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    2023, Volume 30 Issue 1
    25 January 2023
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    Oil and gas exploration potential of the Permian multi-stage platform-margin zone, western Sichuan Basin.
    YANG Yu, WEN Long, CHEN Cong, WANG Hua
    2023, 30(1): 1-10. 
    DOI: 10.13745/j.esf.sf.2022.8.22

    Abstract ( 91 )   HTML ( 44 )   PDF (7168KB) ( 119 )  

    Situated in stable cratonic margins during the Permian the western Sichuan Basin experienced tectonic-sedimentary differentiation under jointly the actions of the Hercynian movement, Dongwu movement and Emei rift. Adopting the research idea that tectonic movement controls basin, basin controls facies, facies control reservoir-forming elements,a new round of research is carried out in the western Sichuan Basin to investigate the control of petroleum accumulation by Permian tectonic-sedimentary differentiation and determine the exploration potential of marine carbonate rocks in the western Sichuan Basin. During the Permian, the Sichuan Basin is dominated by tensional/extensional tectonic activities and a general sedimentary pattern of “uplift in the south, rift in the west, and tension in the north” is revealed. The Permian multi-stage tectonic movements lead to the formation of a series of intra-platform rifts in western Sichuan, which, combined with the Permian sea-level change, control the multi-stage platform-margin development from the Qixia to Changxing formations. The NE-SW and NW-SE fault systems developed in the northwestern basin control the lithofacies paleo-geographic pattern of this area. Among them, the Longmenshan fault belt largely controls the development of platform margins N-E along the northwestern edge of the upper Yangtze platform from the Devonian to Permian Qixia periods. From the Maokou to Changxing periods, the platform margins—spreading N-E in the western Sichuan Basin and N-W along rifts in the Zitong-Laoguanmiao areas—are jointly controlled by cratonic margins and intra-platform rifts, and the multi-stage platform margins of the Qixia, Maokou, Wujiaping and Changxing formations are superimposed in the Jiange area. In the western Sichuan Basin, the Permian multi-stage platform margins control the superposition of multi-layer, high-quality beach facies reservoirs and reef reservoirs, which is the main factor controlling the development of large-scale Permian porous reservoirs in this area. The coupling of multi-stage paleo-rift depression, multi-stage platform-margin zones and large-scale paleo-uplift in the reservoir-forming stage is conducive to the large-scale oil and gas accumulation. Vertically superimposed and spreading N-W over a wide range the multi-layer platform margins of western Sichuan possess trillion-ton petroleum resource potential and are an important replacement exploration target for marine carbonate rocks in the Sichuan Basin.

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    Natural gas accumulation characteristics and great exploration potential of the Middle Permian in the Sichuan Basin
    CHEN Mangjiao, TAN Kaijun, WEN Long, LE Xingfu, YAO Jun
    2023, 30(1): 11-19. 
    DOI: 10.13745/j.esf.sf.2022.8.23

    Abstract ( 60 )   HTML ( 10 )   PDF (3747KB) ( 74 )  

    In recent years porous dolomite reservoirs are drilled in the Middle Permian of the Sichuan Basin, and several high-yield wells with good exploration prospects are obtained. However, the exploration potential of the Middle Permian is still unclear and it is critical to know if it beholds another trillion cubic meter-scale natural gas field in the Sichuan Basin, in addition to the Anyue natural gas field. Through in-depth analysis of the source-rock, sediment and reservoir characteristics as well as accumulation models, we find the Middle Permian possesses superior reservoir-forming characteristics—including high-quality hydrocarbon source rock, porosity reservoir development and good well-sealing conditions—and two source-reservoir configuration types—“self generation-self storage” and “lower generation-upper storage”. The Middle Permian develops massive porous dolomite reservoirs and porous limestone karst reservoirs over a large area, and its reservoir capacity and single-well testing production are equivalent to that of the Dengying Formation in the Anyue natural gas field indicating great exploration potential. We believe the Middle Permian is the main stratum to search for the next giant natural gas field in the Sichuan Basin.

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    Accumulation characteristics and exploration potential of tight limestone gas in the Taiyuan Formation of the Ordos Basin
    FU Jinhua
    2023, 30(1): 20-29. 
    DOI: 10.13745/j.esf.sf.2022.8.24

    Abstract ( 69 )   HTML ( 13 )   PDF (4272KB) ( 96 )  

