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Table of Content

    28 February 2021, Volume 42 Issue 1
    Previous Issue    Next Issue
    Petroleum Geology
    Differential occurence of shale gas in the Permian Longtan Formation of Upper Yangtze region constrained by plate tectonics in the Tethyan domain
    Zhiliang He, Haikuan Nie, Shuangjian Li, Guangxiang Liu, Jianghui Ding, Ruikang Bian, Zhiyuan Lu
    2021, 42(1):  1-15.  doi:10.11743/ogg20210101
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    Great success has been achieved in marine shale gas exploration and development worldwide.Exploration in marine-terrestrial transitional and terrestrial shales has also been active in China but a large scale commercial development is yet to come.Based on the analysis of the controlling effect of the Permian plate tectonics on the formation and distribution of prototype basins and organic-rich shales in South China, we explored the petrographic, geochemical and mineral assemblages as well as reservoir characteristics of the Longtan Formation shale in the Sichuan Basin.In comparison to typical shale formations of similar sedimentary settings that are under commercial development elsewhere, we clarified the occurrence features and resource prospect of marine shale gas in the formation.The main conclusions are as follows: (1) Under the influence of the subduction of the Qinling and Jinshajiang (ocean) blocks and other tectonic events such as the Emei taphrogeny, multiple rift basins were developed in both the center and edge of the Yangtze platform and a set of continental-transitional-marine sedimentary system was also deposited by the subsequent intensive tectonic-sedimentary differentiation.(2) The organic-rich Longtan Formation in the Sichuan Basin can be lithologically categorized into three types: the mud-shale intercalated with limestone intervals of deep-water shelf facies, sandstone-mudstone-limestone interbeds of shallow-water shelf facies and mud-shale intercalated with coal beds of tide flat-lagoon facies.(3) The Longtan Formation in northeastern Sichuan Basin that was deep in stagnant water with good sealing and reducing properties during the Permian provided an ideal place for organic matter enrichment.In terms of preservation, the formation was not only confined by the regional Lower Triassic gypsolyte cap rocks, but also perfectly clamped between the Changxing Formation limestone (on top) and the Maokou Formation limestone (at the bottom).(4) The high organic carbon content (TOC>2%), thermal evolution maturity (Ro>2%), porosity (Φ>5%), quartz content (>40%), gas content (desorption gas content>3 m3/t), and high pressure coefficient (about 1.5) of the formation indicate favorable conditions for shale gas generation.(5) The Longtan Formation, similar to such typical marine shales as the Wufeng-Longmaxi Formations in the same basin, or as the Barnett, Ohio, Antrim and New Albany shales in the U.S., is geographically favorable to the generation of large-scale shale gas deposits.It is expected to take the lead in making strategic shale gas exploration breakthroughs, especially in the northeastern and eastern parts of the basin where the burial depth is relatively shallow and the siliceous shale is relatively thick.

    Sinopec's shale gas development achievements during the "Thirteenth Five-Year Plan" period and outlook for the future
    Xunyu Cai, Peirong Zhao, Bo Gao, Tong Zhu, Lingyu Tian, Chuanxiang Sun
    2021, 42(1):  16-27.  doi:10.11743/ogg20210102
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    During the "Thirteenth Five-Year Plan" period, Sinopec went full steam ahead with oil and gas exploration and development to ensure national energy security for China.With goals to speed up the development of the Silurian marine shale gas, achieve breakthroughs in the exploration of continental and transitional shale gas, and get prepared for shale gas development in new strata of new areas, Sinopec has exerted great efforts to the integration of exploration with development and geology with engineering as well as the innovation of key technologies.The Thirteenth Five-Year has witnessed a series of achievements made by the company, such as the rapid development of the Fuling gas field, the largest shale gas field outside North America; the successful assessment of the Weirong shale gas field, the first deep shale gas field in China; the discovery of China's first economically viable normal-pressure shale gas field; the identification of five potential deep and normal-pressure shale gas pay zones with resource up to 100 billion cubic meters, and the breakthroughs in the exploration of the Jurassic continental shale, the Permian marine shale as well as other new strata in the Middle Yangtze region.Equipped with a deeper geological understanding of shale gas, Sinopec has upgraded its knowledge of enrichment mechanism of deep and normal-pressure shale gas, built its own EOR tool box for marine shale, worked out technologies for developing shales deeper than 3 800 m and formed cost-effective approaches for normal-pressure shale gas development, which as a whole have successfully spurred the company's growth of shale gas reserves and output and facilitated its leap-forward shale gas industry as well as maintained its leading role in the country's shale gas development.In the future, Sinopec will focus its shale gas development on deep, normal-pressure and new formations in and around the Sichuan Basin and its periphery, strengthen its research on the enrichment mechanisms and fundamental development theories of different shale gas types, and develop key technologies.For the "Fourteenth Five-Year" plan, Sinopec plans to establish a shale gas productivity of a trillion cubic meters in the Fuling area, continue with its exploration activities in deep, normal-pressure shale, and other new strata for gas, and realize more commercial discoveries of shale gas of diverse types and higher growth of shale gas reserves and production.

