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Table of Content

    01 April 2022, Volume 43 Issue 2
    Petroleum Geology
    Advances and trends of fine-grained sedimentology
    Rukai Zhu, Mengying Li, Jingru Yang, Surong Zhang, Yi Cai, Yan Cao, Yuan Kang
    2022, 43(2):  251-264.  doi:10.11743/ogg20220201
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    Fine-grained sedimentary rocks are widely distributed in strata of different geological times and account for approximately two thirds of sedimentary rocks around the globe. Fine-grained sedimentology is a scientific discipline that studies the composition, structure, classification and genesis, sedimentary process and distribution pattern of fine-grained sedimentary rocks. Under the control of tectonic sedimentary background, provenance, climate change, hydrodynamic conditions, volcanic and submarine hydrothermal activities, the rocks vary greatly in terms of age and space, organic matter abundance and lamina structure and combinations, thus making the key issues to the discipline the determination of formation and preservation mechanisms of organic matter, the genesis of lamina structures, the sedimentary hydrodynamic conditions and the sedimentary models in different water environments. Recent major progresses have been made in fine-grained sedimentary petrology and classification, sequence stratigraphic framework setup and temporal/spatial distribution studies, palaeo-water depth and palaeo-environment reconstruction, lamina structure and combination analyses, experimental sedimentology and depositional dynamic condition restoration for shale, as well as establishment of organic matter enrichment mechanisms and depositional models. The future trends of the discipline include the standardization and classification of terminology, the establishment of chronostratigraphic framework for shale, correlation of palaeo-climate and sedimentary models, the determination of organic matter enrichment as well as micro observation and logging evaluation and prediction of macro-distribution of lamina types and combinations. The development of fine-grained sedimentology is of important guiding significance for the distribution prediction of organic-rich shale and the evaluation of shale oil and gas sweet spots/areas, thus further promote the conventional and unconventional oil and gas exploration.

    Deformation characteristics and distribution of Tan-Lu fault zone in Liaodong Bay Depression
    Chengmin Niu, Haifeng Yang, Tao Guo, Wei Li, Zhiping Wu
    2022, 43(2):  265-276.  doi:10.11743/ogg20220202
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    Based on the recent 3D seismic interpretation of the Liaodong Bay Depression, we systematically identify the main strike-slip faults, and further discuss the branches and distribution pattern of the Tan-Lu fault zone in the depression in combination with previous research achievements. The results show that there are three (group) branch faults in the Tan-Lu fault zone: the east branch is located in Liaodong area, labeled as No. 1 Liaodong fault; the middle branch is developed in Liaozhong area, including the Cenozoic branches of No.1 and No.2 Liaozhong faults, which have got merged into one major strike-slip fault in the deep layer; the west branch is located in the western slope of Liaoxi area, the Cenozoic Liaoxi strike-slip fault. The three (group) branch faults root in the deep basement and even the Mohorovicic discontinuity, as obviously manifested on the gravity anomaly. There are significant differences in the structural deformation characteristics of the three branch faults in the Liaodong Bay Depression, and they are also significantly different from the Bozhong-Bodong section and Bonan section of the Bohai Sea, mainly resulting from the extent of strike-slip, reactivity of previous faults, thickness and lithology of strata and the property of basement.

    Development potential of deep coalbed methane: A case study in the Daniudi gas field, Ordos Basin
    Faqi He, Zhaoxiong Dong
    2022, 43(2):  277-285.  doi:10.11743/ogg20220203
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    Guided by coal-derived gas theories, a batch of coalbed methane fields with external sources have been discovered in China. However, “internal-sourced” coalbed methane fields are barely found in the country despite many studies indicating great potential of internal-sourced coalbed methane from deep layers (more than 2 000 m deep). This study carries out experiments on the core samples taken from deep Carboniferous-Permian coal-bearing strata in the Daniudi gas field, Ordos Basin, and finds that all coal measures in the field generally contain gas and sometimes free gas and that the coalbed methane abundance can be as high as 3.86 bcm/km2. Combined with stratigraphic burial history and thermal evolution, it is suggested that these layered heterogeneous strata like mudstone and limestone contain mostly small-sized pore throats and thus form the lithological traps dominated by sandstone and coal rocks due to capillary resistance sealing. The coalbed lithological traps control the accumulation of free gas within. Low water saturation values (6.5 %-30 %) of the cores may explain the phenomenon that the depressurization via dewatering in conventional coalbed methane extraction often ends with depressurization via de-gassing, which is conducive to the producing of deep coalbed methane. The primary-cataclastic coal structure is also a favorable feature for fracturing stimulation. The study concludes that deep coalbed methane can be commercially extracted with suitable techniques and thus is worthy of attention.