    In 2021, a high-yield daily output of more than 5.0×105 m3 industrial gas was obtained in well Yutan*H, signifying a major breakthrough in natural gas exploration in the limestone reservoir of Taiyuan Formation, Ordos Basin. Now, more than 700 exploratory wells have good gas shows in the reservoir, showing excellent exploration potential, which is of great significance for further studying the geological characteristics of the limestone of Taiyuan Formation. Based on comprehensive analysis of the source rock characteristics, sedimentary environment, reservoir characteristics and hydrocarbon accumulation assemblage in the area, we show that: (1) The limestone of Taiyuan Formation is an epicontinental carbonate deposit, in which favorable sedimentary facies belt composed of bioclastic beach and biological mound is developed, and the widespread bioclastic silty limestone and algal-rich limestone can form favorable reservoirs after short-term leaching by atmospheric freshwater in penecontemporaneous period. (2) The limestone of Taiyuan Formation develops several types of reservoir space, where solution pores, submicron to nano intergranular micropores and microfractures superimpose to form a three-dimensional reservoir space; meanwhile across the region, 4-10 m thick bioclastic silty limestone reservoir develops in the Hengshan-Jingbian-Zhidan area, and 2-6 m thick algal-rich limestone reservoir develops in the Jiaxian-Zizhou-Qingjian area. (3) The hydrocarbon accumulation mode has a “sandwich-like” structure, where the limestone reservoir is sandwiched between main coaly source rocks, and the generated natural gas enters limestone reservoir through faults and fractures to accumulate and form a lithologic gas reservoir. With a combined area of 1.5×104 km2, the bioclastic beach in Hengshan-Jingbian area and the biological mound in Zizhou-Qingjian area are the favorable exploration targets for the limestone of Taiyuan Formation, which is expected to bring the increase of natural gas reserve and production.

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    Sedimentary characteristics and gas accumulation potential of the Ordovician carbonate-evaporite paragenesis system in central and eastern Ordos Basin
    BAO Hongping, WANG Qianping, YAN Wei, CAI Zhenghong, ZHENG Jie, WEI Liubin, HUANG Zhengliang, GUO Wei
    2023, 30(1): 30-44. 
    DOI: 10.13745/j.esf.sf.2022.8.25

    Abstract ( 57 )   HTML ( 8 )   PDF (7834KB) ( 88 )  

    A sedimentary system consisting of interactive ultra-thick carbonate rock and gypsum-salt rock developed in the Ordovician Majiagou Formation in the central and eastern Ordos Basin, however, its development process and natural gas accumulation characteristics are not clear. By studying its tectonic and paleogeographic background, sedimentary features, reservoir development, hydrocarbon source supply and trap accumulation, we gain new geological insights into the carbonate-evaporite paragenesis system, which can be summarized as follows: 1) Affected by its surrounding ancient uplifts, the carbonate-evaporite paragenesis system is formed in a restricted sea with periodic isolation from the open sea, therefore its lithologic distribution has obvious “facies-control” and “strata-bound” characteristics. 2) The key factor controlling the development of the Ordovician “interior dissolved pore” type reservoir is “nonstratiform sedimentary differentiation”—a novel concept, proposed here for the first time. 3) Controlled by regional lithologic transition and later structural inversion, the Ordovician sub-salt stratum has favorable conditions for forming lithologic trap system on a large scale. 4) Although the sub-salt stratum has limited hydrocarbon-generation capacity from its own marine source rocks, it still has the potential of large-scale hydrocarbon supply/accumulation due jointly to the Yanshanian tectonic uplift in its eastern part, hydrocarbon supply from Upper-Paleozoic coal-bearing source-rock on its west side, and regional continuous distribution of gypsum-salt capping strata.

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    Formation conditions and reservoir forming models of Ordovician buried hill reservoirs in Jizhong Depression
    ZHANG Ruifeng, TIAN Jianzhang, HUANG Yuanxin, TIAN Ran, REN Yi, BIAN Yingying, WANG Yuanjie, CHEN Ling, LU Shan
    2023, 30(1): 45-54. 
    DOI: 10.13745/j.esf.sf.2022.8.15

    Abstract ( 38 )   HTML ( 4 )   PDF (3362KB) ( 49 )  

    Buried hills in the Jizhong Depression are important exploration targets for petroleum production/reserve enhancement. Ordovician buried hill, however, hinders petroleum discovery due to its strong reservoir heterogeneity and complex reservoir-control factors. Here, based on abundance of core, thin section, logging, seismic and geochemical data, and on the basis of geological analysis, the reservoir-forming conditions/modes of the Yangshuiwu and Wen’an-slope buried hills are systematically analyzed to guide explorations for major breakthroughs. The study shows that three sets of source rocks of the 3 rd and 4 th members of the Paleogene Shahejie Formation and upper Paleozoic Carboniferous-Permian are developed in the Jizhong Depression, providing sufficient material basis for the formation of buried hill oil/gas reservoir. The reservoir-control mechanism including three main factors—carbonate dolomitization/karst/fault—is ascertained, and three reservoir models of karst-pore type, fault-pore fracture type and dolomite-pore type—are established, expanding the exploration space. Controlled by multi-stage tectonism in Indosinian, Yanshanian and Himalayan periods, two types of buried hills were formed: one was formed from depressed strata after uplift and the other from uplifted strata after depression. Based on the analysis of reservoir-forming factors of Ordovician buried hills, three models of buried hills are concluded: deep structural and stratigraphic buried hills with layered reservoirs, median buried hills trapped by Mesozoic and Upper Paleozoic mudstones, and shallow buried hills sealed by Paleogene mudstones. In conclusion, the deep structural and stratigraphic buried hills with layered reservoirs in northern Jizhong Depression have the most favorable reservoir-forming conditions, and the Sicundian and Xinzhen buried hills are the favorable exploration targets.