    Evaluation of gas content in shale
    Jinchuan Zhang, Shugen Liu, Xiaoliang Wei, Xuan Tang, Yang Liu
    2021, 42(1):  28-40.  doi:10.11743/ogg20210103
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    The evaluation of gas content as the core of shale gas resource assessment has drawn great attention.Gas accumulation and enrichment in shale is a dynamic geological process that results in a complex occurrence of gas: free gas coexisting with adsorbed gas and their shifting proportions.The accumulation mechanism of shale gas is quite different from that of coalbed methane.Shale gas of direct or indirect origins can have quite different forming conditions and gas-bearing characteristics.The vertical variation characteristics of gas-content-related indicator curves can provide more information on sedimentation, gas content and reservoir preservation.There are, in essence, six kinds of shale gas content evaluation methods, falling into three credibility gradients.The field desorption method, among others, is the major one.According to the method, the lost gas amount of a core sample is physically meaningful only when it is the result of a linear or polynomial regressions of the gas amount desorbed from the sample before restored to its original ambient temperature (formation temperature) during evaluation.The gas content in shale is controlled by the shale's gas generation capacity and gas content.The lost gas, desorbed gas and residual gas are internally related to adsorbed gas and free gas respectively.The adsorbed gas content and total gas content of shale are important evaluation parameters for shale gas content.The ratio of free/adsorbed gas content in shale is an ideal indicator of gas occurrence and recoverability.Assessment targets with both high total gas content and high free/adsorbed gas ratio can be considered as promising.It is recommended to combine these parameters in shale gas evaluation to obtain more accurate results.Machine Learning and big data analysis are also proven to be useful in improving data processing efficiency of shale gas evaluation, indicating intelligent evaluation being one of the important evolving directions of shale gas content evaluation.

    Characteristic differences and controlling factors of pores in typical South China shale
    Zhenxue Jiang, Xin Li, Xingmeng Wang, Guozhen Wang, Hengyuan Qiu, Deyu Zhu, Hongyang Jiang
    2021, 42(1):  41-53.  doi:10.11743/ogg20210104
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    Sweet spot prediction in shale usually depends on a general evaluation of hydrocarbon potential of reservoirs against various sedimentary settings and a thorough investigation into the main controlling factors on reservoir development in different strata context.Despite a large-scale commercial development of shale gas in South China, the marine-continental transitional shale and continental shale in the region have not contributed much to the boom so far.To turn the scale, representative shales in southeastern (marine) and northeastern (continental) Sichuan Basin as well as central Hunan Province (transitional), were sampled and analyzed in their respective sedimentary settings, geochemical attributes, petrological features and pore structure characteristics, to clarify the major factors controlling the reservoir capacity.The results show that marine shale with Type I maceral from southeastern Sichuan Basin is transportable and potentially porous with organic matter pores.The most promising facies is the organic-rich siliceous shale characterized by high pore volume (PV) (0.026 cm3/g on average) and high specific surface area (SSA) (28.99 m2/g on average) and with the abundance of organic matter determining the storage capacity.The marine-continental transitional shale with Type Ⅲ maceral of inertia from the central Hunan Province is dominated by clay mineral pores.The most promising facies is organic-rich clay characterized by high PV (0.023 cm3/g on average) and low SSA (6.33 m2/g on average) and with Si/Al ratios (ideally 2/3) controlling the reservoir capacity.The continental shale with mixing organic matter macerals from the northeastern Sichuan Basin is dominated by clay minerals and organic matter pores.The most promising facies is organic-rich clay mixed with shale, characterized by medium PV (0.017 cm3/g on average) and SSA (11.90 m2/g on average) and with both sapropelic content (>60%) and mineral ratios controlling the reservoir capacity.In all, shales with different sedimentary settings and under differential diagenetic modification present distinctive reservoir properties that require tailor-made exploration strategy.