    Progress in shale reservoir upgrading through in-situ heating
    Guohui Chen, Shu Jiang, Chun Li, Sisi Li, Peng Peng, Lan Mo, Yuying Zhang, Luchuan Zhang, Tianyu Zhang
    2022, 43(2):  286-296.  doi:10.11743/ogg20220204
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    Non-marine oil shale is abundant in China but most of it is not economically accessible due to its low maturity, low API gravity, high viscosity, low-pressure formation fluid as well as low porosity and permeability. In-situ heating technology has been used to improve the recovery rate of the oil and proven quite effective in improving the shale reservoir properties. Based on previous studies on shale pore evolution and reservoir property improvement through in-situ heating of oil shale, this study clarifies that the improvement of shale reservoir properties through heating is realized by both the pore evolution and the thermal cracking. Pore evolution is mainly facilitated by organic matter cracking, inorganic mineral transformation, mineral dissolution and recrystallization. Thermal cracking is mainly induced by thermal stress and hydrocarbon generation supercharging. Despite the insights gained from previous studies on pore evolution pattern of shale of different maturity and on thermal cracking in sandstone and granite, challenge remains in further revealing the effect of reservoir improvement in the in-situ heating process of shale because of the complex mineral composition in shale that leads to a more complex pore evolution process, which is further complicated by pore formation, acid product dissolution and supercharging effect associated with organic matter decomposition. It is therefore suggested that further research be carried out separately on each different effect to reveal the mechanisms and establish models, so as to effectively predict the temporal and spatial evolution of the entire pore and permeability enhancement effect during the in-situ heating process.

    Control effect of paleolacustrine water conditions on mixed lithofacies assemblages:A case study of the Palaeogene Es3, Dongying Sag, Bohai Bay Basin
    Huiming Liu, Huaiyu Yang, Pengfei Zhang, Tongxin Han, Xinjin Liu
    2022, 43(2):  297-306.  doi:10.11743/ogg20220205
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    Turbidite reservoirs in the 3rd member of the Palaeogene Shahejie Formation (hereinafter referred to as Es3) act as an important contributor for the increase of reserves in the Dongying Sag, Bohai Bay Basin. However, their marked richness in lime in the water body during deposition, has severely affected the precision of geophysical prediction and reservoir quality. Based on the genetic link of sedimentary water conditions and mixed lithofacies assemblages rich in lime, we classify the lithofacies assemblages and depict related water conditions, as well as discuss the control mechanism of water conditions on mixed lithofacies assemblages, following the restoration of paleolacustrine water conditions (e.g. salinity, depth) via multiple means. An integration of drilling data, seismic data and core analyses such as paleontology, trace elements, clay minerals, carbon and oxygen isotopes, is applied in the study. The results show that the lithofacies assemblages of the Es3 in the study area can be divided into 7 types, 5 of which are mixed lithofacies assemblages containing lime. Paleoclimate, paleo-provenance and deep-sourced fluids serve to jointly control the paleowater conditions of continental lacustrine basins, while the carbonate content is under the control of water salinity therein. The lime-rich areas are mainly located in the Tuo-Sheng fault zone, northern Caoqiao slope, central anticlinal and slope zones, while the lime-lean areas mainly in freshwater input regions and deep depression zones. Therefore, key turbidite exploration is supposed to be directed towards freshwater-affected areas adjacent to prodelta and freshwater-diluted areas in central depression.

