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Table of Content

    28 August 2020, Volume 41 Issue 4
    Previous Issue    Next Issue
    Petroleum Geology
    Mechanisms and exploitation of deep marine petroleum accumulations in China: Advances, technological bottlenecks and basic scientific problems
    Yongsheng Ma, Maowen Li, Xunyu Cai, Xuhui Xu, Dongfeng Hu, Shouli Qu, Gensheng Li, Dengfa He, Xianming Xiao, Yijin Zeng, Ying Rao
    2020, 41(4):  655-672, 683.  doi:10.11743/ogg20200401
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    Deep marine petroleum reservoirs draw great attention from the oil and gas industry around the world.Despite exploration breakthroughs made in deep and ultra-deep marine carbonate strata in Sichuan and Tarim Basins, Chinese oil companies are still facing many scientific and technologic challenges.This manuscript provides a review of research advances, technological bottlenecks and basic scientific problems relating to the deep marine petroleum accumulations and their exploitation in China; and, to meet the requirement of the national resource strategy, proposes to carry out studies based on analyses of the prototypes and transformation of the Upper Yangtze, Tarim and North China cratons.The studies include integrated multidisciplinary programs on petroleum accumulation and flow mechanisms in deep marine carbonate reservoirs and deep shale gas reservoirs, geophysical theories and methods for imaging and predicting complicated deep reservoirs, and new protocols and downhole tools for deep well drilling and completion.A strengthened analysis of the total factor and life cycle of basin formation, diagenesis, reservoir formation, hydrocarbon generation and accumulation, over a geological and human time scale, is also recommended to help establishing geological models of accumulation and transportation of conventional and unconventional oil and gas in deep marine strata for more reliable petroleum resource assessment and exploration prospect prediction.Future researches are suggested to be focused on solving key geophysical problems, including the low signal-to-noise ratio, low resolution, low imaging accuracy, and low amplitude-preservation, in the exploration of deep marine reservoirs.It calls for efforts made in the imaging and prediction of complicated deep structures and reservoirs, through enhanced technologies such as broad band seismic acquisition, complex structure modelling, anisotropic pre-stack depth migration(PSDM), and quantitative assessment of complicated reservoirs.Such engineering challenges in deep marine reservoirs as how to speed up drilling and maintain wellbore integrity are also suggested to be dealt with first as most of the reservoirs are featured in multiple production zones, strong heterogeneity, and high temperature and high pressure.

    Feature of paleo-oil pools in the Sinian Dengying Formation, northeastern Sichuan Basin, and its significance to exploration
    Xusheng Guo, Dongfeng Hu, Renchun Huang, Jinbao Duan, Zhili Jiang, Xiang Zhu
    2020, 41(4):  673-683.  doi:10.11743/ogg20200402
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    The paleo-oil pools concentrate on the 4th member of Dengying Formation (Deng 4 member) in Hujiaba section in the piedmont of Micangshan Mountain at the northeastern margin of Sichuan Basin, and their abundance ever increases towards the top, with reservoir asphalt dominated by reservoir asphalt and migration asphalt.The oil-source correlation analysis shows that the hydrocarbons in the paleo-oil pools of Dengying Formation in Hujiaba section are mainly sourced from high-quality source rocks in the Qiongzhusi Formation, perhaps together with source rocks in Deng 3 member.The source rocks in the Qiongzhusi Formation, Northeastern Sichuan Basin entered the early oil-generation stage in the Late Silurian and reached the oil-generation peak at the end of Permian.At this time, the hydrocarbons migrated laterally to the reservoir of Dengying Formation, forming paleo-oil pools.During the Middle Triassic, the organic matter entered the stage of high maturity, with the crude oil in the paleo-oil pools being cracked into gas.Since the Late Cretaceous, the tectonic uplifting and denudation have destroyed these paleo-oil pools, leaving the residual paleo-oil pools at present.The discovery of the paleo-oil pools in the study area is of great significance to the hydrocarbon exploration of the Sinian Dengying Formation in Sichuan Basin.The Deng 4 member is composed of mound-shoal deposits formed at the platform margins, developing high-quality reservoirs dominated by TypesⅠandⅡ; while its overlying Qiongzhusi Formation serves as regional and direct caprocks, featuring good sealing capacity.The favorable reservoir-seal combination indicates that high exploration potential exists in the Sinian Dengying Formation, Northeastern Sichuan Basin.

