2023, Volume 30 Issue 6
25 November 2023
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New insights into the formation mechanism of deep-ultra-deep carbonate reservoirs and the direction of oil and gas exploration in extra-deep strata
MA Yongsheng, CAI Xunyu, LI Huili, ZHU Dongya, ZHANG Juntao, YANG Min, DUAN Jinbao, DENG Shang, YOU Donghua, WU Chongyang, CHEN Senran
2023, 30(6): 1-13. 
DOI: 10.13745/j.esf.sf.2023.2.35

Abstract ( 650 )   HTML ( 23 )   PDF (11552KB) ( 200 )  

In deep carbonate strata of the Sichuan and Tarim Basins a series of large scale oil and gas fields have been discovered successively, including the Jingbian, Tahe, Puguang and Yuanba fields, and a “ternary reservoir control” theory for reef-shoal carbonate reservoirs has been developed. In recent years, high-quality carbonate reservoir rocks are discovered in ultra-deep layers at depths of greater than 7000 m or even 8000 m. The deep reservoirs show diverse rock types and complex interactions between the reservoir controlling factors such as facies belt, fluid, pressure, and fault. Using the newly obtained ultra-deep drilling data we carried out a systematic investigation into the formation mechanism of the ultra-deep carbonate reservoir rocks. In-depth studies of the Ordovician strata of Tahe-Shunbei-Shunnan area, Tarim Basin and the Permian Qixia and Maokou Formations in the Sichuan Basin provided further insights into reservoir formation by mechanisms of fault-related fracturing and coupled multi-type fluid alteration. Results for the Upper Sinian Dengying Formation in the Sichuan Basin and the Upper Sinian Qigebulake Formation in the Tarim Basin suggested that the Neoproterozoic “dolomite sea” depositional environment, early dissolution, and early hydrocarbon charging are the key factors for reservoir formation and preservation. Physical simulation experiment on dolomite reservoir formation in gypsum-bearing facies revealed that dolomite recrystallization can significantly improve reservoir properties under gypsum-salt capping and overpressure and relatively closed environment. The new insights enriched the “ternary reservoir control” theory and provided guidance for oil and gas exploration in ultra-deep and even extra-deep strata. In superimposed basins in central and western China, fault-controlled and fault/dissolution dual-controlled reservoirs, ancient microbial mound-shoal dolomite reservoirs, especially those with Cambrian thick gypsum-salt caprocks, are widely distributed in marine carbonate rocks and can be considered the main exploration targets at > 10000 m depths.

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The botryoidal-lace texture and its role in dolomite reservoir control in the 2nd member, Sinian Dengying Formation in Sichuan Basin
LU Pengda, LI Zeqi, TIAN Tengzhen, WU Juan, SUN Wei, QIAO Zhanfeng, WANG Yongsheng, LIU Shugen, DENG Bin
2023, 30(6): 14-31. 
DOI: 10.13745/j.esf.sf.2023.2.15

Abstract ( 576 )   HTML ( 12 )   PDF (20603KB) ( 89 )  

The genesis and depositional environment of the unique botryoidal-lace dolomite in the 2nd member of the Sinian Dengying Formation are widely debated. Through detailed petrological and geochemical analyses we show that (1) the botryoidal-lace texture is formed by karst activities under high-frequency sea-level fluctuations quasi-contemporaneous with Sinian “aragonite-dolomite seas”, while the spatial heterogeneity of microbialite across the karst zone ultimately controls the textural differentiation to result in various fibrous dolomite cement types. (2) There is an obvious correlation between the botryoidal-lace texture and microbiolite matrix (MM) abundances. The botryoidal texture is mainly composed of FSDs (fascicular slow dolomite) and FFDs (fascicular fast dolomite), usually developed in sections of granular dolomite at the top and thrombolites dolomite at the bottom of the fourth-order cycles. The lace texture is composed of fascicular slow dolomite and RFDs (radial fast dolomite), mainly developed in the middle of the fourth-order cycles, in sections of laminated-stromatolites dolomite. Therefore, the longitudinal multi-spinulite feature and intermittent dissolution of microbialite by atmospheric water, together form the multi-stage, three-dimensional architecture of the high-quality botryoidal-lace dolomite in the Dengying Formation.

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Reservoir diagenesis and porosity evolution of the 4th member of the Dengying Formation in northeastern Sichuan Basin
LI Bisong, JIN Mindong, ZHU Xiang, DAI Lincheng, YANG Yi
2023, 30(6): 32-44. 
DOI: 10.13745/j.esf.sf.2023.2.16

Abstract ( 549 )   HTML ( 4 )   PDF (9996KB) ( 70 )  

Based on abundant field outcrop and core data, combined with scanning electron microscopy, cathodofluerescence microscopy and geochemical analysis, carbonate diagenesis, diagenetic sequence and pore evolution in the 4th member of the Dengying Formation in northeastern Sichuan Basin were studied in detail. Widely influenced by pore fluid and diagenetic changes during sedimentation and diagenesis, the carbonate reservoir underwent submarine cementation, syngenetic-parasyngenetic atmospheric freshwater dissolution, karstification controlled by sedimentary facies in the eogenetic stage, cement filling under shallow burial, dissolution under mid-deep burial, recrystallization and tectonic rupturing. Due to complex diagenetic fluids, the main cement fillings featured five periods of dolomite cements and two periods of siliceous cements related to volcano-hydrothermal activity, and the damage to the reservoir was most serious. However, facies-controlled eogenetic karstification was obviously different from typical epigenic karstification, and was the most critical, constructive diagenesis impacting the reservoir development in the study area. And dynamic superposition of different diagenesis eventually resulted in the current reservoir features. Results from this study provided a geological basis for the discovery of high-quality reservoirs in the region.

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Characteristics and formation mechanisms of dolomites in the Middle Permian Maokou Formation in southeastern Sichuan Basin
HAN Yueqing, ZHANG Juntao, PAN Lei, LI Huili, LIU Guangxiang, LI Rangbin, WU Chongyang, HAO Yunqing, HE Zhiliang, HUANG Zhenkai
2023, 30(6): 45-56. 
DOI: 10.13745/j.esf.sf.2023.2.19

Abstract ( 556 )   HTML ( 3 )   PDF (12971KB) ( 99 )  

Breakthroughs in oil and gas exploration have been made recently in dolomite reservoirs of the Middle Permian Maokou Formation in southeastern Sichuan Basin. However, reservoir heterogeneity in the Maokou Formation is high, which makes it difficult to identify high-quality reservoirs without a clear understanding of the dolomitization mechanisms. Based on sedimentary facies of the region and hydrothermal events associated with the Emeishan Large Igneous Province, dolomites in the Maokou Formation are characterized and their formation mechanisms are summarized through lithological and geochemical analyses. In the Maokou Formation fine-medium-crystalline dolomite is the main type, followed by micritic dolomite and dolomite cements. In the Qijiang area stratified, fine-medium-crystalline dolomite is mainly found. The euhedral to subhedral dolomite crystals appear cloudy at the center and bright around the edges, with obvious graining lines; under cathodoluminescence (CL) microscope they appear brown-red at the interior and dark-red toward the exterior. The δ18OV-PDB, δ13CV-PDB and 87Sr/86Sr ratio values of fine-medium-crystalline dolomite indicate it was formed by seepage-reflux dolomitization at medium salinity during shallow burial and later underwent recrystallization and seawater-metasomatism. The Tailai area mainly occur porphyritic- micritic and fine-medium-crystalline dolomites, along with dolomite cements found in rock fractures. Dolomites are compact with no obvious grain structures, while dolomite cements are mainly saddle dolomite with wavy extinction, with multiphase ring-and-ribbon patterns visible under CL microscope. Isotopic data indicate dolomites in this area were formed during shallow burial under seawater environment, and late-stage thermal fluids flowing over clastic rocks directly contributed to the precipitation of dolomite cements and calcite. In general, the key reservoir-controlling factors of the Maokou Formation in southeastern Sichuan Basin are epigenetic dissolution, dolomitization during shallow burial, metasomatism and karstification; and seepage-reflux dolomitization during shallow burial is the main formation mechanism of dolomites.