    Advancement and trends of shale gas reservoir characterization and evaluation
    Ruyue Wang, Zongquan Hu, Li Dong, Bo Gao, Chuanxiang Sun, Tao Yang, Guanping Wang, Shuai Yin
    2021, 42(1):  54-65.  doi:10.11743/ogg20210105
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    Breakthroughs in shale gas exploration and development are inseparable from advancement in exploration theories and technologies.The complex micro-nano scale pore systems prevalent in shale and the unique occurrence of shale gas that distinguishes itself from conventional gases require the characterization of shale gas reservoirs to be more pertinent, accurate and applicable in terms of evaluation objects, methods and techniques.On the basis of a thorough investigation on shale gas reservoir characterization and evaluation status quo around the world, this paper focuses on the advancement of such technologies as lithofacies division and prediction, mineral composition and pore structure analyses, fracability evaluation and multi-scale/technique integration.It proposes that future shale gas reservoir characterization and evaluation pivots around (1) accuracy and pertinence enhancement; (2) further integration of multiple-scale techniques; (3) in-situ condition; (4) dynamic monitoring and assessment; and (5) application of big data.In view of geological conditions, exploration and development practices of shale gas in China, the paper again proposes four aspects as the focuses of future shale gas reservoir characterization and evaluation: (1) fine-grained sedimentary characteristics; (2) temporal and spatial evolution traits; (3) fracability evaluation of reservoirs; as well as (4) integration of different technologies, introduction of foreign technologies, and most of all, innovation.

    Characterization and genesis of deep shale reservoirs in the first Member of the Silurian Longmaxi Formation in southern Sichuan Basin and its periphery
    Hongyan Wang, Zhensheng Shi, Shasha Sun, Leifu Zhang
    2021, 42(1):  66-75.  doi:10.11743/ogg20210106
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    Deep reservoirs are among the future major targets for shale gas exploration and development in China. A study on the first Member of deep Longmaxi Formation (Long 1 Member) shales in the Sichuan Basin and its periphery was carried out by means of whole rock component X-ray diffraction (XRD), TOC determination, large thin section and argon ion (Ar+) milling SEM imaging. The results show that, compared with shallow shales, the deep shale reservoirs are characterized by relatively higher silica content, lower TOC, lower silty lamina fraction, higher porosity, and more effective pore networks. From shallow to deep, the silica content increases from 30% to 62%. Accordingly, the contents of carbonate minerals, clay minerals, and TOC decrease from 32%, 33%, and 7.1% to 14.3%, 7.8%, and 4.25% respectively. The stripe-shaped siltstone-bearing lamina developed in deep shale reservoir is significantly lower in content and thickness compared with shallow shales. Organic/inorganic-matter pores micro-fractures are widespread within the black shales. From shallow to deep, the thin section porosity increases from 1.6% to 10.8% along with significantly increasing organic and inorganic pores, as well as micro-fractures (with a proportion increase from 1% to 12%). In deep shales, the organic/inorganic-matter pores and micro-fractures are interconnected to form an effective pore network. In addition, the higher silica content and porosity, as well as effective pore network are possibly associated with the biogenic silica; the relatively lower TOC may be due to the long distance from provenance; and the low silty lamina fraction is related to deep water environment. Biogenic silica can produce and preserve a large amount of organic/inorganic-matter pores and well as micro-fractures during diagenesis. The supply of nutrient substance determines the generation of organic matter, and the distance to provenance is in negative correlation to nutrient supply. Silty lamina is mainly composed of carbonate minerals, which, however, are hard to form in deep reservoirs.