    Lamina characteristics and their influence on reservoir property of lacustrine organic-rich shale in the Dongying Sag, Bohai Bay Basin
    Yang Chen, Qinhong Hu, Jianhua Zhao, Mianmo Meng, Na Yin, Xiaobei Zhang, Gefei Xu, Huimin Liu
    2022, 43(2):  307-324.  doi:10.11743/ogg20220206
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    The shale of the upper 4th member of Shahejie Formation in the Dongying Sag, Bohai Bay Basin (Es4 U) is studied to clarify the characteristics of shale laminae and their influence on reservoir properties through organic matter analysis, thin section observation, scanning electron microscopy (SEM) observation, high pressure mercury intrusion experiment and low-temperature nitrogen adsorption (LTNA). The results will be of a theoretical basis to shale oil exploration and development. This shale interval (Es4 U) can be divided into four lamina units different in composition and pore feature, and consequently seven types of lamina assemblages are concluded. The assemblage of granular calcite lamina, mixed lamina and organic lamina is of the best reservoir capacity in this interval with well-developed nano- and micro- pores. Its nano-pores are mainly composed of clay-mineral intergranular pores in the mixed lamina and constricted fissures of organic matters in the granular calcite lamina; while the micro-pores are mainly composed of micro-cracks in the mixed lamina and at the boundary of the granular calcite lamina. The assemblage of micrite calcite lamina and organic lamina ranks second in terms of reservoir property. Its nano-pores are dominated by rigid mineral intergranular pores in the micrite calcite lamina, and the micro-pores by the micro-cracks formed on inter-lamina weak surfaces. Under the effect of burial compaction, clay mineral transformation, calcite recrystallization and organic acid dissolution, the lamina assemblages are different in reservoir properties.

    Evolution and genesis of organic pores in Triassic Xujiahe Formation shale, Western Sichuan Depression, Sichuan Basin
    Liang Xu, Wei Yang, Zhenxue Jiang, Dongxia Chen, Yaohua Wang, Jiankang Lu, Mingzhu Zhao, Lan Li
    2022, 43(2):  325-340.  doi:10.11743/ogg20220207
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    Samples from non-marine organic rich shale in the Xujiahe Formation, Western Sichuan Depression, Sichuan Basin, were analyzed with field emission scanning electron microscope and EDS energy spectrum. Three maceral components, including solid bitumen, vitrinite and intertinite, as well as microscopic subcomponents such as oil bitumen, tar bitumen, structured vitrinite, unstructured vitrinite, detrital-vitrinite, fusinite, semi-fusinite, detrital-intertinite, and sclerotinite were identified in these organic-rich shale samples. The organic pores of different types vary greatly under scanning electron microscopy: the organic pores develop the best in solid bitumen in organoclay complex, well in individual solid bitumen flakes, and the worst in vitrinite and intertinite mainly with residual primary pores. Controlled by difference in hydrocarbon generation potential, the three macerals have their weight percentages of carbon increasing and hydrocarbon generation and pore-forming potential deteriorating successively from solid bitumen, vitrinite to intertinite. Higher content of illite means higher specific catalytic activity, which promotes the development of pores in solid bitumen near the illite. Thermal simulation experiments show that the pore evolution of different macerals varies greatly. With maturity increasing, solid bitumen responds differently from other two maceral groups, it develops a large number of honeycomb pores with increasing cross-sectional porosity, which later gradually decreases due to the exhaustion of hydrocarbon generation. Meanwhile vitrinite and intertinite develop less pores and have their primary pores decreasing rapidly. However, the widely developed microcracks in the latter two groups connect bituminous pores and other inorganic pores, forming a microscopic pore-fracture network and improving the storage and seepage capacity of shale reservoirs.

    Reinterpretation of gas sources in the Middle Triassic Leikoupo Formation in Western Sichuan gas field, Sichuan Basin
    Chengpeng Su, Ying He, Xiaobo Song, Bo Dong, Xiaoqi Wu
    2022, 43(2):  341-352.  doi:10.11743/ogg20220208
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    The Western Sichuan gas field with proven geologic reserves of over 1 000 × 108 m3 of natural gas ranks third in scale in the Sichuan Basin following Puguang and Yuanba gas fields. Recently, there have been some controversies regarding to the sources of gas in the Leikoupo reservoir, rendering the necessity to make further analysis. The natural gas in the upper and lower pay zones in the 3rd submember of the 4th member of Leikoupo Formation (hereinafter referred to as Lei 43) in different structures is analyzed for natural gas components and alkane isotopic compositions, which indicate that the lower and upper gas reservoirs in the Lei 43 shale the same gas source. While the gas sources vary with structural belts. Natural gas from Shiyang and Yazihe structural belts, is characterized by high content of H2S and CO2, low δ13C2 (mean: -32.5 ‰), and carbon isotope reversal of alkane, typical of oil-derived gas. However, in the Jinma Structure, the natural gas features relatively low content of H2S and CO2, high δ13C2 (mean: -27.5 ‰), and normal carbon isotope sequence pattern, showing the characteristics of mixed gas. In addition, based on the analysis of gas-migration conditions and previous studies, we consider that the natural gas in the Shiyangchang and Yazihe structural belts is mainly derived from the Leikoupo source rocks, and that in the Jinma structural belt mainly derived from the source rocks of Leikoupo and the Permian Longtan Formations, though without excluding minor contributions by the Ma'antang-Xiaotangzi source rocks.

