rock-forming types: the lower generation - upper reservoir type, and the self generation - self reservoir type. In the isochronous sequence stratigraphic framework of the study area, we identified two types of stratigraphic traps: stratigraphic erosion unconformity trap and stratigraphic overlap unconformity trap. The top interface of the sedimentary facies SQ5 in the western part of the study area is a regional erosive unconformity surface. It forms an erosive unconformity triangle with the underlying strata, creating a favorable stratigraphic erosive unconformity trap developing area. The sedimentary facies SQ4 in the northwestern part of the study area overlaps the unconformity surface, forming an overlap unconformity triangle zone—a favorable stratigraphic overlap unconformity trap developing area.
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Features and control factors of gentle-sloped fluvial sandbodies in rift basins: an example from the Wen'an Slope, Baxian Sag
ZHANG Zili, ZHU Xiaomin, LIAO Fengying, LI Qi, ZHANG Ruifeng, CAO Lanzhu, SHI Ruisheng
2021, 28(1): 141-154. 
DOI: 10.13745/j.esf.sf.2020.5.14

Abstract ( 13 )   PDF (10029KB) ( 28 )  
The gentle-sloped fluvial sandbodies in rift lake basins are important oil and gas reservoir units. Based on core, well logs and seismic sedimentology, we analyzed the fluvial sedimentary system and the spatiotemporal distribution of gentle-sloped sandbodies in rifting lake basins to provide reliable basis for fine exploration of oil and gas. The results show that there are four NE-SW trending meandering-braided fluvial successions in the Dongying Formation, Wen’an slope, Baxian sag, consisting of channel, channel bar and floodplain subfacies, which can be further subdivided into six microfacies, including braided channel, meandering channel, sandy channel bar, muddy channel bar, crevasse fan and floodplain. The river spreads out 40°-65° to the north (angle (α)); the apparent width (l) of the river course is 1.47-2.64 km; the apparent width (w) of the main river course is 0.03-0.58 km; the measured thickness (H) of the river course belt is 16.0-52.0 m; the measured thickness (D) of the main river course is 8.0-23.0 m; the drilling interpretation thickness (d) of the main river course is 1-16.5 m; and the average thickness is 6.5 m. In the fault-depression transition period, the gentle-sloped fluvial facies were controlled by many factors, such as basin structure, climate, provenance, sediment compaction and velocity. Differential subsidence of rift basin led to unbalanced subsidence of basin, which was the main factor controlling the fluvial facies type and sandbody spatial distribution in the gentle slope zone. By influencing flow, periodic climate change controlled the river type, adjusting and transforming the early fluvial sediments, thence determining the combination of microfacies and sandbodies. The study also suggests that the development of thick mudstone upslope may limit hydrocarbon charging and migration, whereas the thick channel and channel bar deposited mid and downslope, cross-cut by faults and conduits for migration, most likely have reservoir potentials in the Bohai Bay Basin.
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Sedimentary characteristics of the shallow water delta in rifted lacustrine basin: A case study in the Aradeiba Formation, Unity Sag, Muglad Basin
FAN Leyuan, WU Jiapeng, DIAO Wan, LI Yang
2021, 28(1): 155-166. 
DOI: 10.13745/j.esf.sf.2020.5.15

Abstract ( 20 )   PDF (3770KB) ( 184 )  
Shallow water delta is a hot topic in sedimentology and hydrocarbon exploration with much attention given to depression lacustrine basins. Some scholars have studied the sedimentary evolution of shallow water delta in rifted lacustrine basins and proved that it can also develop in the basins' recess or early rift stage; however, as a whole, the sedimentology of shallow water delta in rifted lacustrine basins was poorly introduced. Here, based on the core, well logging, seismic and laboratory analysis data, the favorable conditions of sedimentation, sedimentary characteristics and vertical evolution of the shallow water delta in Aradeiba Formation are studied in this paper, and the sedimentary evolution model of the shallow water delta in the early stage of rifting period is proposed in combination with the changes of lake level and the sediment supply. Relatively simple structure with a wide gentle terrain inherited from little basement fluctuation, warm humid paleoclimate conducive to source area weathering and sufficient source materials, and frequently fluctuating shallow paleolake level (inferred by core mudstone data) are the favorable structural and sedimentary conditions for shallow water delta development during the early 2nd rifting period in the Cretaceous Aradeiba Formation. Comprehensive analysis results of core depositional cycles, sedimentary structures and components, grain size features and mud logging characteristics show that shallow water delta developed in the study area. As it is shown, the core is segmented by several small scour surfaces or sedimentary sections in an upward rhythm. The lower part of each segment contains medium-coarse sandstone with strong hydrodynamic sedimentary structures, such as trough and wedge cross-beddings, while the upper part deposits mostly variegated mudstone. The content of quartz, well sorted, is very high, and the grain size probability curve of sandstone mainly has the two-segment shape, showing high saltation and a certain amount of suspended population consistent with the sedimentary characteristics of shallow water traction flow. The development of distributary channels is a significant feature of the shallow delta front, featuring a variety of logging facies types, such as high amplitude box, high amplitude bell and medium/high amplitude thin box type. Another important type of reservoir sandbody, mouth-bar, shows medium/high amplitude funnel type on the logging curve. The Aradeiba Formation in the study area is divided vertically into 5 sublayers based on drilling and logging data of several oil fields in the Unity Sag. Each sublayer exhibits obviously different main sedimentary microfacies revealed by logging interpretation results, core facies, logging facies analysis and sandstone percentage maps. Taking into account of factors such as lake level and tectonic revolution, we propose a sedimentary evolution model for the Aradeiba Formation as follows. In the 2nd early rifting stage, mainly distal bars and sandsheets deposited in the 5th sublayer. As rift activity strengthened, with enriching sediment supplies, distributary channels and mouth bar developed in the 4th and 3rd sublayers, respectively, enlarging the shallow-water delta. Strengthening lake hydrodynamics at the 2nd sublayer deposition resulted in sheetlike distributary channel dominance, while rapidly rising lake level led to deposition of mainly shallow lake drape mudstone in the 1st sublayer. This study deepens the understanding of the sedimentology of the Muglad Basin and provides guidance for expanding hydrocarbon exploration in this basin.
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Sedimentary characteristics of the Jurassic shelf-edge delta and oil and gas exploration in the Papuan Basin
ZHANG Yina, CAI Wenjie, YANG Songling, ZHANG Ke, CHEN Jingyang
2021, 28(1): 167-176. 
DOI: 10.13745/j.esf.sf.2020.5.16

Abstract ( 10 )   PDF (4182KB) ( 27 )  
In order to ascertain the sedimentary characteristics of the Jurassic and their relationships with oil and gas exploration in the Papuan basin, on the basis of previous studies of the sedimentary background and paleosource in the basin, we performed joint analyses of seismic, logging and core facies in comparisons with the surrounding Browse basin, from which we identified microfacies and described the vertical and horizontal spreads of the basin's sedimentary systems. Then by comparing the hydrocarbon indices and reservoir physical properties of different sedimentary deposition zones, we defined the favorable source rock and reservoir developmental areas to guide oil and gas exploration. The results show that (1) the Papuan basin featured broad and gentle paleogeomorphology during the Jurassic of the Mesozoic era sloping downward from SW to NE. The main provenance was from the Australia paleocontinents and the peripheral bulge provided local material sources. The Jurassic Formation deposited thick passive continental margin delta sediment, and the microfacies were distributary channels, underwater channels, mouth bars and inter-distributary channels. Its sedimentary characteristics were similar to that of the Plover group of the Jurassic Browse basin also in the northern edge of Australia. (2) The dominant components of the delta came from the southwest, with four delta phases developing vertically in succession through two major regressive and transgressive cycles dominated by an ascending half cycle. The Magobu formation of the middle-lower Jurassic was dominated by a progradational stacking sequence and had sufficient provenance and strong hydrodynamic force, while the Imburu formation of the upper Jurassic mainly consisted of retrogradational sequence with stable mudstone deposition on the top. On the plane, the sedimentary sequence was delta plain-delta front-semi deep water-deep water, with the delta distributing widely. The Papuan fold belt was mainly in the delta front, and the front-end sandbodies were transformed into finger- or point-like ones by the tidal action. (3) The source rocks from the Jurassic Magobu and Koi Iange prodeltas were relatively good, while the winnowed and strongly transformed mouth bars in the delta front had the best physical properties to be the optimal zones for reservoir, and the thrust traps in this belt can be potential exploration targets.
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Jurassic sequence framework and sedimentary system distribution in the Wudun Sag, Dunhuang Basin
DONG Yanlei, ZHU Xiaomin, WEI Minpeng, LI Shunli, ZHAO Ruixing, JIA Tianpeng, GUAN Weifeng, ZHANG Yaxiong
2021, 28(1): 177-189. 
DOI: 10.13745/j.esf.sf.2020.5.17