    Types and characteristics of tight sandstone sweet spots in large basins of central-western China
    Xunyu Cai, Guiqiang Qiu, Dongsheng Sun, Hongquan Zhu, Wei Wang, Zhiping Zeng
    2020, 41(4):  684-695.  doi:10.11743/ogg20200403
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    The major basins of central-western China are rich in tight (sandstone) oil and gas deposits, which are the key exploration and development targets of the petroleum industry.However, the formation and distribution of these resources are complex and has put economic restrictions on their extraction.One way to solve this problem is to evaluate, screen and exploit "sweet spots" (promising reservoirs) to reduce uncertainties.Based on a characterization of tight oil and gas in clastic sequences of these basins, this study focused on classifying sweet spots by analyzing geological factors that controlled the formation and distribution of the sweet spots and by taking into consideration the applicability of prediction technologies to field data, a relative consistency between forming mechanisms and distribution patterns, and an integration of economic and technical strategies.The sweet spots were generally classified into the lithofacies-restrained, fracture-dominated, and unconformity-regulated types.Of which, the lithofacies-restrained sweet spots were controlled by diagenesis during burial and developed inside certain lithofacies; the fracture-dominated sweet spots were controlled by fracture networks and mainly formed in faults and folds with strong structural-deformation; and the unconformity-regulated sweet spots were controlled by leaching layers inside weathering crust and formed during long-term uplifting and strong weathering.The Jurassic "narrow channel" in the east slope of the West Sichuan Depression, the Xujiahe "fault-fractured complex" in Malubei area of northern Sichuan Basin, and the Carboniferous System in Chepaizi area of western Junggar Basin were selected to respectively analyze the formation, distribution and controlling factors of the three types of sweet spots.The results show that the classification by reservoir-forming geological processes and products is helpful for further studying the sweet spots and formulating more suitable geological, geophysical and engineering strategies to better evaluate, optimize and develop the sweet spots.

    Geological condition and exploration prospect of shale oil and gas in the Bohai Sea
    Yong'an Xue, Feilong Wang, Guomin Tang, Xinqi Li
    2020, 41(4):  696-709.  doi:10.11743/ogg20200404
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    The Bohai Sea is the Cenozoic subsidence center of the Bohai Bay Basin.Three sets of hydrocarbon source rocks (i.e., Ed3, Es1+2, Es3) developed in the Paleogene of the Basin indicate significant exploration potentials of conventional and unconventional oil and gas resources, especially the shale oil and gas.To further determine the shale oil and gas potential of the area, the exploration and development activities in the Bohai Sea were investigated and the comparison of the petrogeological conditions in the Basin with those in typical shale basins both in China and other countries, was combined with analyses of geochemical parameters and logging data, microscopic observation of thin sections and X-ray diffraction results of whole rock mineral of samples from the Basin, as well as basin simulation, to carry out an in-depth study.It comes out with four conclusions.(1) There are four high-quality hydrocarbon-rich sags in the Bohai Sea.Of which the Bozhong sag has the highest potential of harboring shale gas with gas-prone and highly matured source rocks.The Huanghekou and Liaozhong sags are more oil-prone with relatively low maturity.And the Qikou sag is both oil and gas prone with its low matured Ed3 and Es1+2 source rocks more likely generating oil and highly matured Es3 producing gas.(2) The three sets of source rocks have good reformability with an average brittle mineral content of more than 63.5% and an average clay mineral content of less than 34.7%.The mud shale below 3 200 m was once at the stage B of middle diagenesis, during which the montmorillonite in the montmorillonite-illite mixture was largely converted to illite, thus increasing the plasticity of the shale reservoir.Considering exploration activities in other areas with similar conditions, layers buried between 3 200 m and 4 200 m are suggested to be the targets for shale oil and gas exploration and development in the Bohai Sea.(3) The Bozhong sag has the greatest potential of shale oil and gas resources with an estimated resource of up to 95.8 billion tons, accounting for 45.2% of shale oil and gas resources in the Bohai Sea.The major source rocks in the sag are under overpressure as a whole.Given the economic cost of shale gas and oil exploration and development in the Bohai Sea, the Ed3 and Es1+2 source rocks of the Bozhong sag that are characterized by moderate burial depth and high maturity, are the most promising shale gas exploration targets in the Bohai Sea.(4) The west slope (at the edge) of the Bozhong sag is considered to be the most promising part in the area for current exploration stage as the three sets of source rocks with an average TOC value of more than 3.04% in the slope are under abnormally high pressure and have relatively shallow burial depth.Furthermore, gas logging shows a total gas content as high as 99% and X-ray diffraction results indicate that the clay mineral content in the shale reservoirs is the lowest compared with that of other areas in the Bohai Sea, making them ideal candidates for future fracturing.