    Controlling effect of compaction upon organic matter pore development in shale: A case study on the Lower Paleozoic in southeastern Sichuan Basin and its periphery
    Qian Chen, Xiangbin Yan, Chaoying Liu, Xiaoliang Wei, Zhe Cheng, Weijun Qin, Taiyuan Hong
    2021, 42(1):  76-85.  doi:10.11743/ogg20210107
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    Pores in organic matter are important storage space for gas or oil in shale. Present researches are focused more on the genesis and description of the pores and less on the secondary compaction that also plays a role in the modification of the pores under diverse geological conditions. This paper studies the modification of the pores under compaction with samples from the Lower Paleozoic black shale of southeastern Sichuan Basin and its periphery being observed with scanning electron microscopy (SEM) and analyzed with quantitative pore statistics and gas adsorption techniques. The results show that the deformation and orientation arrangement of pores commonly seen in the organic matter samples can be related to the actual occurrence of the organic matter: the pores are generally destroyed under compaction when in organic matter along beddings, or squeezed to deform locally when in organic matter serving as fillings in shale or just deformed by clay minerals when in organic matter-mineral aggregates. In addition to the occurrence, the pores are also affected by microscopic mineral framework, rock plasticity determined by organic matter content, and their own size.

    Characteristics of shale gas enrichment in tectonically complex regions-A case study of the Wufeng-Longmaxi Formations of Lower Paleozoic in southeastern Sichuan Basin
    Zhiyuan Lu, Zhiliang He, Chuan Yu, Xin Ye, Donghui Li, Wei Du, Haikuan Nie
    2021, 42(1):  86-97.  doi:10.11743/ogg20210108
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    The Dingshan area in southeastern Sichuan Basin is one of the focal points for shale gas exploration and development in China.Despite some promising test results achieved from several wells drilled in the area, the subsequent rapid production decline rate and low EUR estimations make their development uneconomical.To tackle the problem, efforts were exerted to the characterization of shale gas accumulations and predication of potential exploration targets in such areas.The representative Wufeng-Longmaxi Formation shale in the Dingshan area was chosen to conduct analyses of the shale rock types, distribution characteristics, reservoir types and gas content by means of core observation, identification and correlation of graptolite belts and scanning electron microscope (SEM) observation.The results show that there are four types of shale: siliceous shale, lime-calcareous shale, clayey shale and silty shale.Among them, the siliceous shale in the graptolite belt of the second member of the Wufeng Formation and the fourth member of the Longmaxi Formation (WF2-LM4) is potentially high-quality shale rocks with a thickness of 6 m to 10 m.The reservoir space is mainly organic-matter pores, mineral pores and micro-fractures.The dissolution pores that are well developed near the margin of the basin are found to be no signs of asphalt, indicating a poor preservation condition for shale gas.Compared with the Fuling shale gas field, the siliceous shale rocks in the graptolite belts in the Dingshan area are thinner, with those buried shallow in the faulting zone near the edge of the basin containing shallow normal-pressure gas reservoirs of low formation pressure coefficients, and those distributed basinward having deep, over-pressure gas reservoirs of high pressure coefficient.It is suggested that the exploration of shale gas should be focused on parts with large thickness, moderate burial depth and good preservation conditions in the WF2-LM4 zones of southeastern Sichuan Basin.

    Division of shale lithofacies associations and their impact on fracture network formation in the Silurian Longmaxi Formation, Sichuan Basin
    Cheng Shen, Lan Ren, Jinzhou Zhao, Mingpei Chen
    2021, 42(1):  98-106, 123.  doi:10.11743/ogg20210109
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    A commercial development of shale gas reservoirs largely depends on the effect of hydraulic fracturing.However, shale of different lithofacies associations responds differently to the treatment.The study used the Silurian Longmaxi Formation shale in the Sichuan Basin as an example to conduct a division of lithofacies associations and an analysis on the impact of the associations upon fracture networks.The roles of mineral composition, physical properties, beddings and bedding-parallel fractures of the shale in the formation of fracture networks were analyzed and then integrated with characterization of crops, cores and stimulation effect of hydraulic fracturing in the shale to define the association divisions.The results show that there are four types of lithofacies associations in the shale.Type Ⅰ association (FA1) is obviously the result of diagenesis modification and contains well-developed calcite-filled fractures, which can be activated preferentially to generate fracture networks but are also likely to confine the expansion of fractures during the earlier stage of fracturing.Type Ⅱ (FA2) shows evident signs of sedimentary control and tends to contain interbeds of promising gas reservoirs with crackable reservoirs.Type Ⅲ (FA3) and Type Ⅵ (FA4) are dominated by terrigenous components in comparison and are difficult to crack to from fracture networks.In all, FA1 and FA2 are the potential sweet spots for fracturing.The study is of theoretical support and on-site guiding significance to target formation selection and scheme design in fracturing.