    Reservoir characteristics and evolution mechanisms of the Upper Ordovician Wufeng-Lower Silurian Longmaxi shale, Sichuan Basin
    Ruyue Wang, Zongquan Hu, Shengxiang Long, Wei Du, Jing Wu, Zhonghu Wu, Haikuan Nie, Pengwei Wang, Chuanxiang Sun, Jianhua Zhao
    2022, 43(2):  353-364.  doi:10.11743/ogg20220209
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    The characterization of the Upper Ordovician Wufeng - Lower Silurian Longmaxi shale in terms of mineralogy, organic geochemistry, storage space, physical properties and gas-bearing potential, is applied to discuss the evolutionary mechanisms and their influence on shale gas exploration and development, proposing a shale reservoir evolution mechanism of “framework development as controlled by biogenetic silica-calcite, pore generation as promoted by co-evolution, and pore preservation as controlled by pressure evolution”. The results show that: (1) lithofacies along with the type and occurrence of organic matters (organic pore carrier) plays an important role in controlling the development of organic pores. The intergranular pores (with a size of greater than 3-5 μm) filled with organic matters in the siliceous shales are highly developed, the most conducive to the development of organic macropores. The development of organic pores in argillaceous shales is affected by the TOC content and the structure of organo-clay complexes, and the organic pores are characterized by a wide range of distribution, large pore size on average, and small total number.(2) The rigid framework composed of bio-quartz, microbial dolomite and pyrite formed in the early of contemporaneous-early diagenetic stage contributes to the preservation of original pores. In the early stage of the middle diagenesis, the production and consumption of organic acids, unstable mineral dissolution, clay mineral transformation and oil generation from kerogens are synchronized, serving to provide spaces favorable for the charging and retention of liquid hydrocarbons during the oil generation period. From the late mesogenetic stage to the late diagenetic stage, gas generation, organic pore formation and pressure increase from cracking of kerogen and retained hydrocarbon jointly promote the development of organic pores and micro-fractures.(3) The compaction mitigation by overpressure allows the maintenance of organic pore morphology and shale physical properties. Its influence on highly stress-sensitive argillaceous shale at upper interval is obvious, while on the other hand, unapparent on highly brittle shale intervals at the bottom. With the tectonic modification and pressure relief intensity getting enhanced from the basinal center to edges, the physical properties of siliceous shale at the bottom largely remain the same, while the physical properties of the upper argillaceous shale become poor resulting in an enhanced sealing capacity.

    Sedimentary characteristics and model of platform-trough shale in the Lower Carboniferous, Guizhong Depression, Dianqiangui Basin
    Jinyu Tao, Zongquan Hu, Baojian Shen, Anyang Pan, Chuxiong Li, Ruihu Wang, Meiling Zhang
    2022, 43(2):  365-377.  doi:10.11743/ogg20220210
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    The sedimentary characteristics of shale in the Lower Carboniferous Luzhai Formation, Guizhong Depression are discussed based on observation and description of cores from typical wells, outcrop observation and laboratory tests. A sedimentary model of the platform-trough facies is thereby established, and the potential zones favorable for shale gas exploration are proposed. In comparison, the differences between shales of shelf facies (as represented by the Wufeng-Longmaxi shale, Sichuan Basin) and platform-trough facies (as represented by shale in the study area) are clarified. In addition to facies markers of lamellation, deformation, and grain sequence typical of the bedding structure, large amount of pyrites are widely enriched in the trough. A calcareous and a siliceous turbidites, paleontological fragments like brachiopods together with some plant fragments, could be identified in the slope between platform and trough. According to the lithologic combination, facies markers and other hydrodynamic conditions, platform-trough facies could be subdivided into 4 subfacies and 13 types of lithofacies. Among others, the siliceous lithofacies and siliceous shale lithofacies in the platform-trough facies, together with the argillaceous siliceous lithofacies and carbonaceous mudstone lithofacies in trough slope subfacies, could be of the potential facies belts for shale gas exploration. The siliceous rocks with some lime and shale contents can be of potential targets for shale gas exploration.