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Geometry, kinematic characteristics and evolution of No.15 strike-slip fault zone in Fuling area, eastern Sichuan
DUAN Jinbao, PAN Lei, SHI Siyu, JIANG Zhenxue, LI Pingping, ZOU Yutao, ZHANG Wenrui
2023, 30(6): 57-68. 
DOI: 10.13745/j.esf.sf.2023.2.18

Abstract ( 565 )   HTML ( 7 )   PDF (20134KB) ( 106 )  

There are many NW-trending hidden strike-slip faults in the central and northern Sichuan Basin, and their control on the carbonate reservoirs is revealed by the recent oil and gas explorations. In order to better understand the geometry, kinematic characteristics and evolution of such faults, the No. 15 strike-slip fault in Fuling area, eastern Sichuan is investigated to determine the deformation characteristics and fault structure styles of different structural layers using the latest processed 3D seismic data. Based on the temporospatial variation patterns of stratum thicknesses on both sides of the strike-slip fault, the mechanism of strike slip faulting is analyzed, and the strike slip displacement is determined. Finally, the evolution of the hidden strike-slip fault zone is described in combination with the tectonic deformation events. The structural layer of No. 15 strike-slip fault below the Cambrian salt is mainly a single vertical fault with linear distribution on the plane, showing weak compressive torsional strength in the middle-Lower Cambrian to Ordovician with the development of branch faults at the top and a narrow pressure uplift section. The branch faults are most developed in the structural layer below the Silurian to Lower Triassic salt, with positive and negative flower-like structural styles with echelon distribution on the plane. According to kinematic analysis, the strike-slip fault experienced three main evolutionary stages: right strike slip in the middle-late Caledonian, synsedimentary activity in the late Hercynian, and left strike slip in the late Yanshan-early Himalayan periods. The research results provide a basis for in-depth studies of the strike-slip faults in Fuling area, and a reference for the study of reservoir control models and oil and gas accumulation in strike-slip faults in general.

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Scope and mechanism of deep fluid circulation in karst systems, northern Awati-Manjiaer transition zone, Tarim Basin
YOU Donghua, PENG Shoutao, HE Zhiliang, LIU Yongli, HAN Jun, XIAO Chongyang, LI Yingtao
2023, 30(6): 69-79. 
DOI: 10.13745/j.esf.sf.2023.2.29

Abstract ( 540 )   HTML ( 6 )   PDF (14958KB) ( 90 )  

The transition zone between the Awati and Manjiaer depressions, Tarim Basin is an important area with the development of large-scale fault-associated petroleum reservoirs. Focusing on the fluid events within the fault zone, this study aims to gain deeper insights into the scope and mechanism of deep fluid circulation in karst systems, through comprehensive geological and geochemical analyses of wells TS6, YJ1-1x, SHB1-3x, and SHBP1. The secondary and dolomitic limestones, formed from fine-crystalline dolomite via dedolomitization, and macrocrystalline calcite cements in the Penglaiba Formation of well TS6 are typically 13C, 18O depleted, indicating the depth of karstification in the Shaya uplift can reach ~1200 m (away from the $\mathrm{T}_{7}^{4}$ unconformity). Caves in well YJ1-1x contain four rock layers and are filled with argillaceous siltstone and silty mudstone, with characteristic minerals such as limonite, indicating the presence of lateral fluid flow and weak hydrodynamic conditions. In well SHB1-3x, caves with three rock layers contain argillaceous fillings and volcanic rock debris, and the structural characteristics of calcite and limonite as secondary fillings reveal sediment transport in the underground rivers. The distance between wells YJ1-1x and TS3 (near the erosion line of the Sangtamu Formation) is about 76 km, and the linear distance between wells SHB1-3x and TS3 is about 90 km, indicating long distance lateral fluid flow. The dolomite reservoir of the Yingshan Formation in well SHBP1 is characterized by fault-fluid transformation and continuous deterioration of reservoir performance from bottom to top. The secondary macrocrystalline calcite cements in dolomite are obviously 13C, 18O depleted and 87Sr enriched, which are strongly comparable to the calcite of karst origin in the Tahe area, suggesting the calcite cements may originate from the bottom-up intrusion of deep circulating fluids. The geological model of deep fluid circulation and the existence of subvolcanic rocks in the Yingshan Formation of well SHBY1 indicate the driving force behind fluid migration may come from stratum warming and/or geothermal events. The water-rock interaction during deep fluid circulation is conducive to the development of deep and ultra-deep carbonate reservoirs in the Awati-Manjiaer transition zone under certain conditions.

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Fault zone architecture of strike-slip faults in deep, tight carbonates and development of reservoir clusters under fault control: A case study in Shunbei
LI Yingtao, DENG Shang, ZHANG Jibiao, LIN Huixi, LIU Yuqing, QIU Huabiao, HUANG Cheng, LIU Dawei, YAO Yili
2023, 30(6): 80-94. 
DOI: 10.13745/j.esf.sf.2023.2.32

Abstract ( 545 )   HTML ( 5 )   PDF (15402KB) ( 79 )  

Combining seismic attribute analysis, well/imaging logging, coring calibration, and production performance data analysis, the fault zone architecture of the Shunbei No. 4 fault zone is determined and statistically analyzed to obtain insights into the characteristics of reservoir development under strike-slip fault control. The results show that the type of reservoir rock space is determined by the fault zone architecture which, in turn, is controlled by the segmentation structural style of the fault zone, as are the locations of favorite fractures and caves. In fact, the development of reservoir rocks in the interior of the fault zone is highly inhomogeneous, as fault braccias and damage zones usually develop into clusters within a single seismic-scale fault plane, with wall rocks in between the fracture zones. The fault zone architecture differs under different segmentation structural styles (e.g., push-ups, pull-aparts, strike-slip, etc.). The push-up structure is characterized by high fracture intensity, with densely distributed fault breccias and fracture zones; whereas in the pull-apart structure fault braccias/fracture zones are mostly localized in fault plane boundaries where the fault zone architecture is less developed. In comparison, the strike-slip structure is characterized by lower fracture intensity and the least number of fault zones and greater spacing between them. To maximize the production of effective reservoirs and obtain higher single-well productivity, the well trajectory should be designed based on the fault zone architectural characteristics under different structural styles. As the productivity of a single well per unit pressure drop is taking into account, it is clear that the size of the reservoir is not the only factor controlling the single-well productivity but also the reservoir connectivity across different segments.

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Characteristics and formation mechainism of the strike-slip fault networks in the Shunbei area and the surroundings, Tarim Basin
LIU Yuqing, DENG Shang, ZHANG Jibiao, QIU Huabiao, HAN Jun, HE Songgao
2023, 30(6): 95-109. 
DOI: 10.13745/j.esf.sf.2023.2.31

Abstract ( 545 )   HTML ( 1 )   PDF (18245KB) ( 81 )  

Intracratonic strike-slip fault systems in the Tarim Basin are characterized by small displacement, low strain deformation and large burial depth. These attributes make it challenging to study the formation mechanism of these fault systems. Based on the recently obtained high-density 3D seismic data, 3D mapping and detailed analysis of strike-slip faults are conducted in Shunbei and its surrounding area. The distinct characteristics of fault array development in the east and west of the Shunbei area are analyzed and clarified, and the formation mechanism of the strike-slip arrays is also discussed. It was found that a set of ~30NE trending strike-slip faults, including the major and secondary strike-slip faults, are developed in eastern Shunbei. At depth, strike-slip deformation is stronger in the major faults than in the secondary faults, and the compressive strain increases gradually eastward in the major faults. In the shallow strata en echelon fault activity intensifies eastward. The strike-slip fault systems differ between the northwest and southwest of Shunbei. The northwest mainly develops a set of NW-trending faults spreading southward from the Tabei uplift, whereas in the southwest a set of ~15NE trending faults with high density and multistage activity are developed. The strike-slip fault systems in Shunbei are characterized by “north-south zonal distribution, east-west differential activity and multi segments coalescing growth”, indicating the fault systems consist of multiple sets of strike-slip faults formed in different areas under distinct regional stress fields and later coalesced in the interior Tarim Basin, and their formation is probably related to multi-directional compressional tectonic event at the basin boundaries in the Late Ordovician.

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Storm deposition characteristics and models for the Middle and Upper Cambrian in Xiaweidian, Xishan area, Beijing
WU Chun, LIU Hangyu, LU Feifan, LIU Bo, SHI Kaibo, HE Qing
2023, 30(6): 110-124. 
DOI: 10.13745/j.esf.sf.2023.2.17

Abstract ( 527 )   HTML ( 1 )   PDF (25345KB) ( 66 )  

Based on field survey and thin section analysis of tempestites in the Xiaweidian section, western Beijing, 59 sets of storm deposits are characterized. The storm sedimentary sequences are determined, and an epicontinental sea deposition model is established. There are four sedimentary facies types in the Cambrian, Xiaweidian section: micritic limestone, pebbled micritic limestone, wackstone and packston, where in-situ, proximal and distal tempestites are developed in sequences of oolitic-gravel limestone, mudstone/siltstone-gravel limestone and micritic-gravel limestone, respectively. In-situ tempestites develop erosion surfaces and have high grain content and large axial ratio; proximal tempestites contain flat peebles with good orientation and show graded bedding; and distal tempestites have high matrix content. The three tempestites types are deposited in the subtidal zone under epicontinental sea environment, with the in-situ, proximal and distal types deposited from top to bottom and separated by normal- and storm-wave bases, respectively, in the order of increasing distance from the storm center and thus increasing transport distance. Results of this study not only complement the storm deposition records of the Cambrian in Xiaweidian, but also provide evidence for the study of palaeoenvironment in this period.