    Characteristics and exploration potential of the Wufeng-Longmaxi shale gas reservoirs of Lower Paleozoic in Yichang area, western Hubei Province, China
    Quansheng Cai, Xiaohong Chen, Guotao Zhang, Baomin Zhang, Jing Han, Lin Chen, Peijun Li, Yangui Li
    2021, 42(1):  107-123.  doi:10.11743/ogg20210110
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    During the past few years, several shale gas discoveries were made in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in Yichang of western Hubei Province, China.The shale gas potential of the formations was then explored by combining the previous studies with updated analyses of drilling data and outcrops, geophysical information and testing results.A summary of the development characteristics, reservoir distribution and resource potential of the formations shows that the targeted dark, organic-rich rocks at the lower part of the formations have high quartz content (more than 50% on average) and TOC(2.47% on average), and high maturity (Ro of more than 2% in general), low porosity (less than 5% in general), extra-low permeability (0.19×10-3 μm2 on average), and relatively high gas content (2.0 m3/t on average).Quality reservoirs are mostly distributed in the lower part (about 11 m thick) of the black rocks and can be corresponded to the WF2-WF3 Katian graptolite zone and LM2-LM3 Rhuddanian graptolite zone.The reservoirs are thinner than high-quality reservoirs in Jiaoshiba and Changning areas, Sichuan Province.However, judging by their preservation conditions and economy, we propose that the rocks in the slope of eastern Yichang with their shallower burial depth, relatively better preservation conditions as well as a geological reserve of over 1 200×108 m3, are worthy of further exploration.

    Geological characteristics and controlling factors of hydrocarbon accumulation in terrestrial shale in the Da'anzhai Member of the Jurassic Ziliujing Formation, Sichuan Basin
    Shasha Sun, Dazhong Dong, Yucong Li, Hongyan Wang, Zhensheng Shi, Shiwei Huang, Yan Chang, Wenhua Bai
    2021, 42(1):  124-135.  doi:10.11743/ogg20210111
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    Hydrocarbon exploration in the terrestrial Jurassic system of Sichuan Basin has been focused on conventional tectonically fractured reservoirs and unconventional coquina and tight sandstone reservoirs.However, shale reservoirs are regarded as the new focal point for making significant breakthroughs in the system of the basin.Based on an analysis of the distribution, generation, reservoir properties and preservation conditions of the terrestrial shale oil/gas in the Da'anzhai Member of Ziliujing Formation, we conclude that the member is favorable for a large-scale shale oil and gas accumulation in terms of its geological features and that three factors serve to control the accumulation.Geologically, the 20-80 m thick dark shale with oil and gas shows in the 2nd sub Da'anzhai Member is both excellent source rock and over-pressured oil/gas reservoir sandwiched between cap and bottom tight limestones with its wide and continuous distribution, large thickness, well-developed pores and fractures, good physical properties, and medium-high maturity.The three factors including the semi-deep and deep lacustrine sub-facies, high-porosity reservoirs between the top and bottom limestones and medium-high maturity, jointly control the generation, preservation, enrichment of oil and gas in the shale.

    Pore characteristics and controlling factors of continental shale reservoirs in the Lower Jurassic Ziliujing Formation, northeastern Sichuan Basin
    Zhongbao Liu, Zongquan Hu, Guangxiang Liu, Zhujiang Liu, Haotian Liu, Jingyu Hao, Pengwei Wang, Peng Li
    2021, 42(1):  136-145.  doi:10.11743/ogg20210112
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    The development characteristics and mechanisms of pores in the organic-rich continental shale in the Ziliujing Formation of the Yuanba and Fuling areas, northeastern Sichuan Basin, were studied based on an integration of thin-section observation, whole-rock x-ray diffraction and organic matter petrological analyses with measurements obtained jointly through high-pressure mercury intrusion and adsorption experiments, and argon ion polishing-scanning electron microscopy and physical property tests.As a result, the structure, type and storage capacity of the pores as well as the factors controlling the pore growth were revealed.It shows that both the Dongyuemiao Member shale in the Fuling area of eastern Sichuan Basin and Da'anzhai Member shale in the Yuanba area of northern Sichuan Basin have high reservoir capacity with an average porosity higher than 4%.Pores in the shales are mostly inorganic mineral pores, followed by organic-matter pores and some local micro-cracks.The inorganic mineral pores show signs of strong influence of clay mineral and compaction intensity.And the organic-matter pores were probably conditioned by the composition type of organic matter as they are found to be abundant in solid asphalt, but rare in most vitrinites.The Dongyuemiao and Da'anzhai Members of the Fuling area in eastern Sichuan Basin and the Da'anzhai Member of the Yuanba area in northern Sichuan Basin contain both types of pores, while the Dongyuemiao Member of the Yuanba area is rich in inorganic mineral pores only.