    Identification and description of fault-fracture bodies in tight and low permeability reservoirs in transitional zone at the south margin of Ordos Basin
    Faqi He, Chengchun Liang, Cheng Lu, Chunyan Yuan, Xiaowei Li
    2020, 41(4):  710-718.  doi:10.11743/ogg20200405
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    The transitional zone at the south margin of Ordos Basin is structurally complicated and unique in terms of reservoiring conditions with its well-developed faults and fractures serving as better channels for hydrocarbon seepage and providing more space for hydrocarbon accumulation in tight and low permeability layers.Based on the determination of regional structural stress and fault-fracture evolving characteristics in the zone, technologies for identifying and characterizing these faults and fractures are developed through a combination of core, drilling and logging data with 3D seismic analysis of fine coherence, amplitude change, curvature attributes and etc.Considering the fact that fractured reservoirs are widely distributed in the zone, the study proposes the concept of fault-fracture bodies to describe the reservoirs.The fault-fracture bodies are defined as composite reservoirs composed of faults, associated brittle fractured belts and low-permeability sandstone layers modified by the belts.They are usually capped or flanked by impermeable argillaceous rocks or tight layers.The discovery of several oil reservoirs in the bodies of the zone has already verified their exploration value.The identification and description of the bodies indicates that the distribution of large-to-medium-scale fracture assemblages determine the skeletal morphology of the bodies, and the superimposition of small-to-micro-scale fracture and sand body boundaries determines the ranges of the bodies.Meanwhile, a set of identification and description procedures is formed to characterize the geology of the bodies.Practices have proven that the procedures are of guiding value to the exploration and development of low abundance and low permeability reservoirs in the zone.

    Formation and accumulation mechanism of shale oil in the 7th member of Yanchang Formation, Ordos Basin
    Shixiang Li, Xiaobing Niu, Guangdi Liu, Jihong Li, Mingliang Sun, Fuliang You, Haonan He
    2020, 41(4):  719-729.  doi:10.11743/ogg20200406
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    Shale oil is a kind of important unconventional hydrocarbon resource in urgent need of exploitation.A systematic analysis of the forming conditions and accumulation mechanisms of shale oil in the 7th member of the Yanchang Formation (Chang 7 member) in Ordos Basin was carried out by using petro-chemistry and source rock geochemistry methods.The results show that the oil shale deposited in the member is extremely rich in organic matter with an average residual organic carbon abundance of 7.4%, and contains well-developed micro- and nano-pores for shale oil accumulation.Three main mechanisms of shale oil accumulation in the member are brought forward in this study:(1) both high organic matter productivity and strong reducing water environment served to effectively preserve organic matters in the super eutrophic lake basin during deposition, and organic shale of high abundance developed thereby provided the material basis for shale oil generation; (2) organic-rich shale characterized by strong oil-generating capacity, high conversion efficiency, and high intensity of oil expulsion in the member provided favorable conditions for the generation of shale oil in large scale; and (3) the dual occurrence mode of shale oil, i.e., organic matter adsorption and pore retention, provided various ways for the enrichment of shale oil in the member.

    Resource evaluation method and influence factors of its parameters for tight sand gas reservoir in southwestern Sulige gas field
    Jinbu Li, Ya Li, Ji Zhang, Tebo Yang, Aiping Fan, Renchao Yang, Mingming Cui
    2020, 41(4):  730-743, 762.  doi:10.11743/ogg20200407
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    The research on reservoir characteristics and their impact on resource assessment is one of the bases for tight gas resource evaluation, production and efficient development.An integration of analyses of logging data, hydrocarbon generation conditions and production data, as well as thin section observation, is applied to investigate the reservoir characteristics, the distribution, thickness and superposition pattern of sand bodies in He 8 member and Shan 1 member in southwestern Sulige gas field, to select an applicable evaluation method according to the reservoir characteristics, and to explore the effect of reservoir characteristics on evaluation parameters.The results show that the reservoirs in Sulige gas field are characterized by low porosity, low permeability and strong heterogeneity, thus the volumetric method is suitable for its resource evaluation.The reservoir thickness and area are controlled by the distribution and superimposition pattern of sand bodies, and the distributary channels and composite sand bodies with large thickness and area are usually high in resource potentials and more productive.The reservoir porosity and permeability are controlled by debris composition and microscopic pore type, and are strongly modified by diagenesis at later stages.Compaction and cementation lower porosity, whereas dissolution may effectively increase it.The intensity of hydrocarbon generation fundamentally controls the distribution of the water producing zone and gas producing zone, while the sweet spots of tight gas are mainly located in areas of relatively high porosity and permeability in favorable reservoirs.For resource evaluation of tight sand gas reservoirs, the sedimentation controls the distribution and superimposition patterns of sand bodies, produces composite sand bodies of various types, determines reservoir area and thickness, and then controls the gas storage potential and production capacity.Diagenesis modifies reservoir porosity and permeability, thus mainly influences oil and gas reserves of the study area in a microscopic sense.