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Reservoir development characteristics and karst models for the “strata bound” karst reservoirs in the Tahe Oilfield
JIN Yanlin, ZHANG Huitao, LIU Yao, JI Yuwen
2023, 30(6): 125-134. 
DOI: 10.13745/j.esf.sf.2023.2.33

Abstract ( 525 )   HTML ( 2 )   PDF (15983KB) ( 49 )  

The oil well productivity in the Tahe Oilfield is determined primarily by the reservoir connectivity. For the oil and gas exploration and development in the greater Tahe area, it is important to clarify the reservoir development characteristics of bedding karst reservoirs and the underlying vertical zoning patterns and karst formation mechanisms. Based on the outcrop, core/rock thin section, drilling, logging and 3D seismic data and the characteristics of water cycling in karst aquifers, combined with seismic attribute analysis, the “strata bound” karst system in the Tahe area is preliminarily defined as a dissolution karst system that is controlled, independently or jointly, by factors such as favorable facies belt, primary layer, secondary interface and intra-formational fault. The karst system develops a set of layered reservoirs that are different from the upper and lower strata in lithology, physical property and oil and gas production. It is clarified that the sequence boundary, fault and sedimentary environment jointly control the development of the “strata bound” karst reservoirs. Two karst models are summarized: synclinal-confluence type in karst canyon areas, and single-branch seepage type in localized wide, gentle platform areas. These two karst models reflect the differences in reservoir control factors including structural background, water system conditions and fracture scales in the Tahe Oilfield, which are key to determining the role of “strata bound” karst in the formation of karst reservoirs.

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Thermal analysis of ultra-deep layers and its influence on reservoir utilization in platform area, Tarim Basin
LI Dan, CHANG Jian, QIU Nansheng, XIONG Yujie
2023, 30(6): 135-149. 
DOI: 10.13745/j.esf.sf.2023.2.25

Abstract ( 542 )   HTML ( 5 )   PDF (4440KB) ( 91 )  

The temperature field is one of the key factors restricting the development of effective reservoirs, however, few studies have focused on the thermal evolution of ultra-deep reservoirs and its influence on reservoir performance in the Tarim Basin. In this paper, the paleothermal history of the Tarim Basin platform area since the Phanerozoic are investigated by vitrinite reflectance modeling using well logs from new wells. The heat flow in the basin platform area has decreased gradually since the Early Paleozoic, from 45-65 to 32-45 mW/m2, and peaked briefly to ~46-62 mW/m2 during the Early Permian in some areas. Combined with the burial history, the thermal history of ultra-deep carbonate reservoirs and the distribution characteristics of key tectonic events are revealed, and the differential evolution of temperature field in different tectonic units and the main controlling factors are analyzed. The Ordovician reservoirs experienced slow heating then rapid heating processes. The reservoir cooling characteristics, i.e., planar cooling from the central area to the north and south sides during the key tectonic episodes, matches the hydrocarbon phase behavior, except for the local high anomaly during the Early Permian. The temperature field conditions for effective reservoir development in the platform area are discussed through building the temperature field and porosity evolution models. The phase behavior of hydrocarbons in ultra-deep carbonate reservoirs is strongly influenced by temperature and pressure, where not only the reservoir temperature can affect pore space for oil/gas charging, but also the long-term low-temperature, high-pressure environment is beneficial to the preservation of liquid hydrocarbons. This study helps to clarify the thermal evolution process of ultra-deep carbonate reservoirs in the platform area of the Tarim Basin, and provides a reference for the next stage of hydrocarbon exploration.

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Paleo-environmental variation and its control on organic enrichment in the black rock series, Cambrian Yuertusi Formation in northwestern Tarim Basin
CHEN Jianfa, XU Jin, WANG Jie, LIU Peng, CHEN Feiran, LI Maowen
2023, 30(6): 150-161. 
DOI: 10.13745/j.esf.sf.2023.2.12

Abstract ( 819 )   HTML ( 6 )   PDF (7394KB) ( 95 )  

Based on the sedimentary and geochemical characteristics of the Lower Cambrian Yuertusi Formation in the northwestern margin of the Tarim Basin, the variation of seawater redox condition and primary productivity during sediment deposition and its control on organic enrichment in the black rock series and source rock distribution in the Yuertusi Formation were investigated. The Yuertusi Formation in the northwestern Tarim Basin can be divided into five members: black phosphorus-bearing siliceous rock at bottom; black mudstone in the lower part; gray dolomite in the middle part; interbedded black shale and gray microcrystalline limestone in the upper part; and gray crystal dolomite near top; this vertical lithofacies sequence incorporates a complete transgression-regression sedimentary cycle. The redox index and hydrothermal tracing parameters revealed the details of paleoenvironmental variation: the bottom siliceous rock is formed in an anoxic environment with strong hydrothermal activity; the lower black mudstone in an euxinic environment with slight hydrothermal activity; and the upper black shale in a sub-anoxic to anoxic environment with weak hydrothermal activity. Comprehensive studies have shown that during the deposition of the black rock series sea level first rises then falls, oxygen fugacity first decreases then increases, and paleo-productivity gradually weakens. The anoxic condition and high productivity influenced by hydrothermal activity jointly control the distribution of ancient, high-quality source rocks represented by the black mudstone in the lower part of the Yuertusi Formation.

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High-resolution sequence-stratigraphic characteristics and filling evolution model of Lower Cambrian fine-grained sedimentary rocks in southwestern Sichuan
YUAN Yuxuan, LI Yifan, FAN Tailiang, DU Wei, CHEN Gege, ZHANG Tan, KUANG Mingzhi, LIU Wangwei
2023, 30(6): 162-180. 
DOI: 10.13745/j.esf.sf.2023.2.34

Abstract ( 701 )   HTML ( 1 )   PDF (31791KB) ( 63 )  

The Lower Cambrian shale in Upper Yangtze has good shale gas prospects and is of great importance for the exploration and research of deep marine shale gas in China and even in the world. This paper carried out high-resolution sequence-stratigraphic analysis of the fine-grained marine sediment rocks in the Lower Cambrian, southwestern Sichuan to establish a high-precision sequence-stratigraphic framework of the rock formation which can be used, by sequence comparison analysis, to effectively predict the organic-rich intervals and guide unconventional oil and gas exploration in the region. The fine-grained sedimentary rocks of the Lower Cambrian Maidiping-Qiongzhusi Formations in well Jinshi 103 were examined through elaborate core description and high-precision thin section analysis. The rock formations were divided into 11 lithofacies types: phosphatic rocks; weak-laminar siliceous mudstone; siliceous silty mudstone; deformed bedding argillaceous siltstone; massive siliceous siltstone; dark-laminar silty mudstone; lenticular sequence argillaceous siltstone; wavy lamellar argillaceous siltstone; lean laminated silty mudstone; laminar-rich argillaceous siltstone; and calcareous cementation siliceous siltstone. On this basis, five sedimentary facies types were identified: mixed tidal flat; muddy deep-water shelf; sandy argillaceous deep-water shelf; muddy shallow-water shelf; and sandy muddy shallow-water shelf. By fine core observation, three third-order sequence boundaries—Sinian-Cambrian unconformable surface, SB1, and lithological interfaces between contrasted lithologies, SB2, and between lithologic assemblages, SB3, within the studied rock formation—were identified. Based on quantitative statistical analysis of lithofacies superposition styles and sedimentary structures, 35 parasequences and 14 parasequence sets were delineated in the studied rock formation. Four parasequence superposition styles were summarized: “phosphorite grainstone+calcareous phosphatic sandstone+calcareous phosphatic argillaceous siltstone” parasequence unit (HST1) controlled by tidal flow; “cemented calcareous siltstone+massive siltstone+deformed bedded argillaceous siltstone+dark laminated silty mudstone” (HST2) controlled by synergy of gravity and bottom flows; “lenticular graded argillaceous siltstone/wavy-laminated argillaceous siltstone/deformed bedded argillaceous siltstone+dark laminated silty mudstone” (TST3) controlled by storm wave and co-influenced by bottom flow; and “cemented calcareous siltstone+richly-laminated argillaceous siltstone+poorly-laminated silty mudstone” (HST3) influenced by wave and gravity flow. Finally, 6 system tracts were identified and 3 complete third-order sequence units were delineated. Combined with sedimentary facies/sequence characteristics, a “mixed shelf, to deep shelf, to shallow shelf” evolution model for the Maidiping-Qiongzhusi Formations was established.