    Geological characteristics and enrichment pattern of Permian Mao 1 Member shale gas reservoirs at the southeastern margin of Sichuan Basin
    Peixian Zhang, Xipeng He, Quanfang Gao, Yuqiao Gao, Bin Sun, Xiao Cai, Guisong He, Zhiping Zhang, Nana Liu
    2021, 42(1):  146-157.  doi:10.11743/ogg20210113
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    To understand the shale gas reservoirs in the first member of the Permian Maokou Formation (Mao 1 Member) in the southeastern margin of Sichuan Basin (southeastern Sichuan Basin), we combined outcrop and core observation, mineralogical and geochemical data, thin section analysis and scanning electron microscopic results of argon ion beam cross sections, with logging and seismic attribute prediction techniques to study their sedimentary facies, source rocks, reservoir properties and types.The geological characteristics and gas enrichment pattern of the Mao 1 Member shale were revealed and summarized as: (1) The member is located in the relatively deep-water gentle slope of a carbonate platform and comprised of well-developed dark gray-gray black carbonaceous mudstone, calcareous mudstone, argillaceous limestone and bioclastic limestone.The carbonaceous and calcareous mudstone, among others, with relatively high TOC (0.5%-5.1%) and moderate thermal maturity (Ro: 2.0-2.3), are potential hydrocarbon source rocks with possible new-type atypical shale gas.(2) The reservoir space in the member including organic matter pores, dissolution pores, grain boundary fractures, and constricted fissures, may form a reservoir complex of "pore-fracture-network".The carbonaceous mudstone and calcareous mudstone are dominated by organic matter pores, clay mineral shrinkage cracks and bedding-parallel fractures, while argillaceous limestone and bioclastic limestone mainly develop dissolution pores, grain boundary fractures, contraction fissures and high-angle cracks.(3) The shale gas enrichment in the member can be generally described as "integrated continuous source-reservoir assemblages controlled dually by structures and fractures", and results in an estimated geological resource volume of more than 500 BCM continuously distributed in an area of more than 7 000 km2.It therefore has great exploration potential for shale gas.The research results are of great significance to improving the understanding of the new-type shale gas geology and guiding the shale gas exploration in southeastern Sichuan Basin.

    Sedimentary environment and organic matter enrichment mechanisms of the Upper Permian Dalong Formation shale, southern Anhui Province, China
    Jianghui Ding, Jinchuan Zhang, Gang Shi, Baojian Shen, Xuan Tang, Zhenheng Yang, Xingqi Li, Chuxiong Li
    2021, 42(1):  158-172.  doi:10.11743/ogg20210114
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    The organic-rich shale in the Upper Permian Dalong Formation in southern Anhui Province is one of the major source rocks in the Lower Yangtze region and also the important shale gas exploration target in China.Its sedimentary environment and organic matter enrichment mechanisms were studied through organic geochemical tests and elemental geochemical analyses on samples from Well Gangdi-1 in the area.Results show that the shale was deposited in a warm and humid paleoclimate in which biotic productivity was high and sedimentary rate was relatively fast.The sedimentation experienced an environmental transition from an overall dysoxic-anaerobic setting with regional rising sea level that enhanced sediment retention during the early and middle stages to an oxydic setting with falling regional sea level (accompanied by regional volcanic hydrothermal activities at the end of the Permian) that reduced sediment retention during the later stage.In other words, the Dalong Formation shale, rather than a child of a single factor, is the result of a mutual configuration and coupling of multiple factors such as paleoclimate, paleoredox, biotic productivity, and sedimentary rate.The rising regional sea level during the early sedimentary stage took with it abundant nutrients from deep up to the surface, leading to bacteria, algae and phytoplankton booms that significantly enhanced the biotic productivity and a dysoxic-anaerobic water body that aided in the preservation of organic matter in a partially reducing environment.The falling sea level that formed an oxydic environment during the late stage of sedimentation was unfavorable in terms of organic matter preservation.However, the organic matter was still well preserved when the Permian volcanic hydrothermal activities brought in rich nutrients that promoted the mass reproduction of microorganisms (both the source of high biotic productivity and the reason of a rapid consumption of oxygen), accelerated sedimentary rate to shorten the exposure time, and fasten the burial and preservation process of organic matter.