    Characteristics and physical simulation of the Upper Paleozoic tight gas accumulation in Linxing area, Ordos Basin
    Dingye Zheng, Xiongqi Pang, Fujie Jiang, Tieshu Liu, Xinhe Shao, Longlong Li, Yuying HuYan, Fangxin Guo
    2020, 41(4):  744-754.  doi:10.11743/ogg20200408
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    The exploration potential of the Upper Paleozoic tight sandstone gas reservoirs in Linxing area, Ordos Basin is huge, but the accumulation patterns and migration pathways are relatively controversial.Based on the characteristics of structure, sedimentary facies and sand-body distribution, the source-reservoir combination relationship of the Upper Paleozoic in Linxing area can be categorized into three types:in-source-, near-source- and far-source-reservoir combinations.Using the data of rock pyrolysis, thermal maturity measurement, core description, thin-section observation, scanning electron microscopy, petrophysical property analysis, seismic and well logging interpretation, we analyzed the development characteristics of the source rock, reservoir, charging dynamics and faults.The results show that there are three sets of hydrocarbon source rocks in Linxing area, including coal, carbonaceous mudstone and dark mudstone source rocks with Type Ⅲ and Ⅱ2 kerogens.The source rocks have high abundance of organic matter and are mostly mature to highly mature, thus are large in hydrocarbon generation potential.The reservoir rocks are dominated by feldspar lithic sandstone of moderate maturity, and are characterized by high heterogeneity and multiple pore throat types, including primary intergranular pores, dissolved intergranular pores, dissolved intragranular pores, intercrystalline pores and micro-cracks.The reservoirs mostly have low porosity and low permeability, with an average porosity of 6.81% and an average permeability of 0.61×10-3 μm2, which reflects a typical tight reservoir.Overpressure is common and gas expansion force is the main driving force for gas accumulation of the in-source- and near-source-reservoir combinations.The well-developed faults can serve as the migration pathways for the natural gas vertically charging to the far-source-reservoir combination and can also improve the reservoir quality and strengthen the lateral migration ability of natural gas.Physical simulation experiments were carried out on the established accumulation model, and it was found that reservoir heterogeneity and faults are the main factors affecting the gas-water relationship.The in-source-reservoir combination is the favorable area for hydrocarbon accumulation, and can be selected as the focus of later exploration and development.

    Geochemical characteristics of noble gases in natural gases from the Tarim Basin
    Daxiang He, Youjun Tang, Jinjie Hu, Shaowu Mo, Jianfa Chen
    2020, 41(4):  755-762.  doi:10.11743/ogg20200409
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    The isotopic characteristics of noble gases have been long used in the study of the genesis and origin of natural gases.A systematic analysis of the abundance and isotopic composition of noble gases in natural gases from different structural units shows that the majority of samples is low in the ratio of 3He/4He, with the R/Ra values generally less than 0.1, indicating a crustal radiogenic genesis of the noble gases and a significant "cold" feature of the basin.No simple correlation exists between the 3He/4He ratios and both of heat flow and paleo-geothermal gradient, revealing little heat sourcing from mantle.The lower level of 21Ne/22Ne ratios influenced by the relatively late hydrocarbon generation and accumulation period, mostly during the Himalaya movement, shows an inconspicuous proportion of mantle-derived gas in natural gases from Kuqa Depression.The relative depletion of 129Xe shows that the mixing of mantle-derived volatile constituents is insignificant.Furthermore, compared with the natural gases from Tabei Uplift and Tazhong Uplift, the Xe in natural gases from Kuqa Depression was more likely derived from the crust due to the influences of lithology and structural features.