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    Fault characteristics and fault control of hydrocarbon accumulation in “new source, old reservoir” type buried hills of the Huanghua depression
    JIN Fengming, ZHANG Feipeng, HAN Guomeng, PU Xiugang, WANG Hua, FU Lixin, LI Hongjun, LOU Da, CHENG Xin, SUN Peipei
    2023, 30(1): 55-68. 
    DOI: 10.13745/j.esf.sf.2022.8.16

    Abstract ( 39 )   HTML ( 10 )   PDF (6947KB) ( 55 )  

    The “new source, old reservoir” type buried hills are an important exploration target in the Dagang Oilfield, Hunanghua depression where an exploration breakthrough is made recently. Controlled by multi-stage fault evolution this type of buried hills is diverse in reservoir types and hydrocarbon accumulation conditions. In this study, the static and dynamic characteristics of the hill-controlling faults are systematically analyzed, and the fault control on the formation of buried-hill reservoirs is revealed. Based on a systematic analysis of the influencing factors in reservoir formation, such as physical properties/reservoir-forming mechanisms of Paleozoic carbonate and clastic reservoirs, source-rock properties, and source-reservoir configurations, the buried-hill types are delineated, and the potential exploration strata and distribution area for each type are identified. Results show (1) the current fault system has the multi-direction, multi-property characteristics resulted by multi-stage superposition evolution. The fault system develops through four main stages: near-EW and NW (W) reverse faults of T3 period continue development in J1+2; NNE (near-SN) thrust-transition fault forms by extrusion thrust in J3+K1; the above thrust-transition fault undergoes negative inversion in K1; and NE (E) extension fault forms in the Cenozoic E period. (2) The buried hill experienced “two-stage extrusion, one-stage block faulting” orogenesis, and the inner reservoir experienced multi-stage diagenesis (early-, supergene, middle- and late-diagenesis). The Ordovician carbonate reservoir has medium porosity/permeability, and the clastic reservoir has medium-low porosity/permeability. (3) With high-quality source-rock developing respectively in the 2nd and 3rd members of the Kongdian and Shahejie formations, source-reservoir configuration becomes an important factor controlling the formation of buried-hill reservoirs, and five source-reservoir configuration types are formed under fault control. (4) According to source-rock property, source-reservoir configuration and internal structural style, the “new source, old reservoir” type buried hills are divided into three categories and seven subcategories. The buried hills have a ring-shaped planar distribution, and the potential exploration area includes three zones: Ordovician strata in the multi-stage structural superposition area; middle-low strata in the basin margin; and deep strata connected to the high buried-hill source rocks outside the basin.

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    Oil-gas geological features and exploration direction in the Qingxi Sag, Jiuquan Basin
    WEI Haoyuan, ZHU Zongliang, XIAO Wenhua, WEI Jun, WEI Deqiang, YUAN Bochao, XIANG Xin
    2023, 30(1): 69-80. 
    DOI: 10.13745/j.esf.sf.2022.8.20

    Abstract ( 35 )   HTML ( 7 )   PDF (4796KB) ( 45 )  

    Petroleum exploration in the Jiuquan Basin begins in the 1920s with the successive discoveries of the Laojunmiao, Yaerxia and Danbei oilfields. The basin is a fault/depression superimposed basin formed through two-stage evolution—Early Cretaceous fault depression and Mesozoic-Cenozoic foreland, where a set of laminar argillaceous dolomite and dolomitic mudstone deposited in the Lower Cretaceous Chijinbao and Xiagou formations during the fault depression period. The relatively developed algal dolomite is high-quality source rock, and rapid basin subsidence during the depression period lead to the quick maturation of source rock and efficient hydrocarbon generation, which provides strong driving force for oil and gas migration. At the same time, compression and thrust play a key role in the generation of structural traps and migration channels as well as improvement of reservoir physical properties. Many positive structural belts, such as the Kulongshan mountain range, are favorable areas for oil and gas enrichment. There are three types of crude oil in the Qingxi Sag according to thermal maturity. The low-maturity crude oil is only found in the upper Xiagou-Zhonggou formations; mature crude oil, capable of large-scale migration, are distributed in the Liugouzhuang, Yaerxia, Laojunmiao and Shiyougou oilfields; and deep high-maturity crude oil, largely confined below the medium and shallow layers, presents promising resource potential. The above results provides a valuable reference for the petroleum exploration in the Lower-Cretaceous Qingxi Sag.