Abstract ( 12 )   PDF (6537KB) ( 31 )  
The Dunhuang Basin is a Mesozoic rift basin with relatively high exploration potential, and is in the early stage of petroleum exploration. As it is known, the basis of petroleum exploration is building the sequence framework and obtaining the sedimentary system distribution. Using the outcrop, core, lithology, well-logging and seismic data, and taking into account of various geological factors, such as tectonic settings and sediment supplies, etc., we identified the sedimentary facies types and determined the sedimentary distribution in the Wudun Sag, Dunhuang Basin. The findings show that the main Middle-Lower Jurassic strata in the study area can be divided into two third-order sequences SQI and SQII. Roughly, SQI corresponds to the Dashankou Formation, and SQII to the 1st member of the Zhongjiangou Formation. Due to the movement of boundary faults, the Wudun Sag is a half graben-like sag with steep slope in the south and gentle slope in the north, so the sequence distribution thins out from south to north. The material sources in the Dunhuang Basin primarily originated from the Bei, Altun-Qilian and Sanwei Mountains. The sedimentary distribution in the Wudun Sag was controlled by the source supplies from the north, south and east. The Middle-Lower Jurassic strata of the Wudun Sag developed three types of facies: braided river delta, fan delta and lakes. The sedimentary type for Wells XC1 and Dun1 is the braided delta front. Fan delta without drilling wells can be identified by outcrops and seismic data. Lacustrine consists of shore-shallow lake (including lake mud and beach and bar microfacies) and sublacustrine fan (including sand and mud flow deposits). The developmental scale and distribution mechanism of sandbodies were primarily subjected to tectonic and provenance controls, and the sedimentary distribution of different sequences are roughly the same. Continued accumulation of terrigenous detrital rocks in the north and northeast left behind braided delta sediments on the gentle slopes, while, affected by material sources from the Sanwei Mountain, the fan delta sedimentary system developed in the south, and littoral-shallow lacustrine sediments deposited in the central depression.
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Pore evolution and genesis of secondary pores in the Paleogene Dainan Formation, Jinhu Sag, Subei Basin
ZHANG Qin, ZHU Xiaomin, MAO Ling, SUN Zuyu, ZHOU Chen, SU Kang, YANG Ligan
2021, 28(1): 190-201. 
DOI: 10.13745/j.esf.sf.2020.5.18

Abstract ( 12 )   PDF (4452KB) ( 37 )  
The reservoir of the Dainan Formation, Jinhu Sag is a set of clastic reservoirs with medium-low porosity and permeability. The secondary pore is the most important and effective pore type in the study area, and the determination of its genesis and distribution is the key step for predicting favorable reservoirs. Based on core, thin section and cast thin section data, scanning electron microscope imaging, and physical property measurements, we analyzed the rock type, reservoir property, pore type and distribution of secondary pores, and discussed the genesis of secondary pores in association with the dissolution conditions in this area, which will provide scientific guidance for predicting favorable reservoir and for reservoir exploration. Our study showed that clastic reservoirs mainly deposited in delta, fan delta and show-shallow lakes. Controlled by sedimentary facies, the reservoir rocks are complex and include mainly unequal-grained sandstone and pebbly sandstone, fine sandstone and siltstone, showing varying lithologies in different areas. According to the triangular plot, the sandstone is mainly feldspathic debris quartz sandstone. The compositional maturity is medium to low and the textural maturity is medium. The porosity and permeability peak values were 12.0%-14.0% and 110 mD, respectively. There are some differences in reservoir properties associated with different sedimentary facies and regions. The reservoir properties of delta front are most favorable, with an average ~19.4% porosity and ~134.2 mD permeability, and the lithofacies is mainly fine sandstone distributing in the western and central areas. The fan delta near Well Bian-1 in the southeast of the Tongcheng fault zone cames in second. The fan sandstone is gravel bearing mix-grained sandstone with an average porosity of ~12.5% and permeability of ~6.2 mD. The reservoir property of shore-shallow lake is least favorable. It deposited mainly siltstone with an average porosity of ~8.3% and permeability of ~2.3 mD. There are two types of pores, primary and secondary pores, in the Dainan Formation. The dissolution pores included inter and intragranular dissolution pores and mold pores. The intergranular dissolution pores such as calcite, dolomite, iron calcite, etc., formed after dissolution of intergranular carbonate cements. The dissolution pores in grains mainly formed by selective dissolution of feldspar and carbonate debris. The mold pores left by the dissolution of dolomite grains could be seen. There are few primary pores in the Dainan Formation, mainly in the form of residual intergranular pores after quartz enlargement and developed in shallow buried wells, such as Wells Xinzhuang-1 and Guan-1-1. The pores evolved from primary to secondary pores in the Dainan Formation. On the whole, reservoir buried less than 1100 m is mainly in the early diagenetic stage A and mainly developed primary pores; reservoir buried between 1100-1500 m is in the early diagenetic stage B with mixed pore section; and reservoir buried below 1500 m entered the middle diagenetic stage A when secondary pores replaced primary pores. There are three secondary pore developing zones in the reservoir. The first one is between 1200-1600 m and distributed in the relatively shallow to moderately buried well area; the second one is between 1800-2800 m; and the third one is between 2900-3000 m. Among them, the absolute secondary porosity of the first and second secondary pore developing zones is relatively high, indicating strong dissolution. The secondary pore development in the Dainan Formation was related to the dissolutions of calcite and dolomite cements, feldspar and carbonate debris. Mature hydrocarbon from source rocks charging into the reservoir was the main controlling factor for the secondary pore development, as carbonate cements provided the material basis for the dissolution, and the dissolution of feldspar in turn contributed to the development of secondary pore. The formation of secondary pore was also related to the transformation of clay minerals, and fault activity further promoted the development of secondary pore.
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Genesis of the tight reservoirs in the upper fourth member of the Shahejie Formation, northern Bonan Subsag, Zhanhua Sag
WU Dong, LIU Xiantai, DU Yushan, ZHU Xiaomin, JIANG Long, GENG Bin, CHENG Ziyan, GUO Shibo
2021, 28(1): 202-213. 
DOI: 10.13745/j.esf.sf.2020.5.19