    Sedimentary geological model and distribution prediction of source rocks in the Saergan Formation(Middle-Upper Ordovician) in Awati fault depression, Tarim Basin
    Yao Yao, Zhiliang He, Huili Li, Yu Zhang, Xiaopeng Gao, Kangning Wang, Cunli Jiao
    2020, 41(4):  763-775.  doi:10.11743/ogg20200410
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    Saergan Formation hydrocarbon source rocks of the Middle-Upper Ordovician are important marine source rocks in the Tarim Basin.Due to the large burial depth of the Middle-Upper Ordovician, a lack of drilling data for horizon calibration in Awati fault depression, and the insignificant seismic response characteristics of hydrocarbon source rocks in the Saergan Formation, it is difficult to accurately predict the distribution of the source rocks with traditional techniques.Based on the analysis of lithology types and stratigraphic configuration in the Saergan Formation together with its overlying and underlying strata on the outcrops in Kalpin area, we established a geologic model for seismic forward modeling.The combination of geological model and seismic model provides a basis for identifying and tracing seismic facies of the source rocks.Following the interpretation of 2D seismic lines one by one in the study area, we analyzed both the seismic and sedimentary facies according to the features of reflection wave groups.As to areas where source rocks occur on the outcrops but are not penetrated by wells emplaced in fault depression, we worked out methods and established the workflow for marine source rock identification, and re-recognized and predicted the spatial distribution of marine source rocks in the Middle-Upper Ordovician Saergan Formation.The results show that the Saergan Formation source rocks, developed under the control of sea level rising, belong to deposits of the deep shelf facies.The Saergan Formation source rocks are widely distributed in the study area with a predicted thickness range of about 0-120 m.It is the thickest in the southwest, thins progressively toward the northeast under the control of the Aqia and Tumuxiuke fault belts, and overlaps to the north till pinches out to the south of Well Yingmai 3;moreover, it still occurs sporadically at the western margin of Aman transitional belt to the east.

    Impact of in-situ stresses on shale reservoir development and its countermeasures
    Jincai Zhang, Yuanchang Qi
    2020, 41(4):  776-783, 799.  doi:10.11743/ogg20200411
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    The degree of shale oil and gas enrichment and rock brittleness are key to the successful development of shale oil and gas in North America; however, in addition to the above factors, the high in-situ stresses in a complex geological structure also serve as a main factor hindering the effective development of marine shale gas in the Sichuan Basin.This study focuses on explaining the mechanism of hydraulic fracture propagation under complex and high in-situ stresses and demonstrating the impact of unfavorable in-situ stress conditions on fracturing effectiveness.The results show that shale reservoirs in the Sichuan Basin have higher minimum principal stresses, causing very high formation breakdown pressures.Through theoretical analysis and case study, the optimized design of drilling and completion is proposed, including horizontal well drilling direction, wellbore trajectory, and selection of hydraulic fracturing intervals.Recommendations for drilling and completion of the Fuling, Weirong, and Weiyuan shale gas fields are proposed based on the various in-situ stresses in different areas of the Sichuan Basin.

    Tectonic-sedimentary evolution of Guangyuan-Liangping paleo-rift in Sichuan Basin
    Ping Gao, Shuangjian Li, Zhiliang He, Yujin Wo, Yueqin Han, Xinchen Li
    2020, 41(4):  784-799.  doi:10.11743/ogg20200412
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    During the Permian-to-Triassic transition, a NW-SE-trending paleo-rift, also known as Guangyuan-Liangping paleo-rift, was developed in the Guangyuan-Liangping area of northern Sichuan Basin.The paleo-rift is of great significance to the oil and gas exploration and development in the northern and northeastern parts of the Basin and therefore calls for an in-depth study on its tectonic-sedimentary evolution.A systematic collection and study of outcrops and drilling data from both inside and outside of the paleo-rift, combined with analyses of corresponding sedimentary records, including unconformities, sedimentary filling, abrupt log response changes, and sediment geochemistry, as well as seismic profiles, shows that the sediments of the 3rd and 4th members of the Maokou Formation extend into the paleo-rift.The absence of an extensive karstification on top of the Maokou Formation in the paleo-rift suggests that, it was sedimentary facies changes, rather than karst erosion, that caused the thinning of the Maokou Formation in the paleo-uplift.The paleo-rift was initially taking shape during the deposition of the 3rd and 4th members of the Maokou Formation and continued to grow during the Wujiapingian stage.It entered the peak development phase during the Changxingian stage when the extensional rifting and hydrothermal activities intensified and tectonic-sedimentary differentiation escalated.It wasn't until the deposition of the 4th member of the Feixianguan Formation that the paleo-rift was completely filled up.The paleo-rift had strong control over the development of the Middle-to-Upper Permian reservoir rocks around it, including the biodetritus beach facies of the Maokou Formation and the grain beach facies in the Wujiaping Formation.Moreover, signs of hydrocarbon migration and accumulation and residual pores in the Wujiaping Formation indicate that the Formation is promising in terms of further hydrocarbon exploration.