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    Structural architecture and energy resource potential of Gonghe Basin, NE Qinghai-Tibet Plateau
    HE Bizhu, ZHENG Menglin, YUN Xiaorui, CAI Zhihui, JIAO Cunli, CHEN Xijie, ZHENG Yong, MA Xuxuan, LIU Ruohan, CHEN Huiming, ZHANG Shengsheng, LEI Min, FU Guoqiang, LI Zhenyu
    2023, 30(1): 81-105. 
    DOI: 10.13745/j.esf.sf.2022.8.35

    Abstract ( 61 )   HTML ( 10 )   PDF (10936KB) ( 73 )  

    The Gonghe Basin is located at the junction of the South Qilian, East Kunlun and West Qinling orogenic belts in the NE Qinghai-Tibet Plateau where high temperature Hot Dry Rocks and multiple sets of source rocks are discovered. However, without a clear understanding of the genesis, resource potential and distribution pattern of geothermal and oil and gas reservoirs, it is difficult to carry out accurate resource evaluation and effective resource exploration and development in the Gonghe Basin. In this study, the regional tectono-sedimentary sequences, deep tectonic architecture and tectonic deformation are systematically analyzed, the dynamic process of basin evolution is discussed, the main factors controlling reservoir formation in geothermal and hydrocarbon accumulation are analyzed, and the favorable distribution zones and exploration directions are proposed. In the Gonghe Basin, the periodically active Wahongshan-Wenquan, Tanggemu, Waligong and Duohemao NNW-striking dextral strike-slip and thrust faults and the Qinghai Nanshan NWW(E-W)-striking thrust and sinistral strike-slip fault intersect (or in tangent) in the same or opposite direction, and the superposition of the intersecting faults and mantle upwelling result in the unique strike-slip stretching environment during most of the Cenozoic, and control the distribution and properties of seven tectonic units featuring faulted-depressions and uplifts. The Gonghe Basin region experienced six stages of evolution: 1) Development of main basement rock in the Early-Middle Triassic in the northern parts of East Kunlun forearc basin and volcanic arc belt; 2) Collision orogeny and formation of folded basement in the Late Triassic; 3) Post-collisional extension and development of local continental facies in prototype basin at the end of Triassic; 4) Formation of local faulted depressions in the Jurassic-Cretaceous under regional extension; 5) Development of strike-slip pull-apart and faulted basin in the Late Paleogene-Miocene; and 6) Development of intracontinental foreland basins in the Pliocene to the present. During the Meso-Cenozoic periods three large tectonic-sedimentary sequences and eight secondary sequences are formed in the Gonghe Basin, along with the developments of the deep-water shelf and carbonate platform facies, igneous facies, as well as the multi-cyclic terrestrial sedimentary sequences of the alluvial-fluvial, littoral-shallow lacustrine and semi-deep lacustrine facies. The superimposed evolutionary processes record the basin response to the near field effects of the northward subducting, retraction-subducting, and post-collisional extension of the Anemaqen Paleotethyan ocean slab, and to the remote effects of the opening, subducting and closure of the Neotethyan Ocean and the India-Eurasia continental collision.
    The unique tectono-sedimentary evolution results in the favorable oil/gas and geothermal energy resources conditions in the Gonghe Basin, where three sets of hydrocarbon source rocks are developed in the Lower Jurassic, Lower Cretaceous and Neogene, and they can—pending further fine correlation, play evaluation and favorable trap confirmation—form three favorable oil and gas exploration strata. The five-layer deep structural architecture of the Gonghe Basin and its genetic dynamics reveal the basin’s favorable geothermal resources potential. Mantle upwelling leads to the upward movements of each layer in the crust, which results in the short-path, multi-source heating action under the strike-slip and extensional development environment of the basin. The Hot Dry Rock geothermal reservoirs are formed in the basin’s fault intersection zones. For the medium-high temperature reservoirs the main controlling factors are: mantle upwelling and lower-middle crustal partial melting respectively providing the regional and local heat sources; continental margin arc and collision-related granitoids, superimposed with multi-stage fault, fracture and hydrothermal activities, controlling the heat storage and heat conduction; and the thermally stable overlying ultra-thick fine-grained sedimentary rock layers providing the reservoir caps. These factors may also control the heat source and heat conduction in shallow hydrothermal geothermal reservoirs. This study provides a reference for the understanding of geothermal reservoirs in small-medium size intracontinental basins with locally high terrestrial heat flows.