    Characteristics of horizontal and vertical permeability of continental shale oil reservoirs in China: A case from Jiyang Depression in Bohai Bay Basin and Qianjiang Sag in Jianghan Baisn
    Yunqi Shen, Zhijun Jin, Jianzheng Su, Zhiming Li, Jun Niu
    2022, 43(2):  378-389.  doi:10.11743/ogg20220211
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    At present, commercial development of shale oil has been achieved in North America, and a major breakthrough has been made in the exploration and development of shale oil in China. In practice, it has been found that segments with horizontal beddings contribute more in petroleum production compared with those without beddings, probably indicating a larger horizontal permeability compared with the vertical permeability. The study focuses on quantitative analysis of horizontal and vertical permeability variation pattern in typical continental shale oil reservoirs in China by using pressure-pulse decay method. The results indicate that: (1) under atmospheric pressure, the horizontal permeability is 5 orders of magnitude higher than the vertical permeability in shale oil reservoirs with bedding?parallel fractures, while it is about 20 to 50 times higher than the vertical permeability in those without bedding-parallel fractures. When vertical structural micro-fractures cutting through bedding exist, the ratio of horizontal permeability to vertical permeability is less than 1. (2) Under confining pressure, the variation of horizontal and vertical permeability in continental shale oil reservoirs is more complicated in comparison with marine shale reservoirs, and the horizontal permeability is the more sensitive to the confining pressure. (3) The ratios of horizontal permeability to vertical permeability in laminated calcareous shale/laminated arenaceous shale facies, lamellar shale facies, and massive shale facies, vary from 1.5 to 90.0, from 1.4 to 46.0 and from 1.3 to 40.0, respectively. (4) In comparison, the permeability of shale oil reservoirs in Qianjiang Sag in Jianghan oilfield is 10-100 times lower than that in Jiyang Depression in Shengli oilfield due to smaller pore throat, complex mineral composition and the existence of pore-filling anhydrite.

    Influence of the Cretaceous fine-grained volcanic materials on shale oil/gas, Luanping Basin
    Xiaoning Liu, Zaixing Jiang, Xiaodong Yuan, Chen Chen, Cheng Wang
    2022, 43(2):  390-406.  doi:10.11743/ogg20220212
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    Industrial shale oil/gas flow was successfully tested in Well Luanye 1 after fracturing, and the Cretaceous Xiguayuan Formation in the Luanping Basin has currently been a key zone of hydrocarbon exploration and research in this area. Recent petrological studies on Well Luanye 1 show that fine-grained volcanic materials have been discovered in the source rocks and are closely related to the generation and accumulation of shale oil/gas. It has become a new proposition to explore the relationship between fine-grained volcanic materials and shale oil/gas. The lower member of Xiguayuan Formation in the Luanping Basin is the target of the study with a combination of analyses and laboratory data such as cores, microscopic thin sections observation, logging, X-ray diffraction and geochemical test. The sedimentary characteristics and organic matter development of the fine-grained tuffaceous sedimentary rocks are discussed to provide theoretical support for further exploration and development of shale oil/gas in the area. On the other hand, the development of lower member of Xiguayuan Formation is controlled by volcano-lacustrine interaction, with lacustrine shale, fine-grained tuffaceous sedimentary rocks under the effect of volcanic activities, as well as andesite and pyroclastic rocks of volcanic facies well developed within. The results show that in the shale, the average TOC content is 0.64 %, and the average S1+S2 content is 0.41 mg/g; in the fine-grained tuffaceous sedimentary rocks, the average TOC content is 1.92 %, and the average S1+S2 content is 2.64 mg/g, indicating that the abundance of organic matters is markedly greater in the fine-grained tuffaceous sedimentary rocks. Pyrite framboids are widely developed in the intervals where fine-grained volcanic materials are enriched; meanwhile, Pr/Ph is smaller and V/(V+Ni) is larger, also indicating a strong reduction of water environment after volcanic eruption. The rapid deposition and burial of fine-grained volcanic materials is conducive to the preservation of organic matters. Lacustrine carbonate rocks are well developed and interbedded with laminated organic-rich mudstone and tuff. The interlayer fractures are conducive to hydrocarbon migration, while the tuff and dolomite laminae provide reservoir spaces. The fine-grained volcanic materials play a positive role in the complete process from the enrichment and hydrocarbon generation of organic matters to the storage and preservation of oil/gas.