Abstract ( 8 )   PDF (4039KB) ( 25 )  
The Bonan Subsag is an important oil and gas-rich region of the Jiyang Depression, Bohai Bay Basin. The deeply buried tight glutenite reservoir in the northern Bonan Subsag is significant for hydrocarbon recovery from the upper fourth member of the Shahejie Formation, and understanding its genesis is the basis of exploring new reservoirs in this area. Here, using various techniques, including drilling, thin section analysis, scanning electronic microscope, physical property measurement, mercury penetration and nuclear magnetic resonance, we show that the tight reservoirs can be recognized in two kinds of fan delta deposits and they were influenced significantly by differential depositions. The fan large delta formed in the early stage hosts an extensive deposition of well separated coarse and fine-grained sandstones due to strong mechanical differentiation, and showed a relatively good correlation between porosity and permeability. Whereas the small fan delta formed in the later stage is composed of poorly separated mix-grained sandstones that accumulated rapidly near the source of clasts, and there was no apparent correlation between porosity and permeability for this reservoir. The source sediment should contain sufficient amount of hard and soluble rock fragments which can effectively improve reservoir property. We show that the tight reservoirs are mainly composed of lithic arkose and feldspathic litharenite and generally immature both texturally and compositionally. Commonly observed were dissolved and primary pores, and flaky or curved lamellar throats, with poor pore connectivity. Five types of pore structures resulted in strong reservoir heterogeneity. We further show that compaction and cementation were important factors for destroying the reservoir properties of the upper fourth member of the Shahejie Formation in the northern Bonan Subsag. Cementation was multitype, multistage and had strong destructive impacts, and the degree of lillite cementation, specially, had a significant influence on the reservoir. Dissolution, fracturing and overpressure were beneficial for improving reservoir properties, however, their overall influence was limited. We then discuss the genesis of tight reservoirs in the upper fourth member of the Shahejie Formation, northern Bonan Subsag, and reveal that the coarse-grained sandstone, containing roughly equal amounts of quartz, feldspar and lithic fragments and a little illite, was beneficial for making good reservoirs in the study area. Besides porosity and permeability, pore size and pore to throat size ratio are key parameters in reservoir evaluation. As it has been demonstrated, higher porosity and permeability, larger pore size and smaller pore to throat size ratio can result in better reservoirs.
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Characteristics and control mechanism of high quality reservoir of lacustrine dolomitic rocks from the Lower Cretaceous of the Erennaoer Sag, Erlian Basin, northeastern China
WEI Wei, ZHU Xiaomin, ZHU Shifa, HE Mingwei, SUN Shuyang, WANG Mingwei
2021, 28(1): 214-224. 
DOI: 10.13745/j.esf.sf.2020.5.20

Abstract ( 14 )   PDF (3749KB) ( 24 )  
The Lower Cretaceous layer of most sags in the Erlian Basin develops a set of lacustrine dolomitic rocks. The dolomitic layer not only serves as an important stratigraphic correlation marker, but also is an effective hydrocarbon reservoir with a highly heterogeneous complex pore structure. The key of hydrocarbon exploration is finding high quality reservoir. In this study, we took lacustrine dolomitic rocks from the Lower Cretaceous of the Erennaoer Sag as an example to investigate the characteristics and genesis of high quality reservoir of dolomitic rocks. We utilized a multifaceted approach including core observation, petrography, scanning electron microscope (SEM), X-ray diffraction and stable carbon/oxygen isotope geochemistry to better understand the rocks petrologic and diagenetic characteristics and their influence on reservoir quality. In order to analyze their reservoir characteristics in-depth, we divided the dolomitic rocks into three types: dolomitic siltstone, dolomitic tuffacecous rocks and dolomitic mudstone. The results are summarized below. Dolomitic siltstone often occurs adjacent to dolomitic/calcareous mudstone. It shows relatively good physical properties with an average porosity of 10.2% and permeability of 1.03 mD, and belongs to low-ultra low porosity and extra-ultra low permeability reservoir. While microcrystalline dolomite can fill intergranular pores or replace matrix or calcite cement, calcite cementation was the major factor for decreasing reservoir porosity and permeability. On the other hand, organic acid dissolution of tuffacecous rich minerals or dolomitic siltstone (adjacent to dolomitic tuffacecous rocks) resulted in intra and intergranular pores. Dolomitic tuffacecous rocks are often interbedded with mudstone and siltstone, occurring mainly in the fore-fan delta subfacies as microcrystalline dolomite or fine-coarse crystalline calcite. They mostly distribute in the tuffaceous matrix as aggregates, with an average porosity of 9.85% and permeability of 0.34 mD, and belong to ultra-low porosity and extra low permeability reservoir. The decreasing reservoir porosity and permeability were primarily attributed to the tuffaceous materials, whose alteration released abundant ionic species into the formation water, resulting in not only early carbonate cementation enhancement but also clay mineral pore filling. In contrast, dissolution contributed the most to reservoir quality improvement as dissolution of carbonate aggregates led to intra and intergranular pores. In dolomitic mudstone, dolomitization is weak and dolomite occurs mainly as micritic crystal along the laminae. The physical properties of dolomitic mudstone are rather poor with an average porosity of 2.5% and permeability of 0.01 mD, resulting in poor reservoir quality. Furthermore, strong carbonate cementation in the late diagenetic stage resulted in the dense reservoir which relies mainly on microfractures for improvements. Although microstructures are prone to develop near the fault zone, their likelihood decreases with increasing distance from the main fault.
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Meteoric water dissolution controls on microbial carbonate reservoir formation in the penecontemporaneous stage: insight from the Lower Cambrian formation of the Tarim Basin
LIU Wei, HUANG Qingyu, BAI Ying, SHI Shuyuan
2021, 28(1): 225-234. 
DOI: 10.13745/j.esf.sf.2020.5.21

Abstract ( 22 )   PDF (5276KB) ( 231 )  
Microbial carbonates are an important part of Sinian and Cambrian formations in China. They are a significant reservoir type, and it is of great significance for carbonate propecting to understand their genesis and reservoir forming mechanisms. However, the formation mechanism of the Cambrian microbial carbonate reservoir in the Tarim Basin is still controversial with various explanations proposed, such as simultaneous and penecontemporaneous dissolution, burial hydrothermal dissolution or multiple fluid interactions, and so on. In this study, we investigated the Xiaoerbulake Formation of the Shiairike and Xiaoerbulake sections in the northwest margin of the Tarim Basin. We analyzed the effect of fresh water on the formation of microbial carbonate reservoir in the early diagenetic period through means of outcrop observation, thin section identification, cathode luminescence, carbon/oxygen isotope analysis and others. The results show that (1) four microbialite facies can be identified in the Xiaoerbulake Formation: stromatolite dolostone, thrombolite dolostone, foam spongy dolostone and microbial-related dolograinstones. The pore system in microbial carbonate rock is complex and the pore types in microbial mounds include intergranular pores, dissolution pores, various scale dissolution vugs/caves and fractures, among which the dissolved pores and vugs/caves are the main pore types. (2) Short-term subaerial exposure and penecontemporaneous dissolution of carbonates by atmospheric fresh water predominantly contributed to pore formation as evidenced by three observations. Firstly, there are some small-scale tracks formed by dissolution on the surface of microbial buildups. Secondly, meniscus cement and unconformity are found in slices, indicating short-term exposure. Meniscus calcite cement is a sign of vadose freshwater diagenesis; and unconformity means a partial absence of fibrous cementing materials between grains and drusy cement that generally formed in burial environment. Thirdly, luminescence of the cement is mottled medium bright, distinctive from that of cement formed in meteoric or burial environment as bright and orange-red or dark red rings in general. In addition, affected by atmospheric fresh water, carbon and oxygen isotopes in the outcrops are slightly lower than in seawater. (3) The microbial carbonate reservoir is heterogeneous and its distribution was controlled by the sedimentary architecture of microbial mounds and sea level fluctuations. It is clear that thrombolite dolostone, foam spongy dolostone and cyanobacteria related dolo-grainstone are developed successively from bottom to top, constituting a complete sedimentary assemblage of mound base, core and cap, respectively. The porosity of the mound core (average 5.47%) is much favorable than that of mound cap (average 3.51%) and mound base (average 2.01%). Difference in rock types in microbial mounds is the main cause of disparity in reservoir quality.
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Characteristics of and main factors controlling the karst shoal reservoir of the lower Cambrian Longwangmiao Formation in the Anyue gas field, central Sichuan Basin, China
ZHANG Manlang, GUO Zhenhua, ZHANG Lin, FU Jing, ZHENG Guoqiang, XIE Wuren, MA Shiyu
2021, 28(1): 235-248. 
DOI: 10.13745/j.esf.sf.2020.5.22