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Comparative study on the organic enrichment mechanisms between western Hubei and northeastern Guizhou during the Early Cambrian
ZHANG Liyu, CHEN Qianglu, LI Maowen, YUAN Kun, MA Xiaoxiao, XI Binbin, YUE Yong, HUANG Taiyu
2023, 30(6): 181-198. 
DOI: 10.13745/j.esf.sf.2023.2.11

Abstract ( 572 )   HTML ( 3 )   PDF (9408KB) ( 80 )  

Early Cambrian petroleum source rocks are widely distributed in the middle and upper Yangtze region. Understanding the coupling relationship between source-rock deposition and paleogeography/paleoclimate/paleo-ocean and the mechanistic differences in organic enrichment under different depositional environments is critical for understanding the intrinsic relationship between environment and resources, so as to provide a geological basis for resource potential analysis. In this study, Early Cambrian source rocks from outcrops in western Hubei and wells in northeastern Guizhou are characterized to reveal the main controlling factors of organic accumulation in intrashelf/interplatform basins and slope facies, using petrological, organic geochemical (rock pyrolysis, kerogen carbon isotope, total organic carbon) and inorganic geochemical (whole-rock major and trace elements, X-ray diffraction) methods. The Early Cambrian source rocks in the study area are mainly composed of type I kerogen. Along the Yazikou basin cross-section the Al contents, nutrient elements (Ba, Cu, Zn, Ni) and clay contents remain nearly constant in the organic rich interval. The redox sensitive elements (Mo, U, V) and TOC are higher in the lower Lower Cambrian Shuijingtuo Formation and show an upward decreasing trend. The Th/U (< 2) and V/(V+Ni) (> 0.54) ratio data suggest continuous anoxia in the lower Shuijingtuo Formation. In contrast, in well ZK513 slope facies, there is no obvious increase of Al and clay contents in the organic rich interval but co-variation and slight increase of nutrient elements, redox-sensitive elements and TOC in the Lower Cambrian Niutitang Formation and upper Lower Cambrian Jiumenchong Formation, indicating an increase of primary productivity and marine euxinia; such euxinic condition is further conform by the Th/U and V/(V+Ni) ratio data. Based on the paleogeographic features and previous researches, we propose that the main controlling factors of organic enrichment in well ZK513 (slope facies) are high productivity and favorable preservation conditions, where high nutrient elements brought by upwelling promote the primary productivity and lead to organic accumulation and subsequent bacterial sulfate reduction (BSR) and seawater uxinia. Whereas in the Yazikou section of the intrashelf/interplatform basins with no significant primary productivity enhancement, organic enrichment is mainly controlled by the preservation conditions.

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Maturation history of deep and ultra-deep source rocks, central and western basins, China
QIU Nansheng, CHANG Jian, FENG Qianqian, ZENG Shuai, LIU Xiaoyu, LI Huili, MA Anlai
2023, 30(6): 199-212. 
DOI: 10.13745/j.esf.sf.2023.2.37

Abstract ( 582 )   HTML ( 3 )   PDF (9325KB) ( 114 )  

There are significant differences in the oil and gas phase behavior, distribution depth, and content in deep and ultra-deep strata in the central and western basins in China where the temperature field plays an essential role in hydrocarbon generation and conservation. Focusing on the Cambrian of the Tarim and Sichuan Basins, this study summarizes and contrasts the present-day and ancient temperature fields, and clarifies the maturation history and differences of the Lower Cambrian source rocks. The effect of thermal evolution on hydrocarbon generation and phase behavior is also discussed. The average heat flows in the Tarim and Sichuan basins are (42.5±7.6) mW/m2 and (53.8±7.6) mW/m2, respectively, reflecting the characteristics of "cold" and "warm" basins. In the Tarim Basin the paleo heat flow decreased since the Early Cambrian and only peaked abruptly in the Early Permian in response to the Tarim Large Igneous Province. Whilst the Sichuan Basin experienced steady, low heat flow in the Cambrian-Early Permian, rapid rising heat flow at the end of the Early Permian due to the Emeishan mantle plume, and declining heat flow since the Late Permian. The Sichuan Basin's consistent higher thermal regime, and the differential effect of Permian magmatic activity, resulted in the differential oil and gas generation and conservation in the ancient, ultra-deep strata between the two basins. The maturation history of source rocks of the Lower Cambrian Yuertusi Formation in the Tarim Basin can be classified into three kinds: (1) Paleozoic rapid maturation and finalization; (2) Early Paleozoic rapid maturation followed by continuous heating; and (3) Early Paleozoic and Mesozoic rapid maturation. Similarly, three maturation patterns are identified for the source rocks of the Lower Cambrian Qiongzhusi Formation in the Sichuan Basin: (1) continuous maturation; (2) Silurian-Permian cessation followed by Mesozoic maturation; and (3) Silurian-Permian and Triassic cessation and later continuous maturation. Due to its low thermal regime, the Tarim Basin has favorable temperature conditions for the liquid hydrocarbon preservation in ultra-deep strata.

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Hydrocarbon generation, residual hydrocarbon and petroleum expulsion characteristics of Ordovician carbonate source rocks in northern Tarim and its surrounding areas
PANG Hong, PANG Xiongqi, WU Song, CHEN Junqing, HU Tao, JIANG Fujie, CHEN Dongxia
2023, 30(6): 213-231. 
DOI: 10.13745/j.esf.sf.2023.2.24

Abstract ( 558 )   HTML ( 2 )   PDF (10008KB) ( 41 )  

The Ordovician carbonate rocks in northern Tarim Basin and its surrounding areas are considered effective source rocks, however, their hydrocarbon generation, residual hydrocarbon and petroleum expulsion characteristics and the relative contribution to petroleum accumulation are not well understood. Based on the logging data and thermal simulation parameters for 21 wells in the study area, combined with geological and geochemical statistical analysis and numerical simulation, this paper aims to address the above issues and reveal the hydrocarbon expulsion behavior o f source rocks in different evolutionary stages. The results show that the hydrocarbon generation center is located in the gentle slope area of Shuntuogol-Shunnan region. The hydrocarbon expulsion occur broadly in the water-soluble, oil-soluble, diffusive and free phases in proportion of 6.5%, 0.4%, 9.8% and 83.3%, respectively. Combined with the dominant migration channels and high-quality reservoir distribution in the study area, the fault zones of the Shuntuogol low uplift/Shunnan gentle slope and southern Shaxi uplift/Shunbei gentle slope zone are identified as the next key exploration areas.

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The hydrogen isotopic composition of methane from Lower Paleozoic natural gases, cratonic platform areas, Tarim Basin and its geological significance
CHEN Zeya, CHEN Jianfa, LI Maowen, FU Rao, SHI Xiaofei, XU Xuemin, WU Jianjun
2023, 30(6): 232-246. 
DOI: 10.13745/j.esf.sf.2023.2.14

Abstract ( 538 )   HTML ( 1 )   PDF (5740KB) ( 51 )  

The Lower Paleozoic marine carbonate strata of the cratonic platform area, Tarim Basin are a deep oil and gas exploration hotspot in China. However, the origin of deep and ultra-deep natural gases and their accumulation process are still difficult problems in natural gas geoscience. In this study, the chemical and isotopic compositions of natural gases in the Lower Paleozoic were determined. Combined with previous results and regional geology we believe the hydrogen isotopic composition of methane (δ2H1) is a more reliable maturity indicator for source rocks in the study area, and a new way of discussing the origin, generation mechanism and secondary alterations of natural gas reservoirs is established using δ2H1 value, δ13C values of alkane gases and gas dryness coefficient. The Lower Paleozoic accumulated gaseous hydrocarbons generated from source rocks of different maturities, genetic types and geochemical characteristics. Moreover, most of the natural gas reservoirs were formed under relatively open environments, with low contribution of oil cracking gases. Nevertheless, there are also some natural gas reservoirs formed under relatively closed environments that received large amounts of oil-cracking gases due to high heterogeneity in carbonate reservoirs and transport systems, where natural gases of complex, diverse origins have undergone secondary transformations, mainly mixing and oxidative alteration, during the accumulation process.

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Geochemical characteristics and phase behavior of the Ordovician ultra-deep reservoir fluid, No.4 fault, northern Shuntuoguole, Tarim Basin
MA Anlai, QI Lixin
2023, 30(6): 247-262. 
DOI: 10.13745/j.esf.sf.2023.2.21

Abstract ( 557 )   HTML ( 0 )   PDF (4742KB) ( 44 )  

The phase behavior of the Ordovician ultra-deep reservoir fluid, No. 4 fault (F4), northern Shuntuoguole, is complex. From NE to SW, the fluid phase changes from volatile oil to condensate, with the solution GOR increasing from 358 m3/m3 to 3200 m3/m3 then decreasing to 1800 m3/m3. Using geochemical methods the geochemical characteristics of the reservoir fluid and the cause of its phase change were investigated. It was found that while the biomarker concentrations in oil from F4 were low to non-measurable, the source rocks were similar for oils from F1 and F4; and oil from F4, with Rc of 1.14%-1.60%, was of higher maturity. For natural gas from F4, its dryness coefficient showed an increasing trend from NE to SW, with Rc ranging between 1.3%-1.7%, while its CO2 and H2S contents showed the same spatial variation trend. The reservoir fluid experienced no evaporative fractionation as inferred from the relationship between the mole fraction and carbon number of n-alkanes. In F4, the concentration of diamondoids in oil ranged between 27.26-523.31 μg/g, indicating 20.5%-95.8% thermal cracking which is higher than in F1; while the concentration of thiodiamondoids in oil from wells SB4, SB41X-C and SB42X ranged between 33.76-76.92 μg/g, suggesting occurrence of thermochemical sulphate reduction (TSR). Thus, two factors contributed to the phase change in F4: one is geothermal gradient, where relatively strong thermal gradient in the middle compared to either sides of the fault led to high-maturity oil and gas generation and high degree of cracking in the middle of the fault; the other is TSR, which took place in the middle and SW of the fault and further increased CO2, H2S and GOR.