    Geochemical characteristics of bitumen and tracing of gas source in the Middle Triassic Leikoupo Formation, Western Sichuan Depression
    Xiaoqi Wu, Yingbin Chen, Changbo Zhai, Lingfang Zhou, Xiaojin Zhou, Jun Yang, Yanqing Wang, Xiaobo Song
    2022, 43(2):  407-418.  doi:10.11743/ogg20220213
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    The Middle Triassic Leikoupo Formation (T2 l) is one of the exploration targets for natural gas in western Sichuan Basin. However, the development and geochemical characteristics of bitumen in the formation is barely studied and understood. Analyses are therefore carried out on the content, reflectance, carbon isotopes, and biomarkers of bitumen samples from the formation to determine the genesis and source, and a gas source tracing is also performed based on geochemical characteristics of the bitumen, thus shedding some light on finding new ways of gas-source correlation in the area. The results indicate that the bitumen distributes sporadically along sutures and fissures, with δ13C values ranging from -28.6 ‰ to -24.6 ‰. The bitumen from core samples is of typical thermogenic origin as suggested by an average equivalent vitrinite reflectance of 3.14 %. The bitumen from outcrops has low equivalent vitrinite reflectance values, suggesting diversified origins: samples from the Huanglianqiao outcrop, Xiangshui Town, and the Ma’antang outcrop, Shiyuan Town, are the result of biodegradation, whereas those from the Xiejunmen outcrop, Tianchi Town, are derived from de-asphalting of crude oil. It also indicates a close affinity between the bitumen and carbonate rocks in the Leikoupo Formation. However, the oil cracking intensity in the formation of the area and the total gas amount generated through cracking in the whole trap calculated according to bitumen contents do not match the scale of current large gas pools. It is suggested that the pools are sourced from direct charging of gas from cracking of oil in a high maturity stage rather than the in-situ cracking of paleo-oil, and that most of the gas is from the underlying Upper Permian source rocks, rather than from the carbonates in the Leikoupo Formation. These understandings may help revealing the accumulation mechanisms and specifying exploration direction in the Leikoupo Formation of western Sichuan Basin.

    Geochemical characteristics and hydrocarbon generation history of Mesozoic-Cenozoic lacustrine source rocks in the South Yellow Sea Basin,offshore eastern China
    Zhiqiang Li, Bo Yang, Jun Wang, Zijun Han, Qingxun Wu
    2022, 43(2):  419-431.  doi:10.11743/ogg20220214
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    No commercial oil or gas discovery has ever been made in the South Yellow Sea Basin (SYSB), offshore eastern China so far. This is probably due to the lack of study on the development and hydrocarbon generation history of the three sets of Mesozoic-Cenozoic lacustrine source rocks in the basin. Rock pyrolysis, trace elements, X-ray diffraction and chromatography-mass spectrometry analyses on 41 samples collected from the source rocks show that the second member of the Taizhou Formation (K2 t2) (one of the three sets) is rich in sapropel of Type Ⅱ1 kerogen, mainly the result of aquatic organic matter (OM) input as indicated by a dominant C27 regular sterane. The member has high content of carbonate minerals, suggesting a transgression influence that caused an anoxic and saline paleoenvironment, favorable for OM preservation. The second member of Funing Formation (E1 f2) shares the paleoenvironment with K2 t2 but has dramatically higher content of C28 regular sterane and a dual OM input of dominant algae with a small amount of terrestrial material, suggesting typical lacustrine source rocks. The fourth member of Funing Formation (E1 f4) contains mostly C29 regular sterane and has high terrestrial /aquatic ratio (TAR), indicating terrestrial OM input and oxic ancient water body with low salinity that is not favorable for OM preservation. The complex OM input mix and paleoenvironment conditions make it difficult to evaluate the thermal maturity of the source rocks. It is recommended to treat with caution the “immature” characteristic expressed by the ratios such as sterane 20S/(20S+20R) or Ts/(Ts+Tm). The hydrocarbon generation process of potential source beds is established base on analysis of structural background transformation from fault-depression period to depression period. It suggests that volcanic activities facilitated the generation process and that the depressions in the north experienced an early hydrocarbon generation and those in the south went through both early and late hydrocarbon generation but with a major generation period occurring during the deposition of the Sanduo Formation. The study may serve to guide the next step of oil and gas exploration in the basin..