Abstract ( 8 )   PDF (4963KB) ( 25 )  
The Lower Cambrian Longwangmiao Formation gas reservoir in the Moxi Block of the Anyue gas field is the largest complete carbonate gas reservoir discovered in China so far. The gas reservoir, developed in the eastern end of the paleo-uplift in the central Sichuan Basin, has a proven gas-bearing area of 805 km2 and proven gas geological reserves of 4403.8×108 m3. Its high efficiency development is of strategic significance for ensuring natural gas supply in the SichuanChongqing area. Reservoir development in the Longwangmiao Formation shows strong heterogeneity, and the gas well productivity varies greatly across the reservoir. For gas reservoir development and deployment, it is very important to study both macroscopic and microscopic characteristics of the reservoir and evaluate the heterogeneity of the reservoir storage space. Based on core, imaging log and cast slice data as well as CT scan, nuclear magnetic resonance, mercury injection, etc., we conducted a systematic study on the stratigraphic sequence division, correlation, karst pattern, reservoir characteristics and main controlling factors of the Lower Cambrian Longwangmiao Formation in the Anyue gas field of the central Sichuan Basin. Results show that there are three types of reservoir storage spaces in the Longwangmiao Formation: millimeter-sized dissolved vugs, solution pores, and intergrain/intercrystal pores. Reservoir of the Longwangmiao Formation is mainly developed in sand-clastic dolomite, featuring small to medium-sized dissolved vugs or solution pores, with medium-low porosity and medium-high permeability. Porosity of the full diameter samples from the Longwangmiao Formation was between 2%-18.48%, averaging at 4.81%, and the full diameter permeability ranged between 0.01-78.5 mD, averaging at 3.91 mD. Due to fracture and cavity, reservoir permeability acquired at different measurement scales varied greatly. The log interpreted permeability ranged between 0.1-10 mD, which was slightly higher than the core tested permeability; whereas well tested permeability ranged between 3.24-925 mD, about 12 orders of magnitude higher than the core tested permeability. The Moxi area developed four stages of shaols in the vertical and two main shoals with one trench in the horizontal directions, while the second and third stage shaols distribute most extensively. The Longwangmiao Formation experienced three stages of reservoir dissolution, namely the pene-sedimentary, supergene reformation and burial stages. In the pene-sedimentary stage, frequent short time exposures and leaching and dissolution of meteoric fresh water resulted in the formation of early micropores as well as solution pinholes in shoal carbonate deposits. Caledonian period karstification was the key factor for the formation of  the vuggy reservoir of the Longwangmiao Formation. Along the vast karst slope, bed-parallel dissolved vugs and pores were well developed, with the karst vuggy reservoirs superimposing vertically and compacting laterally. The favorable spatial allocation of the bed-parallel dissolved vugs and the high-steep structure fissures resulted in the formation of the apparently homogeneous fractured vuggy reservoir. High-quality reservoir is mainly distributed in the MX11-MX8 and MX10-MX12-MX9 well fields, with a reservoir thickness of 40-50 m.
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Pore types and pore size distribution of the typical Wufeng—Lungmachi shale wells in the Sichuan Basin, China
SHI Zhensheng, WU Jin, DONG Dazhong, SUN Shasha, GUO Changmin, LI Guizhong
2021, 28(1): 249-260. 
DOI: 10.13745/j.esf.sf.2020.5.23

Abstract ( 11 )   PDF (3050KB) ( 118 )  
On the overmature WufengLungmachi marine shale in the Sichuan Basin, China, we carried out a detail investigation through standard thin section observation, argon ion polishing sheet production and SEM imaging, X-ray diffraction of whole rock component, clay mineral analysis, TOC content determination, N2-CO2 adsorption experiment, and RG measurement. The results show that the gas-bearing shale contains organic and inorgainc pores and micro-fractures. The organic pores are bubbly or sponge shaped, have various sizes, and are mostly evenly distributed in organic matters. The inorganic pores are mainly triangular, angular or rectangular shaped and can be divided into quartz intercrystalline and dissolved pores. The micro-fractures are mostly strip-shaped and connected to the organic and inorganic pores to form a network. In the overmature marine shale samples, organic pores exceeded 97% and inorganic pores and micro-fractures were less than 3% in quantity. The overmature shale is composed of more than 80% micro and mesopores, with pore size <10 nm. For meso and macropores with pore size above 10 nm, the organic pores were more than 97% in quantity and more than 50% in plane porosity. In comparison, the inorganic pores and micro-fractures were less than 3% in quantity and less than 50% in plane porosity. The pore size of mesopores ranged from 5 to 400 nm and those with pore sizes below 20 nm exceeded 70% in quantity. Of these, the plane porosity of organic pores increased with increasing pore size and reached the maximum value with pore size between 100400 nm. The inorganic pores are composed mainly of intercrystalline pores and dissolved pores and had the highest plane porosity (nearly 100%) with pore size between 100400 nm. The organic pore porosity increased with increasing TOC content, reaching the 5.5% maximum value and then decreasing progressively. The plane porosity of Layer 1 of sub-Member 1 was the highest, decreasing gradually from bottom to top in the Lungmachi Formation. In addition, plane porosity varied for different areas and the Luzhou and Yuxi areas area had the highest and lowest values, respectively. In the WufengLungmachi shale, the intercrystalline porosity was positively correlated with the siliceous mineral content due to material hardness and organic origin. Besides, the dissolution porosity was positively correlated with the carbonate mineral content as a result of dissolution under burial and compaction. This pattern is possibly relating to reduced pore size caused by collapse under burial and limited resolution of FE-SEM imaging.
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Source-reservoir characteristics and coupling evaluations for the Lower Jurassic lacustrine shale gas reservoir in the Sichuan Basin
HU Zongquan, WANG Ruyue, LIU Zhongbao, LIU Guangxiang, FENG Dongjun, YANG Zhenheng, WANG Pengwei
2021, 28(1): 261-272. 
DOI: 10.13745/j.esf.sf.2020.5.24

Abstract ( 91 )   PDF (4973KB) ( 150 )  
Continental shale gas in the Sichuan Basin has good exploration prospects with obtainable industrial gas flow in the Lower Jurassic formation of the northeastern Sichuan Basin. Compared with marine shale, the characteristics of continental shale include rapid lithofacies variation, multiple types of lithofacies association (often interlayers), low total organic matter content (TOC), and complex source-reservoir relationship. Based on the idea of “source-reservoir coupling”, using macro-micro reservoir characterization methods, we analyzed and evaluated the source-reservoir characteristics and coupling relationship for the continental shale in the Da’anzhai Member of the Low Jurassic Ziliujing Formation. The results can be summarized into four main areas. (1) Lithofacies combinations. The Da'anzhai Member is a set of shallow to semi-deep lake deposits with dark shale and shell limestone interlayers of varying thickness. The main lithofacies combinations of 4 lithofacies and 3 thicknesses are mainly thin (< 1 m) interlayers of shell limestone between high TOC argillaceous shale, thin to medium-thin (< 3 m) interlayers of shell limestone between medium TOC argillaceous/silty shale, and medium-thick interlayers of shell limestone with a small amount of fine-silty sandstone between low-medium TOC silty shale. Shale is the source rock and main reservoir; shell limestone serves as secondary reservoir and interlayer; and tight sandstone with poor physical properties mainly functions as interlayer. (2) Source-reservoir characteristics. The thickness of shale in the northeastern Sichuan Basin is between 30-40 m, with TOC content generally between 1.0%-2.0%. Organic matter types are mainly types II2 and III. The shale reservoir space types are organic and inorganic pores and macro-micro fractures. The most developed pores are marginal and interparticle pores in clay minerals, followed by organic pores, while interconnected micro fractures are also well developed. In the shell limestone interlayers, the inter and intraparticle pores and micro fractures are the main types of reservoir space. (3) The macro source-reservoir configuration and micro source-storage coupling mechanism of continental shale strata in the Da’anzhai Member. Macroscopically, both TOC and porosity decreased with increasing carbonate content. The condition of source-reservoir coupling is better in shale than in interlayers, and the argillaceous shale is the best shale. Microscopically, there are a integrated source-reservoir coupling between organic matter and organic pore and a migration coupling within nm to μm distance between organic matter/inorganic pore and micro-fracture in shale strata; between shale and interlayer there is a migration coupling within mm to cm distance; and in the interlayer, migration coupling occurs within cm to m distance between organic matter (from the adjacent shale)/inorganic pore and micro-fracture. (4) Coupling evaluation. The corresponding source-reservoir coupling evaluation parameters are established for shale strata and interlayers, while coupling conditions are evaluated for two typical wells, and a preferred horizontal well’s target window has been suggested based on the evaluation results.
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Recognizing turbidite sand bodies in thinly interbedded sand-shale reservoir in the Weixinan Depression by energy half-time attributes
LUO Qi, HUANG Shizhuo, SHI Defeng, WANG Rui
2021, 28(1): 273-281. 
DOI: 10.13745/j.esf.sf.2020.5.25