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Multi-stage hydrocarbon accumulation in Cambrian Xixiangchi Group, Pingqiao area, southeastern Sichuan and its implications for hydrocarbon exploration
GAO Jian, LI Huili, HE Zhiliang, CAI Xunyu, LI Shuangjian, LIU Guangxiang, YUAN Yusong, LIN Juanhua, LI Zhi
2023, 30(6): 263-276. 
DOI: 10.13745/j.esf.sf.2023.2.20

Abstract ( 554 )   HTML ( 2 )   PDF (10309KB) ( 45 )  

The hydrocarbon accumulation in deep marine carbonate rocks generally involves multi-source, multi-stage processes, and an accurate analysis of such complex processes is necessary to help guide efficient oil and gas exploration. In order to determine the hydrocarbon accumulation processes in the Cambrian Xixiangchi Group, Pingqiao area, southeastern Sichuan, the multistage cementing materials within the reservoirs were investigated by cathodoluminescence microscopy, elemental and isotopic analysis, paleotemperature/pressure trapping in fluid inclusions, and bitumen analysis by laser Raman spectroscopy. The filling of cementing materials from early to late followed by the order of dolomite, calcite and quartz, with bitumen widely distributed in the dissolved pores of dolomite matrix and the filling dolomite grains. The cementing materials contained bitumen/gas inclusions in the dolomite layer and primary/secondary methane inclusions in the calcite layer. The diagenetic fluids were derived largely from host rocks, and calcite precipitation liked involved organic matter fluids. Based on the bitumen reflectance data (2.90%<BRo<3.59%), combined with the homogenization temperature of the brine/bitumen two-phase inclusions and the hydrocarbon generation history of source rocks, it was inferred that bitumen was derived from pyrobitumen—a thermal cracking byproduct during liquid hydrocarbon charging from the Lower Cambrian Qiongzhusi Formation into the Cambrian Xixiangchi Group. The trapping pressure for the primary methane inclusions ranged between ~55.8-81.7 MPa (at trapping temperature of 198-203 ℃), indicating a pressure coefficient of 1.06-1.56 during thermal cracking in the burial period, consistent with moderate overpressure. The trapping pressure for the secondary methane inclusions ranged between ~42.1-65.0 MPa (at trapping temperature of 150-155 ℃), suggesting a pressure coefficient of 0.94-1.45 during shale gas charging from the Wufeng-Longmaxi Formations during the uplift period while still maintaining moderate overpressure. A multi-stage hydrocarbon accumulation model for the Cambrian Xixiangchi Group, Pingqiao area was established, which included liquid hydrocarbon charging in the Lower Cambrian Qiongzhusi Formation and in-situ cracking gas accumulation during the burial period, and shale gas charging in the Wufeng-Longmaxi Formations during the uplift period. This model provides a new direction and idea for hydrocarbon prospecting in the Cambrian Xixiangchi Group.

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Migration direction and accumulation characteristics of paleo-oil in the Changxing Formation in Yuanba Gas Field, Sichuan Basin
LI Pingping, WEI Guanglu, XU Zuxin, LI Bisong, PENG Yuqing, ZOU Huayao
2023, 30(6): 277-288. 
DOI: 10.13745/j.esf.sf.2023.2.22

Abstract ( 546 )   HTML ( 4 )   PDF (10881KB) ( 50 )  

The natural gas in the Permian Changxing Formation (P2ch) in the Yuanba gas field is mainly derived from oil cracking. Study on the migration direction and accumulation characteristics of paleo-oil can provide valuable insights into the reservoir evolution and fluid contact distribution in the P2ch reservoirs to guide natural gas prospecting and exploration in the region. In this paper, the paleo-oil charging period was first determined using fluid inclusions data, and the reservoir cap structure during oil charging was restored. Based on the reservoir cap structure the oil migration pathway was simulated, and the oil migration direction was traced using alkyl dibenzothiophene markers from solid reservoir bitumen. Finally, the paleo-oil accumulation characteristics and oil/gas reservoir evolution in the P2ch reservoirs were analyzed. The main findings were (1) bitumen inclusions were widespread, and the associated brine inclusions had a homogenization temperature of 110-130 ℃. Combined with the burial history and thermal evolution data, the estimated paleo-oil charging period spaned from the Late Triassic to Early Jurassic. (2) The reservoir cap structure during reservoir deposition was retained during paleo-oil charging. (3) Paleo-oil migrated laterally from the platform margin slope to the reef-shoal traps, while the alkyl dibenzothiophene ratio for solid reservoir bitumen also gradually decreased along the migration direction consistent with lateral oil migration. (4) The degree of paleo-oil filling was higher in reef-shoal traps in the platform margin compared to lower positions. On the whole, the paleo-oil in the Yuanba gas field was characterized by near-source charging and oil accumulation in reef-shoal traps in the elevated platform margin. For paleo-reservoirs that had undergone high temperature thermal evolution, alkyl dibenzothiophenes may be used to trace the paleo-oil migration direction.

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Experimental simulation and characteristics of hydrocarbon accumulation in strike-slip fault zone in Shunbei area, Tarim Basin
ZHU Xiuxiang, CAO Zicheng, LONG Hui, ZENG Jianhui, HUANG Cheng, CHEN Xuyun
2023, 30(6): 289-304. 
DOI: 10.13745/j.esf.sf.2023.2.23

Abstract ( 535 )   HTML ( 2 )   PDF (12492KB) ( 46 )  

An important breakthrough has been made in recent years in the exploration of ultra-deep carbonate rocks in Shunbei area, Tarim Basin with the discovery of fault-controlled fracture-cave reservoirs. These reservoirs show a good combination of source, reservoir and cap, where different strike-slip fault combination styles have important influence and control on the hydrocarbon migration and accumulation. In this paper, a typical strike-slip fault in Shunbei area is selected to analyze the compressive-torsional and tensional-torsional deformation segments, and it becomes clear that the former has a dendritic/dendritic combination style, and the latter has an asymmetric/symmetric style. Combined with seismic analysis of the drilled oil and gas reservoirs, an experimental model is established to conduct physical simulation experiments, and it is found that the main factors affecting the oil, gas, water distributions are the physical properties of the fault zone, the fracture connectivity, the intensity of the hydrocarbon supply and the development of the fracture networks. Being directly connected to the primary and secondary faults, the compressive-torsional deformation segment is superior for hydrocarbon accumulation, and can be a target area for later exploration and development. The above results provide deeper insights into the oil and gas migration and accumulation in deep strike-slip faults and can be used to guide the exploration of fault-controlled fracture-vuggy reservoirs in Shunbei area.

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Differential overpressure distribution in Ordovician carbonates, Shuntuoguole area, Tarim Basin
ZENG Shuai, QIU Nansheng, LI Huili, MA Anlai, ZHU Xiuxiang, JIA Jingkun, ZHANG Mengfei
2023, 30(6): 305-315. 
DOI: 10.13745/j.esf.sf.2023.2.30

Abstract ( 570 )   HTML ( 0 )   PDF (5920KB) ( 51 )  

With the continual advancement in deep exploration in the Tarim Basin, a succession of anomalous high-pressure reservoirs associated with strike-slip faults have been discovered in the ultra-deep carbonate strata of the Shuntuoguole region, but the origin of the overpressure and its distribution pattern have rarely been discussed. Also, the relationship between strike-slip fault and formation pressure in cratons has seldom been studied compared to results for extensional basins and compressive foreland basin systems; whilst strong carbonate reservoir heterogeneity further complicates the overpressure pattern. In this study the characteristics of the present-day pressure field and the main features of overpressure reservoirs are analyzed based on data from drilling tests and production dynamics studies. Further, combined with paleopressure calculation (using fluid inclusions), gas geochemical characteristics and faults activity, the origin of overpressure and its differential distribution in secondary tectonic units of the Shuntuogole area are discussed. The results suggest that the overpressure in the Shunbei slope and Shuntuoguole low uplift is caused by gas charging and tectonic compression, with the former mainly affected by pyrolytic gas generation from crude oil cracking. The gas generation and faults activity determine the differential overpressure distribution, where overpressure occurs widely across the Shunnan area but only locally in the Shunbei and Shuntuoguole areas. The development pattern of the fault-associated reservoirs results in the division of the pressure system in the Shuntuoguole area.