    Geochemical characteristics of crude oil from coal measure source rocks and fine oil-source correlation in the Pinghu Formation in Pingbei slope belt, Xihu Sag, East China Sea Shelf Basin
    Tianjun Li, Zhilong Huang, Xiaobo Guo, Jing Zhao, Yiming Jiang, Sizhe Tan
    2022, 43(2):  432-444.  doi:10.11743/ogg20220215
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    The properties and distribution of crude oil from the coal measure source rocks are complex in Pingbei slope belt of Xihu Sag, East China Sea Shelf Basin. In addition, the sedimentary environment and organic matter origin of coal measure source rocks are highly variable in both vertical and horizontal views. The oil reservoirs in this area are characterized by hydrocarbon supply from multiple sources including western subsag and the slope zone. A combination of geological and geochemical analyses is applied to study crude oil types and fine oil-source correlation. Finally, the relationship between crude oil distribution and hydrocarbon-generating subsag is clarified. The results show that the crude oil in Pingbei area is mainly condensate oil, accompanied by a small amount of light and normal crude oil, and it can be grouped into three types (i.e. TypeⅠ,Ⅱ1 andⅡ2) based on biomarker compound features. TypeⅠcrude oil mainly sourced from coniferales resinites of gymnosperms of terrestrial higher plants, is mainly distributed in Kongqueting area, and is obviously related to the source rocks in the lower member and lower submember of middle member of Pinghu Formation in the western subsag. TypeⅡ1 crude oil mainly occurs in the upper member and upper submember of middle member of Pinghu Formation in Wuyunting area and the NB8 subsag. It is speculated that both terrestrial gymnosperms and ferns, together with high-maturity marine organic matters from lower slope belt and western sabsag, have made contributions to the generation of TypeⅡ1 crude oil, with the contribution of ferns even greater, and this type of crude oil is obviously related to the source rocks in the upper member and upper submember of middle member of Pinghu Formation. In addition, TypeⅡ2 crude oil is mainly distributed in Wuyunting area and Baoyunting-Tuanjieting area, with greater contributions made by coniferales resinites of terrestrial gymnosperms; and it is obviously related to the source rocks in the lower member and lower submember of middle member of Pinghu Formation. The research results are of guiding value to the new understanding of oil source, reservoir prediction and hydrocarbon exploration in Pingbei slope belt.

    Methods and Technologies
    Seismic prediction of fractures and vugs in deep-water sub-salt lacustrine carbonates:Taking F oilfield in Santos Basin, Brazil as an example
    Wensong Huang, Fang Xu, Chengbin Liu, Jixin Huang, Junfeng Zhao, Songwei Guo, Yunbo Li
    2022, 43(2):  445-455.  doi:10.11743/ogg20220216
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    The F oilfield, a deep-water sub-salt oilfield in the Santos Basin of Brazil, is characterized by deep burial, diverse lithology and strong heterogeneity. Unlike commonly-seen porous reservoirs and some large karst cave reservoir clusters in Western China, the reservoirs in the study area are deep-water lacustrine carbonate rocks with small vug size and various fracture types, resulting in extremely unclear seismic responses and wrong interpretations with conventional prediction methods for fractured-vuggy reservoirs. In addition, the massive salt layers overlaying the reservoirs also reduce the resolution of seismic data of well fractured and vuggy parts and pose challenges to the understanding of their distribution pattern through post stack seismic method. To deal with the problem, this study develops a workflow suitable for the seismic prediction of small fractured-vuggy carbonate reservoirs of diversified lithology against a deep-water sub-salt setting based on the development characteristics of fractures and vugs in the study area and a combination of well and seismic data analyses to eliminate the lithologic interference of mudstone and igneous rocks. It features in the optimization of seismic data and a quick responsiveness to geological information conveyed through seismic anomalies by making full use of core, FMI well-logging and production leakage information. Application to the prediction of dissolution pores and vugs has been proven positive as the method is capable of defining the sensitive seismic attributes of fractures and vugs to predict fractures by combining structural stress field and to predict vugs by combining seismic inversion results such as wave impedance, porosity and CGR. The study is of guiding significance to the prediction of lacustrine carbonate reservoirs with fractures and vugs.