Abstract ( 9 )   PDF (4805KB) ( 16 )  
The Weixinan Depression is an important oil and gas accumulation area in the Beibu Gulf Basin of the South China Sea, and turbidite sand body is an important reservoir contributing geological body showing great oil and gas exploration potential. With the exploration and production constantly advancing, it is difficult to identify a true turbid reservoir by conventional geophysical techniques due to severe overlapping of sandstone and mudstones seismic characteristics. For this reason, considering the association of strong amplitude reflection with formation of turbidite sand body, we analyzed the two main causes of the abnormal strong amplitude reflection in the non-reservoir formation, i.e., easy production of strong amplitude reflection by interbedding of thin sandstone and mudstone, and severe interference in strong amplitude response of low velocity mudstone from indistinguishable responses of sweet reservoir and surrounding rock. On this basis, we put forward a new technology for reservoir identification based on energy half-time. Firstly, after the isochronous stratigraphic reflection interface is determined, and a fine isochronous stratigraphic framework is established, the fifth-order sequence is examined as a set of equivalent bodies. Because the densities of sandstone and mudstone are relatively stable, it is established that the slight amplitude variation inside the abnormal body mainly reflects the change of velocity structure. That is, the change of velocity structure is key to distinguish true reservoirs from false ones. Secondly, a wedge-shaped model of sandstone and mudstone interbedding is established, which demonstrates that different lithologic compositions correspond to different velocity structures, and different velocity structures cause the change of the attribute value when the energy half-time is changed. That is, the change of velocity structure in thin sand-mud interbedding can be effectively described by the property of energy half-time. Thirdly, based on the fine velocity analysis of the study area, slight amplitude variation inside the anomaly body can be interpreted by forward modeling according to three combination schemes: same lithology and physical property, same lithology but different physical properties, and different lithologies and physical properties. Hence, the main cause of the impedance change can be determined, and the possible composite lithology of the abnormal body can be predicted synthetically to reduce prediction ambiguity. Applying this technology to the case exploration, the results of lithology prediction agrees with the drilling findings, which strongly supports the exploration and production practice in the Weixinan Depression.
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Classification and combination model characteristics of pathway system in marine faulted basin: Taking the Paleogene Lingshui Formation, Qiongdongnan Basin as an example
CHEN Huanqing, ZHU Xiaomin, ZHANG Gongcheng, ZHANG Yaxiong, ZHANG Qin, LIU Changli
2021, 28(1): 282-294. 
DOI: 10.13745/j.esf.sf.2020.5.26

Abstract ( 18 )   PDF (5165KB) ( 54 )  
Taking the Lingshui Formation of the Paleogene in the Qiongdongnan Basin as an example, the problems associated with oil and gas exploration include limited drilling data, complex seismic data, development of fault system, and difficulty predicting favorable exploration targets. In this study, under the guidance of the classical sequence stratigraphy theory of Vail, we established the isochronal stratigraphic framework by closely combining the shallow water drilling data with the deep water seismic data in delineating the target strata into four three-level sequences. Evidences from rock types, sedimentary structures, authigenic minerals and biological fossils showed that the target strata formed in a marine sedimentary environment. In the isochronous stratigraphic framework, we carried out the single element analysis for each geogenetic pathway system through sediment analysis, fault system interpretation, and unconformity characterization. The pathway system of the target strata was genetically divided into three main typesreservoir, fracture and unconformity, and we described in detail the spatial developmental characteristics of different types of transport systems. The slope and pelvic floor fans in the lower system domain, the fan delta, beach bar and shore sand in the pathway system domain, and the fan delta and braided river delta in the higher system domain are the main sedimentary facies types corresponding to the reservoir transport system. The large controlling concave fault spreading in the northeast can be a favorable channel for oil and gas migration. The unconformity is divided into four categories: parallel, overlap, truncation and overlap-truncation unconformities. The widely developed unconformity in the sequence interface can be a favorable migration channel and plays an important role in oil/gas migration and transmission. We further analyzed the spatial combinations of different pathway systems and summed up the modes of combination into six types: fault+unconformity (reservoir) or “T” type transport, fault+unconformity (reservoir) or comb type transport, fault+reservoir or grid type transport, fault+unconformity or stepped type transport, and fracture type transport (fracture+bedding surface+pore). We then located each type of pathway system in the basin, and established the pathway system model. Based on the analysis of petroleum geological characteristics, such as temperature/pressure field and fluid potential as well as hydrocarbon source rock and cap rock, we predicted the favorable oil and gas exploration zones, which mainly included the north margin of the Yanan Depression, the Songxi Depression, the north margin of the Songnan Depression, the periphery of the North Reef Depression, the south slope of the Lingshui Depression, and the north and south margins of the Changchang Depression. Among the four sequences, sequence Ⅰ was the most favorable zone, followed by sequences Ⅲ and IV, whereas sequence Ⅱ was the least favorable zone.
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Marine frontier basin petroleum resources assessment: A case study of the Carboniferous of the Delingha Depression, Qaidam Basin
LIU Chenglin, ZHANG Yu, YANG Shenghao, LI Zongxing, TIAN Jixian, PENG Bo, MA Yinsheng, YANG Yuanyuan, KONG Hua
2021, 28(1): 295-307. 
DOI: 10.13745/j.esf.sf.2020.5.27

Abstract ( 11 )   PDF (5408KB) ( 19 )  
Petroleum resources assessment for the marine frontier areas must face the challenges of few geologic data, low level of knowledge, and difficult parameter acquisition, which affect the credibility of petroleum resource assessment results. In this case study, we evaluated the Carboniferous of the Delingha Depression, Qaidam Basin using multiple methods, including resources assessment by basin modeling and analogous comparison and key parameter determination by geophysical, geochemical and petroleum geological analyses. Taking into account the source-reservoir-caprock association, we delineated the favorable exploration zones through simulation of hydrocarbon migration/accumulation and calculation of petroleum resources in different zones. There are four types of source rocks, mudstone, carbonaceous mudstone, coal and marl, in the upper Carboniferus, and two types of source rocks, mudstone and marl, in the Lower Carboniferous. In the Crook Formation, the upper Carboniferous source rocks have higher organic matter abundances, with the total organic (TOC) contents equaling >1.0%, 6.0%-10.0%, >60% and >0.3% in most mudstone, carbonaceous mudstone, coal and marl, respectively. The main types of organic matters are types II and III, with a small amount of type I. Most hydrocarbon rocks are mature and high mature. Clastic and carbonate reservoirs are developed in the Carboniferous strata into three sets of regional caprocks and five sets of local caprocks with varying forms of source-reservoir-caprock associations, including self source-self reservoir-self cap, lower source-upper reservoir and upper cap, and upper source-lower reservoir-upper cap. The hydrocarbon generation period of the Carboniferous was mainly from the Late Carboniferous to Middle Triassic, and the hydrocarbon generation peak moment was from the Middle Permian to Middle Triassic, when the basic geological conditions for petroleum formation were generally favorable. The Carboniferous of the eastern Qaidan Basin possesses 0.38 billion of petroleum, 0.16 billion tones of oil and 276 billion cubic meters of gas resources, with petroleum resources mainly distributing in the Ounan Sag and Oulongbuluke Uplift.
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Big data science from informationization to modelling to intelligentization: New paradigm of applied geochemical research
XI Xiaohuan
2021, 28(1): 308-317. 
DOI: 10.13745/j.esf.sf.2020.8.12