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Spatial-temporal distribution of fluid activities and its significance for hydrocarbon accumulation in the strike-slip fault zones, Shuntuoguole low-uplift, Tarim Basin
LI Huili, GAO Jian, CAO Zicheng, ZHU Xiuxiang, GUO Xiaowen, ZENG Shuai
2023, 30(6): 316-328. 
DOI: 10.13745/j.esf.sf.2023.2.36

Abstract ( 559 )   HTML ( 1 )   PDF (8636KB) ( 38 )  

The strike-slip faults in the Shunbei oil and gas field, Tarim Basin are essential for the reservoir formation and hydrocarbon accumulation. This research selects carbonate core samples containing fractured calcite veins from 23 drilling wells in eight major strike-slip faults in Shuntuoguole area to investigate fault-fluid interaction in the oil and gas field. Based on the petrography (diagenetic mineral types and cathodoluminescence characteristics) and lithogeochemistry (rare earth elements, strontium isotopes, carbon and oxygen isotopes) of calcite veins and laser ablation in situ U-Pb dating of calcite, the fractured calcite veins are divided into three types based on their formation environment and stage. Calcite veins formed in the first stage (474-444 Ma) are widely distributed in many faults, and their lithogeochemical characteristics indicate the diagenetic fluids are derived from formation water of the Middle-Lower Ordovician. Calcite veins formed in the second stage (441-403 Ma) are mostly distributed in fault zones east of Shunbei No. 5 fault, and Cambrian formation water and deep thermal fluids of the burial period are the sources of the diagenetic fluids. Calcite veins formed in the third stage (374-326 Ma)—with diagenetic fluids likely originated from deep thermal fluids—are only observed in a few samples from Shunbei Nos. 4 and 5 strike-slip faults and Shunnan area. In situ U-Pb dating results suggest the three calcite phases coincide with faulting events in the Middle Caledonian, Late Caledonian-Early Hercynian, and Late Hercynian, respectively. Moreover, the occurrence and characteristics of hydrocarbon-bearing inclusions in calcite veins imply liquid hydrocarbon charging during the Late Caledonian-Early Hercynian and Late Hercynian and crude oil cracking in the study area. The method of direct dating of strike-slip faults by in-situ U-Pb dating of carbonate minerals has broad applications, and the combination of in situ calcite U-Pb dating and hydrocarbon inclusion analysis can directly establish the time of hydrocarbon charging and evolution of dynamic hydrocarbon accumulation in a multi-cycle superposition basin.

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Mechanisms of liquid hydrocarbon evolution and preservation in ultra-deep Ordovician reservoirs, northern Tarim Basin: Insights from laboratory simulation experiments
CHEN Qianglu, MA Zhongliang, LI Maowen, XI Binbin, ZHENG Lunju, ZHUANG Xinbing, Yuan Kun, MA Xiaoxiao, XU Jin
2023, 30(6): 329-340. 
DOI: 10.13745/j.esf.sf.2023.2.13

Abstract ( 544 )   HTML ( 0 )   PDF (4894KB) ( 44 )  

A number of oil and volatile oil reservoirs have been discovered in the Cambrian-Ordovician in the northern Tarim Basin expanding the depth scale of hydrocarbon-liquid window. The deep and ultra-deep reservoirs span multiple tectonic periods and have undergone complex tectonic, burial and thermal evolution. To understand the mechanisms of liquid hydrocarbon formation and maintenance, the burial history of key stratigraphic sequences, oil and gas formation and dating, hydrocarbon inclusions and crude oil stability have been investigated. However, in-depth studies on the interactions between hydrocarbons and mineral matrix/formation water and their effects on reservoir evolution and preservation are still lacking. In this paper, laboratory simulation experiments are carried out using constrains from the burial, thermal and paleopressure history of the Shuntuo Guole region, Tarim Basin, and the thermal history of reservoir fluids is reconstructed under conditions mimicking the natural occurrence state, overburden pressure, fluid properties and pressure regimes of the underground reservoirs. The results show that the reservoir temperature controls crude oil cracking, where the highest paleo-temperature and duration of hydrocarbon accumulation dictate the phase behavior of reservoir fluids today. When VRo is less than 2.0%, crude oil cracking is delayed under effects of formation water and limestone matrix, which is conducive to liquid hydrocarbon preservation in a reservoir. Based on preliminary evaluation, the estimated Oil Preservation Indexes (OPI) for early filled crude oil in Shunbei and Shunnan are 67%-100% and 17%-50%, respectively, which suggests the depth limit for significant hydrocarbon accumulation in Shunbei can be extended to ~10000 m.

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Numerical simulation of water intrusion in wet gas reservoirs: A case study of the Changxing gas reservoir in Yuanba
LI Yudan, YOU Yuchun, ZENG Daqian, SHI Zhiliang, GU Shaohua, ZHANG Rui
2023, 30(6): 341-350. 
DOI: 10.13745/j.esf.sf.2023.2.26

Abstract ( 526 )   HTML ( 1 )   PDF (8827KB) ( 33 )  

The intrusion of edge and bottom waters in water drive gas reservoirs can severely impact the production capacity of gas wells; however, the law of water intrusion in wet gas reservoirs is not well understood. To fill this knowledge gap, the production characteristics of vertical and horizontal wells drilled through the Changxing Formation in the Yuanba gas field is investigated in combination with regional geological conditions, water properties and gas-water interfacial relationship to establish a mechanistic model of water intrusion via numerical simulation, and the influencing factors of gas production and water intrusion such as gas production speed, water body size, tight layer connectivity and height of water avoidance are analyzed. The results show that (1) the gas production speed is the main influencing factor of water intrusion as higher production speed leads to faster water intrusion and shorter stable production period, while the horizontal well can effectively inhibit water intrusion to a certain extent. (2) The size of the water body is another important influencing factor, the larger the water body size, the more serious the water intrusion and the shorter the stable production period. (3) The tight interlayer can inhibit water intrusion to a certain extent, and the bottom water cone, corresponding to the permeable interlayer (Kiz = 100%Kz), has the fastest advance speed and the shortest stable production period. (4) As the avoidance height increases, the water/gas ratio increases slowly and water intrusion is delayed. Therefore, for water drive gas reservoirs, maintaining a reasonable production pressure difference or optimizing production practice is the key to controlling the rapid coning of bottom water. If the natural tight layer in the reservoir is ineffective, the submersible water cone can be inhibited by establishing an artificial compartment (dense interlayer) with low vertical connectivity. At the same time, when designing well drilling or later replenishing holes, it is necessary to take into account the impact of the injection hole size and height of water avoidance on production. The results of this study can be used to guide the development of submersible wet-gas reservoirs.

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Stress sensitivity characteristics of deep carbonate fractured porous media
SUN Ke, LIU Huiqing, WANG Jing, LIU Renjie, FENG Yabin, KANG Zhijiang, ZHANG Yun
2023, 30(6): 351-364. 
DOI: 10.13745/j.esf.sf.2023.2.27

Abstract ( 574 )   HTML ( 0 )   PDF (7231KB) ( 39 )  

The deep, vuggy, fractured porous carbonate reservoirs have three main characteristics, namely complex burial environment, rock-fluid interaction and storage space, which hinders the effective reservoir utilization and development. In order to reveal the stress sensitivity characteristics of this reservoir type under high temperature, pressure and stress conditions, several porous media models with different fracture development, fracture roughness and pore/filling type were constructed using an improved test method for stress sensitivity evaluation; the impacts of different porosity development characteristics on the stress sensitivity of carbonate fractured porous media with vuggy porosity were investigated in depth. Study results show that (1) the effect of stress sensitivity hysteresis generally exists at depth in the studied media, and the measured absolute permeability in the pore-pressure recovery stage is always smaller than in the pressure reduction stage. (2) There is a good positive correlation between the maximum and irreversible permeability-loss ratios for the studied media with similar porosity development characteristics. (3) The stress sensitivity performance of the studied media with different filling types is consistent with that of porous media, with small permeablity-loss ratios for both pore-pressure reduction and recovery stages. (4) Fractures are the main structural control factor affecting the stress sensitivity of the studied media, and the stress sensitivity performance of fractured media is significantly stronger compared to vuggy media. (5) With the increase of fractures and complexity of media morphology, the stress sensitivity of the studied media decreases significantly, while the presence of dissolved vugs and caves causes further reduction in stress sensitivity. This study clarifies the stress sensitivity characteristics and major differences between carbonate vuggy fractured porous media with different porosity developmental characteristics at depth, and provides a theoretical basis for reservoir development decision making.