    Sedimentary geochemical characteristics of the Early Cambrian source rocks in Well Jinye 1 in southern Sichuan Basin
    Jian Cheng, Lunju Zheng
    2020, 41(4):  800-810.  doi:10.11743/ogg20200413
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    An in-depth study was carried out on the settings and sedimentary geochemical characteristics of source rocks in the lower Cambrian Maidiping-Qiongzhusi Formations of southern Sichuan Basin.The analysis of the rock compositions, organic carbon content, hydrocarbon generation biological combinations, and redox indexes as well as their correlations, on source rock samples from the Maidiping and Qiongzhusi Formations in Well Jinye 1 of Jingyan-Qianwei area, shows that the two formations are quite different in terms of mineral composition and hydrocarbon generation environment.The Maidiping Formation is dominated by siliceous dolomite and has a moderate organic carbon content.Hydrocarbon was mostly generated by microbial mats composed of small shallow-water shell creatures.A strong reducing environment can be inferred from its high Uranium index (greater than 1.7).It is suggested that, instead of being caused by deep water, the strong reducing environment is due to the thriving and rapid consumption of oxygen of the shell creatures in the sea bed.A combination of these understandings with a correlation analysis of sedimentary geochemical parameters indicates a tidal flat sedimentary setting for the deposition of the formation.The Qiongzhusi Formation is dominated by siliceous and clayey shale and varies in organic carbon abundance from the lower to upper members.Different from the Maidiping Formation, its hydrocarbon-generation organisms are mainly amorphous flocculent aggregates of organic matter.The Uranium index varying between 0.8 and 1.7 and a positive correlation between the organic carbon content and the redox properties of the samples all indicate a shallow-to-deep shelf setting of oxidation to strong reduction for the formation.In summary, various factors, including the tectonic-sedimentary-lithofacies paleogeographical environment, primitive biological type, and rock mineral composition during the Early Cambrian in southern Sichuan Basin, serve to jointly determine the quality of the source rocks and their sedimentary and geochemical characteristics.

    Formation mechanism and environmental evolution of fine-grained sedimentary rocks from the second member of Kongdian Formation in the Cangdong Sag, Bohai Bay Basin
    Yuan Deng, Shiyue Chen, Xiugang Pu, Jihua Yan, Jia Chen
    2020, 41(4):  811-823, 890.  doi:10.11743/ogg20200414
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    The development of unconventional oil and gas has triggered an upsurge of research in fine-grained sedimentary rocks.However, more research on the formation mechanism and depositional environment of lacustrine fine-grained sedimentary rocks is still needed.Through detailed description of core, thin section identification, X-ray diffraction analysis, automated mineral identification and characterization system (AMICS) and geochemical analysis, this paper establishes a classification scheme for lithofacies based on rock types and sedimentary structures.Fine-grained sedimentary rocks in the second member of the Kongdian Formation in the Cangdong Sag of Bohai Bay Basin was divided into five lithofacies: the laminated felsic fine-grained sedimentary rocks, massive mixed fine-grained sedimentary rocks, laminated mixed fine-grained sedimentary rocks, laminated dolomite and massive dolomite.The depositional environment and formation mechanism of each lithofacies type were analyzed.Four types of lithofacies assemblages were identified based on the superimposition patterns of different lithofacies.These assemblages vary in climate conditions, water body characteristics and terrigenous clastic input; and correspond to four depositional modes characterized respectively by intensive terrigenous sediment input, seasonal climate influence, semiarid seasonal influence, and prolonged arid.Analyses of the classification and geochemical indexes of the lithofacies assemblages show that the depositional environment in the second member of the Kongdian Formation can be divided into three stages from bottom to top: the seasonal climate stage from Ek2(4) to Ek2(3), the humid climate stage for Ek2(2) and the arid-humid transitory stage for Ek2(1).