Abstract ( 43 )   PDF (1426KB) ( 243 )  
Geosciences are in the era of information science and informationization when as modern geosciences driven by the big data strategy are booming, and geochemistry has come into being in modern times as having the natural attribute of structured information science. The overall idea of conducting applied geochemical research, based on the big data science from informationization to modeling to intelligentization, is as follows: foremostly, build the information system on the basis of the geochemical big data; secondly, establish the zonal enrichment model of ore-forming geochemistry and the cumulative effect model of ecological geochemistry using the method and theory of the earth system science; thirdly, use intelligent system and techniques to analyze, judge and predict large amount of geological and geochemical nonlinear phenomena according to patternized thinking; and lastly, based on the geochemical research and application, evaluate and solve the important problems concerning the resources and environment which will influence the nationwide socioeconomic development. Big data and earth system science are two pillars of the applied geochemical research. Aimed at solving the main problems facing us today, this paper discusses the geochemical big data informationization, information modelling, model intelligentization, etc. The key point is to construct the intelligent system, with the geoscientific model as the main body, and study the scientific problems of metallogenic system and ecosystem. The world socioeconomic development and resources-environmental processes are in the age of globalization, therefore, we must look at the ore-forming geochemistry and ecological geochemistry from the perspective of global resources allocation and global environmental changes. Nowadays, the key computer techniques are becoming increasingly mature and perfect, and the frontier studies of the intelligent technology are making breakthroughs continuously. Furthermore, it is a great advantage for geochemistry to have the attribute of structured information science. All these contexts indicate that we have the preconditions to establish the geochemical big data information science such that it may stand out from the field of geosciences and become an emerging research paradigm advancing continuously with broad vision.
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Cambrian Qimantagh island arc in the East Kunlun orogen: Evidences from zircon U-Pb ages, lithogeochemistry and Hf isotopes of high-Mg andesite/diorite from the Lalinggaolihe area
WANG Bingzhang, PAN Tong, REN Haidong, WANG Tao, ZHAO Zhiyi, FENG Jianping, ZHANG Jinming
2021, 28(1): 318-333. 
DOI: 10.13745/j.esf.sf.2020.11.30

Abstract ( 37 )   PDF (5012KB) ( 63 )  
The Qimantagh belt is an important component of the East Kunlun orogen and plays a key role in reconstructing the early Paleozoic convergent history of the northern Tibetan Plateau. The Qimantagh belt used to be interpreted as part of early Paleozoci arc or back-arc basin, however, the geological records of the early-stage of subduction have seldom been reported and the age of subduction initiation still remains poorly constrained. In this paper, we studied the petrology, geochemistry, zircon U-Pb ages and Hf isotopes of volcanic rocks and diorite from the Qimantagh Group of the Lalinggaolihe area and discussed their petrogenesis and tectonic significance. Volcanic rocks are dominated by andesites with moderate SiO2 contents (54.02%-63.99%), low TiO2(0.19%-0.24%), Al2O3(10.67%-12.86%) and P2O5(0.03%-0.05%) contents and low FeOT/MgO ratios (0.80-1.04), high MgO contents (7.86%-11.44%) and Mg# (63-69). Diorites have moderate SiO2 contents (53.62%-55.81%), low TiO2(0.19%-0.25%), Al2O3(10.58%-11.52%) and P2O5(0.02%-0.04%) contents and low FeOT/MgO ratios (0.9-1.0) with high MgO contents (9.86%-11.63%) and Mg# (64-66). The high-Mg andesites and diorites share similar trace-elements compositions and have low Th(0.22-1.23 μg/g), Rb(4.11-16.11 μg/g) and Ba(66.7-166 μg/g) contents, high Cr(286-668 μg/g) and Ni(57-181 μg/g) contents, moderate δEu (0.91-1.47) and low Sr/Y ratios (19.7-36.1). They also have low total REE contents (11.8-27.3 μg/g) and (La/Yb)N values (1.3-4.3), indicating weak LREE/HREE fractionation. The primitive mantle-normalized trace-element diagram shows positive Sr, Ba and Pb anomalies and negative Ta, Nb and Ti anomalies. The LA-ICP-MS U-Pb zircon ages of high-Mg andesites and dorites are 519 and 520/516 Ma, respectively. Both zircons share similar Hf isotopic compositions with εHf(t) of +10.5 to +15.8. These results indicate that the andesites and diorites from the Lalinggaolihe area originated from partial melting of depleted mantle wedge in island arc setting during the early stage of subduction of the Proto-Tethyan Ocean. This study further demonstrates that the subduction of paleo-ocean basin in the Qimantagh belt predated 520 Ma, which improves the understanding on the early-stage subduction history of the Proto-Tethys Ocean in the East Kunlun orogen.
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An alternative perspective of the origin of oxide gabbros from ocean ridge: The case of the ODP 735B core from the Southwest Indian Ridge
JIN Ye, FANG Nianqiao, YUAN Xiaobo, HU Ke
2021, 28(1): 334-352. 
DOI: 10.13745/j.esf.sf.2020.6.32

Abstract ( 17 )   PDF (4704KB) ( 19 )  
ODP 735B core from the Southwest Indian Ridge is highly abundant in Fe-Ti oxide-rich gabbros, which is particularly significant in lothologic Unit Ⅳ of this core. Meanwhile, the primary silicate minerals in Unit Ⅳ display unique trend of compositional variations. The formation and the unusual compositional variations of the oxide gabbros are commonly considered to have occurred through modifications by highly differentiated Fe-Ti-rich melts forming beneath or adjacent to Hole 735B, which migrated into the section along shear zones and reacted with an upwardly differentiated olivine gabbro protolith. Here, we report the detailed petrographic features and structure-controlled mineral profiles of the Unit Ⅳ oxide gabbros to provide a new perspective on the their formation. It can be seen that when some evolved silicate minerals formed due to the injection of exotic materials, Fe-Ti oxides should have remained unsaturated in the liquid. Additionally, the clinopyroxenes follow the crystallization sequence of augite → pigeonite → augite during crystallization, and near contacts with the later-formed minerals in the oxide gabbros (e.g., Fe-Ti oxides or orthopyroxene), the clinopyroxenes have significantly evolved compositions. Based on the detailed petrographic features and mineral compositional microprobe data, combining with discussion on Fe-Ti-rich melts by former researchers, we propose exotic mantle-derived hydrous fluids or liquid mixtures of hydrous fluids and basaltic melts, rather than just highly differentiated Fe-Ti-rich melts, as the suitable exotic materials involved within the formation of the oxide gabbros in Unit Ⅳ. Theoretically, these fluids might not originate from the same source region as the basaltic melt, which means that these fluids might migrate not only vertically, but also laterally over long distances. Therefore, it is reasonable to speculate that oxide gabbros in Unit Ⅳ should have unique isotopic features, which would be important clues in testing the mechanism of the formation of oxide gabbros in Unit Ⅳ, even in the whole 735B core.
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Discussion on the features and mechanism of cryptoexplosive breccia in Wu'an, Hebei Province
LI Ruipeng, CUI Xiaoliang, SU Shangguo, ZHANG Yanan, LIANG Cuntao, CHEN Xuegen
2021, 28(1): 353-362. 
DOI: 10.13745/j.esf.sf.2020.11.29