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    Redevelopment potential of NNW oil field, Ordos Basin—an analysis
    HU Guonong, HAO Shiyan, FAN Pingtian, LI Yu, GAO Qinghua
    2023, 30(1): 106-115. 
    DOI: 10.13745/j.esf.sf.2022.8.34

    Abstract ( 36 )   HTML ( 8 )   PDF (3957KB) ( 58 )  

    The main producing zones of NNW oil field, Ordos Basin, are the Chang 6 and Chang 4+5 members of the Triassic Yanchang Formation, which consist numerous amalgamated lenticular and some sheet sands of fluvial and lacustrine deltaic facies and are classified as unconventional tight sand reservoirs. Past well completion was geared toward completing only a single pay sand in each well, however, such insufficient reservoir opening greatly suppressed daily production rate. Through modeling using an elliptoconic drainage model we found there is still a large quantity of oil left between the existing vertical wells that have a drainage radius seldom beyond 50 m. We propose to replace the single pay mining practice with multistage fracking aiming toward completing all pay sands simultaneously. We further suggest that oil field redevelopment using horizontal wells presents the most important future development opportunity, as horizontal drilling and multistage fracking has the potential to multiply oil field production and economic profit. According to our analysis the NNW oil field has sufficient resource base for redevelopment.

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    Lacustrine medium-high maturity shale oil in onshore China: Enrichment conditions and occurrence features
    ZHAO Wenzhi, ZHU Rukai, LIU Wei, BIAN Congsheng, WANG Kun
    2023, 30(1): 116-127. 
    DOI: 10.13745/j.esf.sf.2022.8.31

    Abstract ( 56 )   HTML ( 15 )   PDF (2484KB) ( 96 )  

    A series of breakthroughs in shale oil exploration have been made in several onshore non-marine basins in China showing good exploration and production potentials. Here we summarize our current understandings on the lacustrine shale oil enrichment and distribution characteristics as well as recent research achievements. The main enrichment conditions are the following: (1) Stable and widely distributed shales with high organic content and appropriate thermal maturity act as the material base for shale oil retention. The critical shale parameters are organic carbon (TOC) greater than 2% (optimal 3%-4%); kerogen Ⅰ and Ⅱ1 as the dominant organic matter type; and vitrinite reflectance (Ro) greater than 0.9% (0.8% for brackish water environment). (2) Diverse reservoir types with brittle accumulation zones and certain volume of micro-nano-scale porous space are critical for shale oil accumulation. The preferred reservoir features are porosity greater than 3%-6%; non-dominating pure shale interval while including tight sandstones and hybrid rocks with clay content less than 20% in the early diagenetic stage; and higher clay content (up to ~40%) in the pure shale interval in the mid-to-late diagenetic stage. (3) High-abundance and good-quality retention hydrocarbons are important for ensuring high-mobility shale oil accumulation. Generally, a S1 threshold higher than 2 mg/g is needed (optimal 4-6 mg/g), and a GOR threshold greater than 80 m3/m3 (optimal 50-300 m3/m3) is desired. (4) Excellent roof/floor sealing conditions in the shale oil enrichment interval is essential for maintaining overpressure and retaining sufficient amount of good-quality hydrocarbons. The distribution characteristics of lacustrine shale oil can be summarized as the following: (1) Major shale oil enrichment zones are concentrated in the semi-deep to deep lacustrine deposition areas that have external material inputs such as falling volcanic ashes, hydrothermal solutions, radioactive substances, etc. (2) Intervals with the aforementioned enrichment conditions, or the so called “four highs, one retention” condition, control the vertical distribution of the enrichment intervals. (3) Favorable lithofacies and lithologic assemblages also determine the distribution of enrichment areas. According to the preliminary estimates China has a total of (131-163)×108 t medium-high thermal maturity shale oil reserves, among which (67-84)×108 t are economically viable resources, which are distributed mainly in the Chang 71+2 interval of the Ordos Basin, the Qing-1, 2 members of the Gulong sag, Songliao basin, the Cangdong sag, Qikou sag and Kongdian-Shahejie formations of the Jiyang depression, Bohai Bay basin, and the Lucaogou formation, Junggar basin.

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    Formation and distribution potential of global shale oil and the developments of continental shale oil theory and technology in China
    ZOU Caineng, MA Feng, PAN Songqi, ZHANG Xinshun, WU Songtao, FU Guoyou, WANG Hongjun, YANG Zhi
    2023, 30(1): 128-142. 
    DOI: 10.13745/j.esf.sf.2022.8.29

    Abstract ( 57 )   HTML ( 13 )   PDF (5107KB) ( 112 )  