    Raman spectroscopy of bitumen from the Sinian Dengying Formation reservoirs, Gaoshiti-Moxi area, central Sichuan Basin
    Chunquan Li, Honghan Chen, Xuewei Xiao, Zecheng Wang, Hua Jiang
    2022, 43(2):  456-466.  doi:10.11743/ogg20220217
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    As an effective technique to characterize the maturity of organic matter, Raman spectroscopy was employed to analyze bitumen in 21 samples from the Sinian Dengying Formation reservoirs in the Gaoshiti-Moxi area, central Sichuan Basin. The equivalent vitrinite reflectance values of the bitumen were calculated with empirical equations to determine the maturity stage of the bitumen and the thermal alternation of reservoirs sampled. The results indicate that most bitumen (81.1 %) filled dissolved pores and fractures are highly to over mature. The Raman spectrums change drastically near the equivalent vitrinite reflectance of 3.0 %, suggesting a two-section evolving trend. Anomalous high equivalent vitrinite reflectance values are typical responses of hot fluid activities, indicating thermal anomalies experienced by the reservoirs. The spatial distribution of the equivalent vitrinite reflectance values also points to the possibility of multiple thermal alteration, which may serve to confirm the assumption that gas is generated from oil cracking and to explain the distribution of current gas reservoirs in the area.

    Reservoir pressure prediction for marine organic-rich shale: A case study of the Upper Ordovician Wufeng-Lower Silurian Longmaxi shale in Fuling shale gas field, NE Sichuan Basin
    Pengwei Wang, Xiao Chen, Zhongbao Liu, Wei Du, Donghui Li, Wujun Jin, Ruyue Wang
    2022, 43(2):  467-476.  doi:10.11743/ogg20220218
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    The pressure prediction for shale reservoirs is essential for evaluating shale reservoir quality, understanding shale gas accumulation mechanism, identifying gas-bearing property and seeking favorable shale gas exploration areas. High-to-over-mature marine shale reservoirs rich in organic matters are commonly characterized by considerable nano-scale organic pores and abnormally high pressure caused by gas generation. At present, shale reservoir pressure prediction is practically based on the development and evolution of inorganic pores while ignoring the nano-scale organic pore growth, resulting in an illusion of “pressure anomaly” or inaccurate prediction of reservoir pressure. This study proposes a reservoir pressure prediction method for marine organic-rich shale. With the assumption that organic pores coexist with abnormal pressure in shale reservoirs, this method serves to calculate the acoustic logging responses of organic pores, correct the acoustic logging curve, and establish a new pressure prediction model. The new method is verified with Wufeng-Longmaxi shale reservoir in Fuling gas field as an example. Compared with the Eaton method, an original calculation method, the results of the revised method match well with the measured data (R2=0.81). Also, the prediction error of the revised method is relatively small and in normal distribution, indicating that this method is suitable for the pressure prediction in the marine shale reservoir. The predicted pressure in Phase I Production Zone in Jiaoshiba area is generally characterized by high value in the center, low value in the surroundings, and locally high value in the southeast. These are well correlated with open flow capacity, further indicating the reliability of the method. It is of great practical significance to improving the unconventional shale reservoir pressure prediction and promoting the exploration in overpressured shale gas.

    Review on the application of nanoindentation to study of shale mechanical property
    Jianfeng Wang, Chao Yang, Yuke Liu, Yongqiang Xiong
    2022, 43(2):  477-488.  doi:10.11743/ogg20220219
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    With the development of mechanical testing technology in recent years, the nanoindentation, a technique wildly recognized in micro-fabrication materials research, has been introduced into shale study, and become an important tool to test the micro-mechanical properties of shale. At present, it has been one of the hot research topics to study the mechanical properties of shale from the microscopic perspective. In this study, we summarize the influences of shale sample preparation methods and nanoindentation test system on test results, elaborate the application of nanoindentation technique in micro-mechanical and creep rupture characterization of shale. The advantages and existing problems of this technique are thereby analyzed and discussed, while looking to its development trend. The results are shown as follows. First, the technique can serve for precisely characterizing the mechanical properties of the bulk shale and its matrix phases. Second, the micro-creep rupture behavior of shale can be obtained by studying the displacement-time curve during the loading stage, which is of significance to having an in-depth understanding on the creep rupture deformation mechanism of shale in micro view. Third, testing the evolution characteristics of micro mechanical properties of fluid/shale interactions can provide basic experimental data for actual hydraulic fracturing or supercritical carbon dioxide fracturing of shale. In all, the nanoindentation technique serves to get an even finer understanding of shale in micro level, which is conducive to analyzing the mechanical behavior of shale in essence, and provide theoretical basis more reliable for the exploration and development of shale gas.