Abstract ( 23 )   PDF (4225KB) ( 55 )  
Cryptoexplosive breccia is widely distributed in Wu'an, Hebei Province. The study of its forming process of is important to the understandings of the interaction between magma and fluid and the formation of Fe deposit. We carried out in situ mineralogical analysis by electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in combination with detailed field and microscope investigation. The results show that the Wu'an breccia is mainly distributed between the Hongshan intrusive complex (mainly diorite) and its host rock (limestone). The gravels are mostly angular or subangular-shaped and could be easily pieced together. Secondary enlargement of calcite is often seen on the edge of the angular gravels of different colors due to different Fe contents. The matrix of breccia is mainly composed of calcite and hematite. A composition zone can be observed in hematite, which has higher FeO content in the core and higher MgO and Al2O3 contents in the edge. The total iron is negatively correlated with SiO2 content. The trace elemental characteristics of hematite show enrichments of Ba, K and U and depletions of high field-strength elements, such as Th, Ta, Nb, Zr and Hf, which may indicate that the magma was formed in a magma-fluid environment. The core hematite zone is enriched in Ti and Pb. The enrichment of Ti indicates the hematite's magmatic origin, while the enrichment of Pb indicates a later involvement of hydrofluid superposition. According to this study, the genetic model of the cryptoexplosive breccia in the Wu'an area is as follows: the trapping of rising deep magmatic fluid by overlying host rock resulted in boiling and overpressure of the fluid, which led to cryptoexplosion as the overlying rock strata suddenly decompressed and broke up and the magmatic fluid cemented the upper rock mass into rocks.
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Geochemical genes related to granite weathering in the Yunmengshan area of Beijing, China
WANG Xuyang, YAO Ning, GONG Qingjie, CHAO Yuede, PENG Cheng, WU Yuan
2021, 28(1): 363-374. 
DOI: 10.13745/j.esf.sf.2020.11.11

Abstract ( 29 )   PDF (3095KB) ( 41 )  
The geochemical gene technique is a new geochemical tracing technique formally proposed over the past two years. In this investigation, samples from two adjacent weathering profiles (B and C) developed in granite in the Yunmengshan area of Beijing, China were collected to test two geochemical genes named lithogene and gold metallogene. In samples from profile B, the chemical index of alteration (CIA) varied between 51.3-58.9, and the weathering index of granite (WIG) varied between 68.6-89.4, indicating incipient weathering in this profile; while in samples from profile C, CIA varied between 52.4-78.4, and WIG between 25.3-84.8, indicating intermediate weathering in profile C. The noticeable difference of weathering degrees between the two adjacent profiles indicates that different micro-environment may result in significantly different weathering products in the same granite. The lithogene similarity between samples and fresh granite varied from 85% to 100% in profile B and 90% to 100% in profile C, while the gold metallogene similarity between the two varied from 85% to 100% in profile B and 80% to 100% in profile C. With respect to top soil, the similarity of lithogenes varied from 80% to 100% in profile B and 90% to 100% in profile C, and the gold metallogene similarity varied from 85% to 100% in profile B and 80% to 100% in profile C. According to the criterion that gene similarity above 80% indicates similar genes, the lithogene and gold metallogene illustrate not only good heredity and inheritance in each profile but also high similarity between the two profiles. Besides, with respect to the ideal gold ore (gold metallogene 12020202020), the similarity of gold metallogenes varied from 25% to 50% in profile B and 30% to 50% in profile C. According to the criterion that mineralization can be recognized when the gold metallogene similarity reaches 40%, one mineralization layer of ca. 1 m thickness can be recognized in each profile. Thus, the gold metallogene may also be used as an integrated geochemical indicator in gold exploration.
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Characteristics and controlling factors of volcanic reservoirs of subaqueous pyroclastic rocks: An analysis of the Miocene Kora Volcano in the Taranaki Basin, New Zealand
TANG Huafeng, WANG Hanfei, Ben KENNEDY, ZHANG Xinyu, Marcos ROSSETTI, Alan Patrick BISCHOFF, Andrew NICOL
2021, 28(1): 375-387. 
DOI: 10.13745/j.esf.sf.2020.11.12

Abstract ( 18 )   PDF (7219KB) ( 44 )  
A clear understanding of the reservoir characteristics and formation process is important for analyzing the distribution of favorable reservoirs and evaluating their qualities. However, the multi-component and multi-feature characteristics of volcanic rocks can complicate the reservoir formation process. Here, we used a dataset of porosity, permeability, pore aperture and thin section from 5 boreholes to analyze the reservoir characteristics, components of void space, fracture aperture and relationship between the primary and secondary porosities in the Kora Volcano. The followings are the results: Firstly, the porosity is mainly contributed from secondary pores, followed by fractures then primary pores. The Kora Volcano has high porosity and permeability and bimodal to unimodal pore aperture. Secondly, the microfractures should have been generated during the magma fragmentation and/or eruptive period(s). The macrofractures were generated by the compressional tectonism and had existed before the oil charging. The sieve and moldic pores should have been generated during the weathering and deep burial stages. The intercrystal micropores were generated by the alteration and/or devitrification diagenesis during the burial stage. Thirdly, the primary porosity controls the secondary porosity. The threshold value of fracture aperture for generating sieve and moldic pores is ca. 9 μm in the Kora Volcano. The bigger aperture of fracture appeared prior to the sieve and moldic pores. And finally, the increasing content of non-juvenile particles could remarkable decrease the total porosity, especially in high porosity rocks. The porosity and permeability of proximal belt are higher than those of distal belt. The calcite cementation also reduced the porosity and permeability of tuffaceous sedimentary rocks, especially in the distal belt of submarine eruption volcano. The findings presented in this work can provide insights into the origin, evolution and distribution of volcanic reservoirs.
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Effects of the lacustrine hydrothermal activity in the Yanchang period on the abundance and type of organic matter in source rocks in the Ordos Basin
JI Liming, LI Jianfeng, ZHANG Mingzhen, HE Cong, MA Bo, JIN Peihong
2021, 28(1): 388-401. 
DOI: 10.13745/j.esf.sf.2020.7.6

Abstract ( 12 )   PDF (3590KB) ( 31 )  
We selected continuous core samples from the Yanchang Formation in Well YK1, southern Ordos Basin to completed a systematic source rock evaluation and geochemical and palynofacies analyses. We discussed the effects of lacustrine lake hydrothermal activity in the Yanchang period on the abundance and type of organic matter in source rocks. Field studies show that the high-quality source rocks developed in the Chang 7-3 interval are mainly carboniferous mudstone and oil shale, the well-developed source rocks from some sections of the Chang 7-1+2 interval are mainly gray-black mudstone, and the individual stratum of the Chang 6 member, interbedded with sandstone and mudstone interlayers, can also form good source rocks. The hydrothermal activity indicators, Al/(Al+Fe+Mn) and (Fe+Mn)/Ti ratios, show that the ancient lake underwent three episodes of significant hydrothermal activity corresponding to the above three stages of source rock development, with the peak intensity in the Chang 7-3 period. The source rock from Well YK1 can be divided into 3 types of palynofacies. Palynofacies A, mainly type III and a small amount of type II organic matter, is distributed in the Chang 89 and Chang 6-2+3 members, where translucent ligno-cellulosic fragments and gelified particles are dominant organic debris. Palynofacies B, distributed in the Chang 7-3 interval, is mainly amorphous type Ⅰ and Ⅱ1 organic matter containing high abundance algae. Distributed in the Chang 7-1+2 interval, Palynofacies C is mainly amorphous type II organic matter with simultaneously developed phytoclasts. The source rock total organic carbon content, organic matter type and fraction of hydrocarbon-generating components correlate well with the intensity of hydrothermal activity. It suggests that hydrothermal activity may play a role in the lake algae and plankton proliferation, thereby improving the abundance and quality of organic matter in source rocks. Indeed, our study show that the algae bloom and high-quality source rock deposition occurred during the Chang 7-3 period of peak hydrothermal activity.
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Genesis of the abnormal formation pressure in the Wufeng-—Longmaxi Formation, Sichuan Basin and a generalized Poisson‘s ratio prediction method: a case study of the Changning area
JING Cui, HAO Long, ZHANG Jing, DENG Xia, YU Wenhui
2021, 28(1): 402-410. 
DOI: 10.13745/j.esf.sf.2020.12.2