    The global unconventional shale formations contain rich oil and gas resources, and worldwide there are four major shale formations namely Upper Jurassic, Oligocene-Miocene, Cretaceous and Upper Devonian formations of the Laurasia and Tethys tectonic domains. In this study, the relationship between production and Ro value of typical North America shale oil is identified by cross-analysis, and a Ro value of 0.7% is proposed dividing the low-maturity and medium-high maturity shale oil. In addition, systematic evaluation is performed on low and high maturity shale oil from 157 shale formations in 116 basins around the world, which involves 251.2 billion tons of technically recoverable resources distributed mainly in North America, South America, North Africa and Russia, where the dominant strata are Miocene formations of foreland basins, Paleozoic formations of cratonic basins, and Mesozoic formations of rift basins and passive continental margin basins. Affected by marine transgression since the Phanerozoic marine shale oil is enriched in stable craton and foreland basins with large-scale stable distribution and moderate maturity; whilst due to greenhouse effect continental shale oil developed mainly in depression and fault-depression basins, where micro and nano inorganic pores and micro laminate fractures serve as the main channels for hydrocarbon migration/accumulation characterized by rapid sedimentary transformation and local enrichment in “sweet spots”. China’s petroleum industry is undergoing a transition from “continental shale oil generation” to “continental shale oil production”, with the initial developments of the geological theory on “in-source exploration” of oil/gas source rocks and the technical system for ‘high-efficiency exploration and low-cost development’ of continental shale oil, seeking major breakthroughs in continental shale oil development in China. Future research should focus on the basic applied theories and key technologies, geology-engineering integration, and “full life cycle” management, aiming to accelerate industrial development of “artificially fractured reservoirs” of medium-high maturity shale oil, enhance technical research and industrial testing on in-situ thermal conversion of low-maturity shale oil, promote “continental shale oil revolution” in China, and solidify resource foundation achieved by stabilizing oil production while increasing gas production.

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    Low permeability tight oil and gas in Yanchang area, Ordos Basin: Advances in accumulation theory and exploration practice
    WANG Xiangzeng
    2023, 30(1): 143-155. 
    DOI: 10.13745/j.esf.sf.2022.8.30

    Abstract ( 32 )   HTML ( 8 )   PDF (4652KB) ( 48 )  

    Petroleum resources (oil, natural gas, shale gas) exploration and development in the Ordos Basin has achieved remarkable results owing to the coordinated resources development as well as theoretical and technical researches promoted by the Shaanxi Yanchang Petroleum (Group) Co., Ltd. In petroleum exploration, a “alternate accumulation of ultra-low permeability oil” theory is proposed, and a new model for Mesozoic multi-stage differential accumulation is establishment, which leads to the discovery of ultra-low permeability oil reserves exceeding 1×108 t per year for the last 10 years. In tight oil discovery, the “sand control by storm and earthquake” theory is proposed for fine-grained sedimentary areas, along with a “sweet spot” prediction method, which leads to the discovery of tight oil reserves totaling 2.1×108 t. In tight gas exploration, the genetic mechanism of sand bodies in tight gas fields and the law of natural gas migration and accumulation are unclear. To overcome this problem a new theory is proposed which considers sand-body distribution is controlled by frequent shoreline migration in shallow water environment, and a formation model for high-quality tight sandstone gas reservoir is established. It is found that pure quartz sandstone and quartz-rich low-plastic granular lithic sandstone are tight-gas “sweet spots”, and a target optimization technique is developed based on multi-element, whole-process natural gas migration and accumulation simulation, which leads to the discovery of the Yan’an gas field with 7635×108 m3 natural gas in reserves. In shale gas exploration, a “ternary accumulation and enrichment of continental shale gas” theory is proposed, which considers that the high-quality lacustrine shale is the material basis for shale gas enrichment and high production, the multi-type, multi-scale complex pore network determines the occurrence conditions of shale gas, and the self-sealing ability and high pressure of thick shale are the keys to gas storage. In the future, the Yanchang Petroleum Group will continue to adhere to the strategy “stabilize oil production, boost gas production, strengthen unconventional resources development”, and ensure sustainable enterprise development by promoting “integrated geological engineering” in resources exploration and development.

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    Preliminary study on nanopores, nanofissures, and in situ accumulation of Gulong shale oil
    HE Wenyuan
    2023, 30(1): 156-173. 
    DOI: 10.13745/j.esf.sf.2022.8.32

    Abstract ( 41 )   HTML ( 11 )   PDF (15400KB) ( 46 )  

    The shale oil resource in the Qingshankou formation, Gulong Sag amounts to 15.1 billion tons, making it an important petroleum “backup” resource. Electron backscattering (HDBSD) shows that nm pores and fissures are well developed in the Gulong shale oil reservoir. The nm pores are mostly 10-50 nm in diameter (median 20-30 nm) and have irregular or polygonal shapes. They are mainly a kind of E-F nano holes, and some E-E nano pores, and connected to the nano fissures that have slit widths mostly between 10-50 nm (median 20-30 nm). The nm fissures are mainly formed in clays via F-F condensation. Clay coagulation is closely related to organic matter, especially algae. The clay colloid is negatively charged due to isocrystalline replacement, and metal cations are absorbed around it to form a positive clay group. The positively charged clays in turn adsorb the negatively charged humic acid (organic matter) and lightly degraded algae to form an organic clay flocculant. When the organic clay flocculates reach the hydrocarbon generation/release threshold, organic matter shrinks up to 87% in volume. Due to capillary resistance (~12 MPa) from nano fissures, the discharged hydrocarbon cannot migrate out, thus forming the special continuous in-situ shale oil reservoir of Gulong.