    Fault distribution patterns and their control on sand bodies in Pinghu Formation of Xihu Sag in East China Sea Shelf Basin
    Zhe Chen, Changmin Zhang, Guowei Hou, Wenjie Feng, Qinghai Xu
    2020, 41(4):  824-837.  doi:10.11743/ogg20200415
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    Xihu Sag in the East China Sea Shelf Basin is an important petroliferous area along the coast of China. Exploration activities in the sag reveal complex distribution of (syndepositional) faults and sand bodies but the controlling factors on the distribution of the sand bodies are still unclear. In this study, core observation, logging and 3D seismic data are combined with fine sand body interpretation based on paleogeomorphology and seismic attributes to study the effect of synsedimentary faults on the sand bodies in the Pinghu Formation of the sag. According to the different development scale, the syndepositional faults can be divided into three levels, which serve to control the sag pattern, slope characteristics and sand body distribution, respectively. In addition, the controlling effect of the three levels of the syndepositional fault distribution patterns on the vertical sedimentation and plane extension of the sand bodies is analyzed. The results show that there are six fault combination patterns controlling the sandstone development in the study area, namely the forward fault-step type, forward fault-step-tilt type, forward fault-step overlying type, corner type, reverse fault step type, and composite type with both forward and reverse faults and steps. Of which, forward types facilitated the transportation of sand bodies to the center of the sag and enlarged the distribution of reservoirs within the sag, and reverse types had significant sand-rich effect and provided superior conditions for hydrocarbon accumulation. The six combination patterns are helpful in guiding the finding of vertical and horizontal enrichment zones of sand bodies, determining the distribution of sand bodies and predicting the locations of reservoirs.

    Geochemical features and organic matter enrichment in the Early Cambrian black shale, northern Jiangsu area
    Yuxi Zhang, Jianwen Chen, Jiangyu Zhou
    2020, 41(4):  838-851.  doi:10.11743/ogg20200416
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    A set of black shale deposited in the Early Cambrian and characterized by a large thickness and high organic matter content is one of the important exploration target strata for marine shale gas in South China.The sedimentary environment and enrichment mechanism of organic matters in the black shale are discussed based on the core and geochemical data from Well XYZ-1 drilled through the Early Cambrian Mufushan Formation in northern Jiansu area.The Mufushan Formation in Well XYZ-1 is a set of depositional sequence of black and dark gray carbonaceous shale with stone coal layers on top and at bottom and interbeds of thin-layered carbonate rocks.The average values of V/(Ni+V), Ni/Co, U/Th, and V/Sc are 0.74, 6.78, 1.46, and 65.43, respectively.These ratios as indicators of redox environment suggest that the sediments were under anoxic conditions and two strong sulfidation events once occurred at the bottom and top.The average total organic carbon (TOC) content of the Mufushan carbonaceous shale is 5.62%, with the TOC value of 72.2% of the samples collected greater than 2%.The TOC-TS correlation coefficient is as high as 0.89.The hypoxic environment with sulfidation events provided favorable conditions for the enrichment and preservation of organic matters, thus laid down foundation for the development of high-quality source rocks and reservoirs in the Mufushan Formation of northern Jiansu.

    Methods and Technologies
    Workflow for pore-type classification of carbonate reservoirs based on CT scanned images
    Peiqing Lian, Wenbin Gao, Xiang Tang, Taizhong Duan, Fuyong Wang, Huawei Zhao, Yiqiang Li
    2020, 41(4):  852-861.  doi:10.11743/ogg20200417
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    Carbonate reservoirs are characterized by storage space of high heterogeneity and complex seepage mechanism.It is difficult to quantitatively characterize the size, shape and connectivity of pores, fractures, and vugs in the carbonate reservoirs.In this study, a new workflow is proposed to identify pore types, quantitatively characterize pore structure, and automatically classify core plugs based on CT scanned images.The CT scanned images of carbonates were pre-processed to perform gray conversion and increase signal-to-noise ratio, and further segmented to binary images containing pores and matrix.Then, the characteristic pore structure parameters were extracted through a series of procedures including morphology analysis and parameter calculation.The support vector machine method was adopted to automatically identify pores, vugs, and fractures in the CT images.Based on the pore identification of all cross-sections through CT images of the core, the coefficient was proposed to identify pore types.Application of the proposed workflow to the M and F reservoirs in the T oil field indicates that the workflow shows excellent accuracy in pore identification, can effectively determin the major pore types in reservoirs, thus is of guiding significance to effective petroleum development.