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A numerical simulation method for deep, discrete fractured reservoirs using a multi-scale fluid-rock coupling model
ZHANG Yun, KANG Zhijiang, MA Junwei, ZHENG Huan, WU Dawei
2023, 30(6): 365-370. 
DOI: 10.13745/j.esf.sf.2023.2.28

Abstract ( 593 )   HTML ( 1 )   PDF (1631KB) ( 36 )  

Deep discrete fractured reservoirs show large variations in porous media type—including both discrete and continuous media—and fracture scale, and the porous media are pressure sensitive and prone to deformation under high pressures. Although the existing numerical simulation methods for oil reservoirs take into account discrete media, they do not consider the impact of fracture deformation and therefore are not applicable for deep, discrete fractured reservoirs. To address this problem we developed a new simulation method using a multi-scale fluid-rock coupling model. First, a subscale model for rock matrix, microfractures, small/medium-sized fractures, and discrete fractures was proposed, and a multi-scale fluid-rock coupling model for discrete fracture network was established. Second, mathematical models for fluid-rock coupling at different fracture scales were created. Pressure sensitivity and fluid-rock coupling were treated separately in consideration of fluid flow and stress sensitivity characteristics in fractures of different scales, while stress-sensitive microfractures and rock matrix were treated together in the pressure sensitivity calculation model to establish a compression coefficient calculation method to describe fluid flow in the fractures. As small/medium-sized fractures are easy to close during oil/gas production, it was proposed to establish a calculation model for fracture opening and closing. Furthermore, as discrete fractures control the direction and scale of fluid flow and are easy to fill, a fully coupled mathematical model was established to treat discrete fractures using the fluid-rock coupling model. Third, finite volume and finite element numerical simulation models were established where fluid flow/ volume were discretized using finite flow/volume methods, and the linear equations were solved by using a fully implicit numerical solution method with relatively good convergence. Finally, the new method was verified by comparing theoretical models with actual reservoir models, which show a good agreement between the numerical calculation results using a two-dimensional consolidation model and the theoretical solution with consideration for pressure sensitivity and fluid-rock coupling. This novel, high accuracy simulation method addresses the issues—such as high-pressure, high-stress, and large variations in fracture scale—associated with deep, discrete fractured reservoirs, and is highly practical and can be used to provide technical support for the efficient development of such reservoirs.

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Mineralogy and geochemistry of gold-bearing sulfides in the Wanken gold deposit, West Qinling Orogen
YANG Mengfan, QIU Kunfeng, HE Dengyang, HUANG Yaqi, WANG Yuxi, FU Nan, YU Haocheng, XUE Xianfa
2023, 30(6): 371-390. 
DOI: 10.13745/j.esf.sf.2023.5.80

Abstract ( 57 )   HTML ( 14 )   PDF (14054KB) ( 125 )  

The West Qinling Orogen, formed as a result of Late Triassic collision between the North and South China Blocks, has undergone complex geological evolution and multi-episodic magmatic activities. It has abundant ore resources, including over 1200 tons of gold by estimation. The genesis of the gold deposits is still debatable, with possibilities including Carlin-type, orogenic, magmatic hydrothermal, and intrusion-related. The Wanken gold deposit in the southwest contains ~3.5 tons of proven gold, hosted in metasedimentary rock of the Lower Triassic Longwuhe Formation. Typical of the region, the gold deposit is structurally controlled by EW and NW-SE trending faults, and mainly exhibits disseminated vein mineralization styles. This study investigates the gold occurrences in the Wanken gold deposit and its genesis through petrological and mineralogical analyses, combined with geochemical analysis of gold-bearing sulfides. Three mineralization stages are delineated: pyrite (Py1)-quartz-sericite-chlorite (Stage I), pyrite (Py2)-arsenopyrite-quartz-sericite-chlorite (Stage II), and quartz-calcite-sphalerite-galena-stibnite (Stage III). According to electron microprobe analysis, the gold content (wt%) is mostly below detection limits in pyrite (0.11%-0.24%) and arsenopyrite (0.11%-0.28%), but relatively high in sphalerite (0.16%-0.37%), with 70% sites above 0.15%, indicating a heterogeneous distribution of Au in sulfides. The Au/As ratio of pyrite (0.007-0.20) is generally greater than 0.004, and there is no evidence of native gold, indicating gold mainly occurs as Au nanoparticles or nanocrystals. A negative correlation between Fe and As or between S and As in As-rich pyrite and arsenopyrite reveals that Au enters sulfide lattice through substitution into octahedral Fe(II) sites, or through chemical adsorption. The Fe (45.41%-46.26%) and S (51.48%-52.79%) contents in pyrite correspond to a Fe/S ratio of greater than 0.875, indicating a metamorphic-hydrothermal origin. Arsenopyrite from the main metallogenic epoch contains 42.80%-46.52% arsenic, and late sphalerite contains 1.77%-2.57% Fe and 63.75%-64.82% Zn. Furthermore, using arsenopyrite/sphalerite geothermometers, the temperatures and sulfur fugacity (lgf(S2)) in the main mineralization stage are constrained at 385±40 ℃ and -7.55±1.45, respectively, and the temperatures in the late mineralization stage are constrained at 251±7 ℃. These observations suggest that the Wanken gold deposit is derived from medium- to high-temperature metamorphic hydrothermal fluids. Combining the mineralogical and geochemical characteristics of gold-bearing sulfides in the Wanken gold deposit with previous thermodynamic modeling results, it is evidenced that the dominant Au-S complexes in the Wanken deposit are $\mathrm{Au}(\mathrm{HS})_{2}^{-}$ and Au(HS). Extensive water-rock interaction and high degree of wallrock sulfidation causes HS- to decrease in the hydrothermal fluids, which results in the decrease of gold solubility and destabilization of Au-S complexes, whereby driving the formation of the gold-bearing sulfides. Therefore, sulfidation is likely the dominant mechanism of gold precipitation in the Wanken gold deposit. In summary, it is considered that the Wanken gold deposit belongs to orogenic gold deposit.

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New method to trace the three-dimensional compositional structure of cratonic lithosphere
YANG Liqiang, HE Wenyan, GAO Xue, WANG Sirui, LI Nan, QIU Kunfeng, ZHANG Liang, MA Qiang, SU Yuping, LI Dapeng, ZHANG Zhiyu, YU Hong
2023, 30(6): 391-405. 
DOI: 10.13745/j.esf.sf.2023.5.81

Abstract ( 51 )   HTML ( 11 )   PDF (2496KB) ( 81 )  

Tracing the three-dimensional compositional structure of cratonic lithosphere is a frontier research topic in earth sciences, and it faces a series of challenges. This paper systematically examines previous results and proposes a new way of tracer study, i.e., changing from point to surface tracing, from local to whole-area tracing, and from single- to multi-method tracing. Taking the North China Craton as an example, urgent research areas and future directions are also proposed. The use of a single constrain and a comprehensive structural interpretation, obtained by multidisciplinary synthesis using various geophysical methods, not only opens up a new way of studying the three-dimensional compositional structure of the lithosphere, but also is conducive to obtaining more reliable results. A more realistic compositional structural model must agree with observations, including rock probe, lithospheric architecture, petrophysical property and mineral composition data, and be consistent with the theoretical geochemical thermodynamic and geodynamic frameworks. To overcome single data/method limitations and to obtain a joint constrain, a multidisciplinary tracer study process adhering to “systematic, gradual progress” and “feedback correction and continuous approximation” is proposed, which aims to recover the compositional structures of cratonic lithosphere in different evolutionary stages and trace its evolutionary process.

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A new granitization theory: Discussion on the four-stage granitization theory
ZHANG Qi, ZHAI Mingguo, WEI Chunjing, ZHOU Ligang, HUANG Guangyu, CHEN Wanfeng, JIAO Shoutao, TANG Jun, LIU Rui, YUAN Jie, WANG Zhen, WANG Yue, YUAN Fanglin
2023, 30(6): 406-435. 
DOI: 10.13745/j.esf.sf.2023.6.11

Abstract ( 110 )   HTML ( 16 )   PDF (11368KB) ( 211 )  

The origin of granite is both an ancient and a frontier scientific problem. One hundred years ago debate on the origin of granite ended with the prevailing view that granite is igneous rather than metamorphic in origin. However, over the past century researchers have shown that the igneous theory is not perfect and the mechanism of basalt crystallization differentiation into granite had been severely challenged. Today it is considered an indisputable fact that granite originated from partial melting of the lower crust, which indicates the source of granite is metamorphic rock. There are many theories on the formation of granites. After many years of testing, the four-stage (melting, melt segregation and ascent, and magma emplacement) theory of granite formation is considered more plausible. Based on detailed study of this theory, this paper proposes a new four-stage theory which divides the granite formation process into two main parts: melt generation and formation (melting and melt aggregation), a heating process, and melt ascent and magma emplacement, a cooling process. The core of this theory is the conjecture of a “lower crustal magma chamber”, which refers to the giant space formed by melt aggregation. This conjecture, first, solves the space problem of a magma chamber in the lower crust. As in situ partial melting of the lower crust only changes the material composition of its products (melt plus remnant), with no space issue involved, the total volume of the lower crust is basically unchanged. And, as there is continuous mantle heating, a lower crustal magma chamber can grow gradually and become very large. Second, we consider the driving force behind magma uplift is not the buoyancy of magma itself. Rather, as the lower crustal magma chamber overflows along the fault zone, the formation pressure from tens of kilometers of strata beneath the magma chamber may transform into great force, driving the magma upward. Therefore, theoretically, granite can rise very quickly, almost instantaneously on the geologic time scale. Third, this conjecture reasonably explains the ancient problem of granite emplacement. It is precisely because the lower crustal magma chamber moves out and ascents, the space it occupied is immediately compacted and filled by the overlying strata, and the subsequence collapse of the underlying strata directly affects the fragile upper crust. A void is then created in the weak part of the upper crustal structure to provide space for the rising magma to complete its emplacement process. Apparently, a space displacement is realized in the disappearing of a magma chamber in the lower crust and magma emplacement in the upper crust. It seems that the concept of a “lower crustal magma chamber” can better resolve many traditional controversies regarding granite formation. The conjecture needs to be verified. Finally, to further study the above issues we suggest using two interdisciplinary approaches-metamorphic-igeous petrology and physical geology.