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    Source-rock reservoirs: Ring structure characteristic, differential accumulation mechanism and ordered distribution pattern
    LUO Qun, ZHAO Xianzheng, PU Xiugang, JIN Fengming, JIANG Wenya, ZHANG Hongli, QIU Zhaoxuan, WEN Pan
    2023, 30(1): 174-186. 
    DOI: 10.13745/j.esf.sf.2022.8.26

    Abstract ( 38 )   HTML ( 9 )   PDF (3288KB) ( 49 )  

    The future direction of oil and gas exploration is to find oil near or inside the source-rock area. However, this type of operation is still risky due to lack of understanding of source-rock properties, mainly its hydrocarbon accumulation conditions, mechanisms and distribution laws. This paper redefines the ‘source-rock system’ and ‘source-rock reservoir’ concepts—it considers the source-rock system is an independent hydrocarbon accumulation unit, and source-rock reservoir is hydrocarbon accumulation within the source-rock system. Hydrocarbon reservoirs are divided into two categories: conventional and unconventional reservoirs. There are six types of hydrocarbon source-rock reservoirs, including mudstone fractured reservoir, lithologic reservoir, shale oil and gas reservoir, and tight oil and gas reservoir. Under structural control, each reservoir type has its unique formation mechanism and specific distribution law and together they form a ‘horizontal parallel ring’ structure, as the source-rock system—with vertical sequence formations, upper and lower stratification transition, differential formation mechanism (conventional vs. unconventional), and ordered symbiosis—developed from deep lacustrine shale and turbidite sandstone reservoirs in the inner circle, to semi-deep lacustrine tight reservoirs in the middle circle, then to shallow lacustrine and semi-deep lacustrine transition facies in the outer circle. The source-rock system is formed by orderly, sequential accumulation mechanism, where the shale reservoirs formed in the inner ring via in situ continuing accumulation and micro migration interruption; the tight reservoirs formed in the middle ring via large-scale primary migration interruption; and the lithologic pinch-out reservoirs formed in the outer ring via buoyancy secondary migration closure. This paper determines that the shale reservoir “sweet spots” are located in the predelta facies belt of deep basin inner circle with “dominant fabric and superior hydrocarbon retention”; for tight oil and gas reservoirs the “sweet spots” are located in the “dominant pore-fracture coupling” region of delta front facies belt in the middle circle of deep basin; and for lithologic pinch-out reservoirs the favorable distribution areas are located in the trap area with dominant migration channel along convergent ridge in delta plain facies belt outside the deep basin. There are variety of source-rock reservoirs with huge oil and gas resources potentials, and their exploration, development and utilization has just begun.

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    In-situ upgrading and transformation of low-maturity shale: Economic feasibility and efficiency enhancement approaches from the perspective of energy consumption ratio
    LU Shuangfang, WANG Jun, LI Wenbiao, CAO Yixin, CHEN Fangwen, LI Jijun, XUE Haitao, WANG Min
    2023, 30(1): 187-198. 
    DOI: 10.13745/j.esf.sf.2022.8.33

    Abstract ( 23 )   HTML ( 5 )   PDF (2516KB) ( 42 )  

    There are few doubts about the technical applicability of in-situ upgrading technology in developing the enormous oil and gas resource potential of low-maturity shale. However, due to the enormous energy consumption required in shale heating, its economic feasibility is still in doubt for lack of quantitative evidence. On the basis of energy conservation law, this paper quantitatively evaluates energy gained from oil and gas generation and energy consumed by organic matter cracking, shale heating, and wallrock cooling during in-situ heating. The energy consumption ratios under different conditions are obtained and the influencing factors are analyzed. The results show that the energy consumption ratio increases rapidly with increasing TOC. For oil-prone shales, energy consumption ratio corresponding to TOC of ~4.2% is 3, indicating shale with high TOC is expected to lower cost through large-scale operation to make energy consumption ratio greater than 1, so that in-situ heating and upgrading technology can achieve economic benefits. Considering the factors influencing the energy consumption ratio, the main ways to improve economic feasibility include (1) explore technologies for efficient shale heating while reducing heat dissipation from wallrock. (2) Explore technologies for efficient organic matter transformation into oil and gas, that is, shorten in-situ heating time through catalysis. (3) Establish a comprehensive horizontal-well deployment technology that takes a comprehensive consideration of the energy consumption ratio, time cost, and engineering cost.