    Pore-throat heterogeneity in dolomite reservoirs based on CT imaging: A case study of the 3rd member of the Ordovician Yingshan Formation in Well GC601 in Gucheng area, eastern Tarim Basin
    Kedan Zhu, You Zhang, Tong Lin, Yachun Wang, Xingping Zheng, Lin Zhu
    2020, 41(4):  862-873.  doi:10.11743/ogg20200418
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    The pore-throat heterogeneity of dolomite reservoirs is mainly reflected by their complex structure and distribution that are difficult to sufficiently and quantitatively analyze with conventional reservoir characterization methods.In this study, nine samples from dolomite reservoirs in the 3rd member of the Ordovician Yingshan Formation (Ying 3 member), Gucheng area, eastern Tarim Basin, were CT scanned to establish a 3D network model for the pore-throats.Such reservoir parameters as pore-throat radius, pore and throat numbers, total porosity, spatial connectivity, and etc., were calculated based on the model.The results show obvious differences among the pore-throat radius distribution curves for the samples.These curves can then be grouped into three categories in terms of shape features, that is, Type Ⅰ, the normal unimodal curve; Type Ⅱ, the unimodal curve for pores and multi-modal curve for throats; and Type Ⅲ, the multi-modal curve for both pores and throats.The genesis of the curves was analyzed and pore-throat radius distribution patterns were set up based on core observation and casting thin section identification of the samples.In conclusion, the pore-throat radius distribution curves for dolomite reservoirs dominated by intra-crystalline pores are generally evenly skewed unimodal distribution, except for some tinny peaks beside the main summits caused by the development of local micro-fractures.The curves may show obvious multiple summits with pores converging along some residual sedimentary structures such as bedding surfaces.All the curves may show the multi-modal distribution with increasing numbers of inter-crystalline dissolved pores under stronger dissolution that indicates high heterogeneity of reservoirs.

    Quantitative analysis on pore evolution in feldspar-rich coarse clastic sandstone: A case study of Es2 in the north part of Liaodong uplift
    Longlong Guo, Hongde Cheng, Xiaobo Huang, Jun Wang, Chong Feng
    2020, 41(4):  874-883.  doi:10.11743/ogg20200419
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    The coupling relationship between pore evolution and hydrocarbon charging plays an important role in hydro-carbon accumulation.However, an accurate quantitative analysis of the pore evolution is difficult.A porosity evolution model was therefore constructed with quantitative calculation parameters determined through analyses of microfabric, authigenic minerals, diagenetic sequences and thermal evolution of feldspar-rich coarse clastic sandstone reservoirs in the second member of the Shahejie Formation(Es2) in the north part of Liaodong uplift.The results show that the water-rock reaction of feldspar together with its products has a dominant effect on the reservoir porosity.While analyses of parameters including reservoir sorting coefficient, cement mass fraction, plane porosity of residual primary intergranular pores, total plane porosity, and plane porosity of dissolved pores, are important to the accuracy of quantitative calculation.The research indicates that compaction caused the strongest damage to the reservoirs from 38 to 23.3Ma, resulting in a porosity decrease of 15%.Cementation, which lasted almost the whole diagentic period, followed up to cut back the porosity by another 10.4% on average.However, erosion from 19 to 2 Ma served to increase the porosity by 9.8%.The physical properties of the reservoirs were gradually improving with hydrocarbon-charging and deteriorating as hydrocarbon charging ended.

    A lithology recognition method based on multi-resolution graph-based clustering and K-Nearest Neighbor:A case study from the Leikoupo Formation carbonate reservoirs in western Sichuan Basin
    Qiangfu Kong, Cai Yang, Hao Li, Chao Geng, Jian Deng
    2020, 41(4):  884-890.  doi:10.11743/ogg20200420
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    Carbonate rocks have the characteristics of strong heterogeneity, changing lithology and various rock types, which make it difficult to recognize their fine lithologic features and seriously restrict the calculation of reservoir parameters as well as subsequent oil/gas development. The carbonate reservoirs in the Leikoupo Formation in western Sichuan Basin were studied to deal with the problem. Core and thin slice observation and other analysis results revealed eight distinctive lithologic facies in the reservoirs: the algal bonded dolomite, crystal powder dolomite, dolomicrite, calcite dolomite, dolomitic limestone, limestone, gypsum dolomite and gypsum. Their log responses were also identified. In addition, machine learning was combined with multi-resolution graph-based clustering to establish a lithology identification training model by using the known and named lithologic samples as training data. Subsequently, the lithology of reservoirs in other wells was predicted with the K-Nearest Neighbor, thus realizing a fine identification of different lithologic facies. Field application of the method shows a 91.3% of overall coincidence rate of lithology recognition, indicating an improved accuracy in lithology identification.