Abstract ( 20 )   PDF (2746KB) ( 47 )  
We compared the measured pressure coefficient of the WufengLongmaxi Formation, Sichuan Basin with the current tectonic division and the eastward movement velocity of the QinghaiTibet Plateau, and found that the formation pressure of the Wufeng-Longmaxi Formation might be affected by the multi-stage tectonic movement since the Yanshan Movement. The overpressure zone is located in the central areas of the high steep tectonic belt in eastern Sichuan and low steep tectonic belt in southern Sichuan. During the YanshanXishan period, the east side of the Huayingshan fault zone was strongly extruded to form a long strip-shaped high steep anticline and an open syncline. The eastward movement of various blocks of the QinghaiTibet Plateau might also have influenced the pressure distribution in the Wufeng-Longmaxi Formation. Drilling results showed that after the deposition of the Wufeng-Longmaxi Formation in the Sichuan Basin, organic matters in the formation were transformed into crude oil and then cracked into natural gas under the action of abnormal geothermal gradient. Therefore, hydrocarbon cracking led to fluid expansion, which was the second causative factor of abnormal formation pressure in the Wufeng-Longmaxi Formation. Meanwhile, local abnormal formation pressure may reflect the shale gas preservation condition. Based on the analysis of the formation pressure anomaly in the WufengLongmaxi Formation, the abnormal formation pressure was attributed to horizontal and vertical forces, and the Poisson’s ratio formation pressure prediction method was developed based on Poisson‘s ratio calculation using seismic parameters. It is proved that this prediction method is suitable for predicting the shale gas formation pressure in the Wufeng-Longmaxi Formation. After years of testing and improvements, this prediction method has been widely applied in the exploration and development of shale gas fields in the Weiyuan-Changning area, bringing huge economic and social benefits. By 2019, the cumulative production of the Changning shale gas field has reached 13 billion cubic meters. The generalized Poisson’s ratio formation pressure prediction method is one of the key technologies propelling the Changning shale gas field to become the No.1 shale gas field in China.
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Variations of Poisson's ratio for tight sandstone and shale under changing confining or pore pressure: characteristics and mechanism
DU Shuheng, LIANG Yaohuan, SHI Yongmin, GUAN Ping
2021, 28(1): 411-419. 
DOI: 10.13745/j.esf.sf.2020.12.14

Abstract ( 34 )   PDF (3037KB) ( 50 )  
When the confining pressure (external pressure) or pore pressure (internal pressure) changes, the variation characteristics and mechanism of Poisson's ratio for tight sandstone and shale still need to be clarified. On the basis of the effective stress theory of Terzaghi and the new concept of effective stress proposed by domestic scholars, this study analyzes the characteristics and mechanism of Poisson's ratio change based on the dynamic and static Poisson's ratio measurement results. The rock types consist of tight sandstone of the Gaotaizi Formation and shale of the Qingshankou Formation in the Songliao Basin, respectively. The anisotropic distribution of Poisson's ratio curves is obvious, which would have some impacts on the propagation law of compression fractures; as far as the scientific definition of “effective stress” is concerned, “Terzaghi effective stress” is suitable for fractured reservoirs while “new effective stress” is suitable for shale with large porosity and uniform pore distribution. The conclusion will provide some important theoretical basis for the effective development of tight oil and gas.
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Identification and quantification of factors controlling hydrogen and oxygen isotopes of geothermal water: an example from the Guide Basin, Qinghai Province
DAI Wan, JIANG Xiaowei, LUO Yinfei, ZHANG Hong, LEI Yude, TONG Jue
2021, 28(1): 420-427. 
DOI: 10.13745/j.esf.sf.2020.7.8

Abstract ( 25 )   PDF (1548KB) ( 33 )  
Hydrogen and oxygen isotopes have been widely used in the study of geothermal fluid. However, due to the influences of multiple factors, the understanding of the processes controlling the hydrogen and oxygen isotopic compositions of geothermal water is often incomplete. Here, we conducted a case study on the geothermal water from the two faults along the margin of the Guide Basin. We collected water samples from five hot spring sites and compared the differences in hydrochemical components and geothermal reservoir temperatures of these samples. We found an approximate relationship between the degree of water-rock interaction and whether the sample deviates from the rainfall line. At the intensively studied Zhacangsi site, previous researchers have quantified the effect of cold water mixing in thermal water and determined the cold water mixing ratio. Based on this ratio, we estimated the magnitude of change in hydrogen and oxygen isotopes caused by cold water mixing and liquid-vapor separation. We also established the isotopic compositions of the geothermal water before water-rock interaction based on the effect of the interaction on the “oxygen shift”. The results were consistent with the isotopic compositions of geothermal water from sites with low degrees of water-rock interaction. The methods proposed in the current study can be applied to analyzing hydrogen and oxygen isotopes in other geothermal fields to enrich our understanding of the circulation process of geothermal fluid.
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Multiscale runoff prediction based on the EMD combined model
LI Fuxing, CHEN Fulong, CAI Wenjing, HE Chaofei, LONG Aihua
2021, 28(1): 428-437. 
DOI: 10.13745/j.esf.sf.2020.10.22

Abstract ( 13 )   PDF (2543KB) ( 21 )  
Affected by global climate change and human activities, runoff sequences increasingly show unsteady and non-linear characteristics. In order to reduce the forecast errors caused by these runoff characteristics, we took full advantages of different models to improve the accuracy of runoff prediction traditionally done by the single model approach. Taking the Manas River, a typical inland river in the arid area as an example, we used empirical mode decomposition (EMD) to extract physically meaningful signals from the runoff sequence to obtain multiple intrinsic mode functions (IMF) at different time scales and a trend indicator. We then used the ARIMA and GRNN models to simulate the IMF components at different time scales and analyze the future runoff changing trends. Next, we used the multiple linear regression, Spearman correlation coefficient and average influence value methods to screen the atmospheric circulation factors and them used as inputs to the neural network model; we then constructed the combined model according to the local frequency characteristics of the sub-sequences. Finally, we reconstructed the prediction results of each IMF component to obtain the runoff prediction. However, a single evaluation index cannot fully evaluate the accuracy of the prediction model. In this paper, we constructed the TOPSIS evaluation model to quantitatively evaluate the runoff prediction model and objectively evaluate the models superiority. The results show that using EMD can effectively extract the multi-timescale signals hidden in the runoff sequence, and the trend indicator indicated that the Manas River runoff is on the rise. Using EMD could improve the passing rate of the ARIMA model by 25%; but the relative errors of high frequency components IMF1, IMF2 and IMF3 in the ARIMA model were more than 70%, indicating the prediction results are not ideal. In the GRNN model, selected predictors could effectively improve the model accuracy, and the predictors selected by the MIV method were shown to be most suitable for the Manas River. Overall, the GRNN-EMD-ARIMA combination model had the highest passing rate, and the TOPSIS model had the highest score. The prediction results can be used as a scientific basis for water resources planning and dispatching, and the modeling ideas can also provide new ways to optimize runoff prediction models.
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Relationship between Cd contents in rice or wheat and soil: Insight from a simulation study
YU Hao, SU Zhijie, ZHU Peitian, CHEN Yong, YANG Qiao, ZHAO Zhongqiu
2021, 28(1): 438-445. 
DOI: 10.13745/j.esf.sf.2020.10.21

Abstract ( 42 )   PDF (1488KB) ( 69 )  
The prevention and control of heavy metal pollution in soil has become one of the important tasks in improving the quality of cultivated land and protecting the ecological security of the land. In order to scientifically predict the relationship between heavy metal contents in soil and staple crops (rice and wheat) in China and reduce the collaborative monitoring of a large number of agricultural products and soil form unpolluted agricultural land, as well as realize the safe use of heavy metal contaminated agricultural land, we studied heavy metal Cd as an example. We selected soil Cd content, pH value, cation exchange capacity (CEC) and organic carbon (OC) as inputs and the Cd contents of rice and wheat as outputs to build the multiple regression and neural network models through simulation. The results showed that the Cd content in rice or wheat was positively correlated with soil Cd content. The predictive abilities of the simulated multiple linear regression model for Cd in rice or wheat and soil were 67.8% and 83.8%, respectively. But the corresponding prediction model based on neural network had higher R-values in the training, validation and test sets than the multiple linear regression model, and the MSE value was small. Thus, the prediction accuracy of the neural network was better than that of the multivariate model for predicting Cd conte
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