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Identification of orbital cycles in coral-reef core from well CK-2, Xisha Islands and insights into coral reef evolution in the South China Sea
ZHANG Niannian, FAN Tianlai, HUANG Chunju, ZHANG Mingwang, LI Yuchun, WEI Lu, YU Kefu
2023, 30(6): 436-450. 
DOI: 10.13745/j.esf.sf.2023.6.10

Abstract ( 37 )   HTML ( 3 )   PDF (9785KB) ( 76 )  

Coral reefs can record past climate change. For long-term climate studies, the establishment of a high-precision stratigraphic chronological framework can provide accurate chronological constraints on the correlation between regional and global climate events. In this paper, coral-reef core samples from well CK-2, Xisha Islands, South China Sea are used in magnetic testing, and anhysteretic remanent magnetization (ARM) is selected as climate proxy to identify astronomical cycles in ancient reefs in the South China Sea since the Miocene. The results of ARM depth series analysis reveal well defined stable depositional cycles corresponding to a ~1.2 Ma obliquity modulation cycle and a ~405 ka long eccentricity cycle; these two cycles are used in orbital tuning of the ARM depth series between 0-878.21 m. The orbital tuning results set the depositional constraints on the coral-reef core at 0-19.2 Ma, with a resolution up to 100000 years, which can be mutually calibrated with Sr isotope dating. The ~1.2 Ma obliquity modulation cycle is most obvious in the reef strata of the South China Sea through the Miocene, and the ~405/~95 ka eccentricity cycles and the ~173 ka obliquity modulation short cycle are discontinuous. These results suggest that the millennium-scale orbital cycles may play a major role in regulating the evolution of coral reefs in the South China Sea. The change of the main control period reflected in the reef records may be an indicator of reef’s timely response to glaciation associated climate change.

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Mechanism of action and evaluation of chelating agents on barite removal
ZHANG Xiong, PAN Lijuan, LI Liang, FANG Junwei, WEI Zhongjin, YANG Juncheng, ZHOU Fengshan
2023, 30(6): 451-462. 
DOI: 10.13745/j.esf.sf.2023.6.4

Abstract ( 52 )   HTML ( 0 )   PDF (4051KB) ( 38 )  

Barite blockage—a result of the formation of a drilling fluid filter cake with barite weighting agent as the main component—is one of the main contributing factors to reservoir damage and causes major problems in oil and gas production. As conventional acidification measures fail to remove the barite blockage, finding cost-effective, non-acid chelating agents and formulating a barite removal scheme are an effective engineering means to solve the barite blockage problem. Aminopolycarboxylic acids as chelating agents bind strongly with barium ions on the surface of barite promoting the rapid dissolution of the barite filter cake. The structural characteristics of common aminopolycarboxylic acids are summarized. From the mechanistic perspective of chelating, we consider DTPA to be most effective for the barite removal in oil and gas wells. In addition, the pros and cons of commonly used evaluation methods for chelating agents are considered, and an evaluation scheme for accurate, simple, fast performance evaluation of chelating agents is established. We recommend using the high-temperature dynamic dissolution-rate measurements to evaluate chelating reagents, as it has the advantages of short experiment time, low requirements on test equipment and intuitive test results.

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Enrichment model and exploration potential for unconventional oil and gas in troughs, Erlian Basin
XIANG Xin, HUANG Chuanyan, CAO Lanzhu, CAO Qiang, JIANG Tao, ZHANG Yufei, SONG Yu, XU Jie
2023, 30(6): 462-472. 
DOI: 10.13745/j.esf.sf.2023.2.71

Abstract ( 32 )   HTML ( 4 )   PDF (11801KB) ( 47 )  

Oil and gas exploration nowadays is increasingly directed toward the center and deep layers of lake basins. In the Erlian Basin the remaining ~800 million tons of unconventional petroleum resources are mainly distributed in troughs of oil-rich depressions, with great shale oil potential. This paper seeks to summarize and improve the theoretical understanding of unconventional petroleum geology in oil-rich lake basins by studying the geological characteristics, accumulation and enrichment mechanism and accumulation model of unconventional oil and gas in the Erlian Basin. The accumulation of interbedded type shale oil in the basin occurs in multiple troughs-preferably at trough edges-via a two-stage process: “accumulation before tightening” (Tenger stage (120 Ma)) and “accumulation after tightening” (Saihan stage (105 Ma)). The degree of shale oil enrichment in high-charge dynamic enrichment areas is controlled by sandbody dimensions, while in low-charge areas it is controlled by “sweet-spot” reservoirs development status. And there are two enrichment mechanisms for interlayer type shale oil: “reservoir control” and “hydrocarbon generation control.” Based on these results a hydrocarbon accumulation model is constructed for the entire oil reservoir series in slope belts and troughs of the Erlian Basin, which is summarized as “conventional/unconventional oil coexistence, hydrocarbon orderly distribution, trough-wide accumulation and differential enrichment.” Results of this study boost our confidence in finding interbedded type shale oil at source-rock edges and interlayer type shale oil within source rocks in the interior of lake basins.

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Multi-dimensional geoelectrical resistivity imaging monitoring for debris flow based on neighborhood domain features
XU Haning, DENG Juzhi, XIAO Hui
2023, 30(6): 473-484. 
DOI: 10.13745/j.esf.sf.2023.2.60

Abstract ( 61 )   HTML ( 5 )   PDF (11754KB) ( 47 )  

As a nonlinear system debris flows exhibit complex slope deformation patterns, and accurate quantification of the key parameters such as slip surface boundary change, rainfall infiltration boundary, head boundary and flow boundary during slope deformation is essential to studying the deformation mechanism of debris flows. Electrical resistivity imaging allows fast and multidimensional profile imaging based on compositional/structural differences between geotechnical bodies and difference in electrical property between strata. In this study, a dataset containing the “time-space-attribute” neighborhood parameters extracted from high-resolution shallow surface resistivity imaging data and various monitoring data was constructed by deep neural network learning, and used as input to generate, with rapid convergence, the relevant spatial weights matrix for monitoring items. On this basis, the deep resistivity imaging data were analyzed recursively, layer by layer, and a multidimensional internal structural model of the studied debris flow was accurately constructed based on the electrical characteristics of the debris flow to quantify its slope deformation process. This method was validated experimentally, which showed effective improvements in the real-timeliness and accuracy of various boundary parameters derived from geoelectrical resistivity imaging monitoring data. Results from this study have theoretical and methodological significances for understanding the slope deformation mechanism of debris flows.

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Soil Sr, Mg, Ca in upstream watersheds of the Guanting and Miyun Reservoirs as climate indicators
ZHANG Chenggang, WEI Jing, ZHANG Yutao, WANG Ran, HOU Zhaoshuo, ZHAO Jingxuan, ZHANG Xinran
2023, 30(6): 485-492. 
DOI: 10.13745/j.esf.sf.2022.12.70

Abstract ( 47 )   HTML ( 4 )   PDF (1134KB) ( 51 )  

Taking the upstream watersheds of the Guanting and Miyun Reservoirs as examples, this paper explores the soil Sr, Mg, and Ca as indicators of climatic factors, such as temperature, precipitation (P), relative humidity (RH), and sunshine duration under supergene environments. Using regional soil data from agricultural land and regional climate data from the last 20 years, the relationships between soil Sr, Mg, Ca and climatic factors are investigated via multivariate statistical analysis. At 95% confidence level, best-fitting multiple linear regression models between soil element ratio (RMg/Ca or RSr/Ca) and climatic factors (P and RH) are established. For equation RMg/Ca=-1.68+(3.59×P+12.62×RH)×10-3, the complex correlation coefficient, R2, is 0.86 after outlier adjustment; for equation RSr/Ca=-3.49×10-2+(4.95×P+47.11×RH)×10-5, R2 is 0.73 after outlier adjustment. A strong positive correlation between Sr, Ca, and Mg in soil is observed under supergene environments, suggesting the three elements may have similar geochemical properties and distribution control mechanisms. The main climatic factors that affect their soil distribution are precipitation and relative humidity, with precipitation having a greater impact. Results show that the Sr, Mg, and Ca contents in soil are inversely correlated with ambient humidity, where higher content usually correlates with drier climates, and vice versa. Compared to soil content, Mg/Ca and Sr/Ca ratios in soil are more effective climate indicators, where areas with low ratios often have relatively dry climates, and vice versa.

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