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    Recent advances in geophysical technology: High-resolution seismic frequency and phase analysis techniques and applications
    JIANG Renqi, WU Jian, John CASTAGNA, ZHOU Gang
    2023, 30(1): 199-212. 
    DOI: 10.13745/j.esf.sf.2022.8.36

    Abstract ( 46 )   HTML ( 4 )   PDF (4424KB) ( 85 )  

    Seismic prospecting for oil and gas exploration and development is limited by seismic data resolution. Improving the accuracy of quantitative interpretation of seismic data in thin layers, thereby identifying effective reservoirs and delineating favorable areas, is often a key factor for successful exploration and development. Historically, the limit of seismic resolution is usually assumed to be about ¼ wavelength of the dominant frequency of the data in the formation of interest. After years of continuous research and testing, this assumed resolution limit of traditional seismic model has been broken, and a series of high-resolution quantitative interpretation methods and techniques have been developed. Case studies in carbonates, clastics, and unconventional reservoirs indicate that the application of quantitative interpretation techniques such as high-resolution seismic frequency and phase attribute analysis has produced remarkable results. Band recovery using high resolution seismic processing technology can greatly improve ability to recognize geological details such as thin layers, faults, and karst caves. Multi-scale fault detection technology can effectively detect small-scale faults in addition to more readily recognized large-scale faults. Based on the traditional seismic amplitude information, high-resolution spectral decomposition and phase decomposition technology expands seismic attribute analysis to the frequency and phase dimensions, boosting the seismic geological information content and reflecting subsurface geological characteristics and hydrocarbon potential, improve the reliability of seismic interpretation, and reduce the non-uniqueness of geological models. These technologies, based on high-resolution quantitative interpretation techniques, make the identification of effective reservoirs more efficient and accurate.

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    Application of OVT-domain 5-dimensional seismic attributes in fracture prediction in the Qixia Formation of the Shuangyushi area
    ZHOU Lu, ZHOU Jianghui, DAI Ruixue, ZHANG Ya, LAN Xuemei, WU Yong, WANG Hongqiu, LIU Shimin
    2023, 30(1): 213-228. 
    DOI: 10.13745/j.esf.sf.2022.8.37

    Abstract ( 30 )   HTML ( 3 )   PDF (10641KB) ( 33 )  

    The ultra deep carbonate reservoir of the Qixia Formation, Shuangyushi area, northwestern Sichuan features strong lateral heterogeneity, complex structure, many drilling wells and great disparity in downhole natural gas production. Using OVT-domain 5-dimensional seismic data, fracture prediction, gas-bearing property assessment and real drilling verification analysis are carried out for this set of reservoirs. The first task is to detect fractures and karst caves in the carbonate reservoirs based on the differences in OVT-domain seismic attributes between different directions. The second task involves analyzing azimuth attribute to find the dominant azimuth with seismic response more sensitive to gas-bearing formations thus more conducive to a positive detection outcome. Both tasks achieved good results in the Qixia Formation, Shuangyushi block in fracture prediction and gas-bearing property assessment. The extent of fracture development in the Qixia Formation, Shuangyushi area is relatively high, and fractures are mainly developed in three directions and oblique to the fault strike. Fault normal seismic data highlight the azimuthal anisotropy and can better detect small faults, while in the fault parallel direction fault-induced seismic anisotropy can be minimized for better oil and gas detection. Targeted pre-stack and post-stack gas detection methods can effectively predict the distribution characteristics of gas-bearing reservoirs of the Qixia Formation in the study area. According to the prediction the natural gas enrichment areas mainly distribute in the middle and southeastern parts of the study area, in contiguous strips along the NE-SW direction.

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    Formation conditions and reservoir-forming models of the Ordovician buried hill reservoirs in the Jizhong depression
    ZHANG Ruifeng, TIAN Jianzhang, HUANG Yuanxin, TIAN Ran, REN Yi, BIAN Yingying, WANG Yuanjie, CHEN Ling, LU Shan
    2023, 30(1): 229-241. 
    DOI: 10.13745/j.esf.sf.2022.8.15-en

    Abstract ( 27 )   HTML ( 8 )   PDF (3859KB) ( 21 )  
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    Enrichment conditions and distribution characteristics of lacustrine medium-to-high maturity shale oil in China
    ZHAO Wenzhi, ZHU Rukai, LIU Wei, BIAN Congsheng, WANG Kun
    2023, 30(1): 242-259. 
    DOI: 10.13745/j.esf.sf.2022.8.31-en

    Abstract ( 19 )   HTML ( 4 )   PDF (1551KB) ( 21 )  
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    Economic feasibility and efficiency enhancement approaches for in situ upgrading of low-maturity organic-rich shale from an energy consumption ratio perspective
    LU Shuangfang, WANG Jun, LI Wenbiao, CAO Yixin, CHEN Fangwen, LI Jijun, XUE Haitao, WANG Min
    2023, 30(1): 281-295. 
    DOI: 10.13745/j.esf.sf.2022.8.33-en

    Abstract ( 22 )   HTML ( 6 )   PDF (1663KB) ( 18 )  
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