    Thermodynamics and kinetics of water vapor adsorption onto shale: A case study of the Permian Shanxi Formation, Ordos Basin
    Wei Dang, Jinchuan Zhang, Fengqin Wang, Pei Li, Chang'an Shan, Ruijing Wang
    2021, 42(1):  173-185.  doi:10.11743/ogg20210115
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    Adsorption is one of the key mechanisms for the occurrence of connate water and formation water in shale reservoirs. A characterization of water adsorption onto organic-rich shale is of great theoretical and practical significance to tackling such geological and engineering issues as the micro-distribution of water and gas, the mechanisms of shale gas enrichment and the improvement of shale gas recovery. Shale-water vapor isothermal adsorption experiments were therefore combined with some theoretical adsorption models (7 thermodynamic models and 4 kinetics models) to study the fundamental principles of water vapor adsorption on shale. Results indicate that the adsorption/desorption isotherms are showing the typical TypeⅡcurves with hysteresis loops extending to very low relative pressure region, which may be explained by the fact of reluctant dehydration of clay minerals. Two of the models, GAB and Dent, are proven to be the most fitting to the shale-water vapor isotherm curves and reveal a two-stage water molecule adsorption on shale from forming monolayers to multilayers and capillary condensation. With p/p0 less than 0.1, water molecules are mainly adsorbed as monolayer on one site of shale. With p/p0 between 0.1 and 0.8, the site is gradually saturating and more layers start to build upon the first layer, thus forming a secondary adsorption site. With p/p0 greater than 0.8, the first site is almost fully saturated and the adsorption on the secondary site continues in such a rate that capillary condensation of water occurs. Moreover, the negative values of Gibbs free energy change, enthalpy change and entropy change of the adsorption, indicate a spontaneous, exothermic and entropy-reduction process. Thus the double first-order rate model is the most suitable for describing the adsorption process of water vapor on shale. It reveals that the adsorption process can be divided into a surface adsorption dominated by an earlier external diffusion and a pore adsorption dominated by a later internal diffusion, and that the internal diffusion serves to control the adsorption rate of water vapor onto shale.

    Evolution characteristics and models of shale pores and fractures under tectonic deformation: A case study of the Lower Paleozoic marine shale in the Sichuan Basin and its periphery
    Hongjian Zhu, Yiwen Ju, Yan Sun, Cheng Huang, Hongye Feng, Raza Ali, Kun Yu, Peng Qiao, Lei Xiao
    2021, 42(1):  186-200, 240.  doi:10.11743/ogg20210116
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    Tectonic stress may deform or damage the shale rocks, thus affecting their macro or micro structures to varying degrees.The marine shale samples from the Lower Silurian Longmaxi Formation and the Lower Cambrian Lujiaping Formation respectively in the southeast and northeast parts of the Sichuan Basin, were studied through focused ion beam scanning electron microscopy (FIB-SEM), gas adsorption, and mercury intrusion, to evaluate the effect of structural type and deformation mechanisms on the reconstruction of shale pore and fracture structures.Results indicate that shale in monoclines contains mostly organic matter pores, while shale in folds or faults resulted from strong tectonic activities contains largely mineral interparticle pores, dissolution pores and fractures.Comparatively, fold-related shale has more micro pores and fault-related shale has more macro pores than monoclinic shale.The pore difference in shale of different structural parts can be attributed to an uneven distribution of local structural stress.Brittle shale contains mainly micrometer-sized pores and fractures, which are conducive to gas migration and accumulation; while ductile shale is dominated by nanometer-sized pores, which are conducive to gas adsorption and occurrence.The total pore volume of the shale increases with brittle deformation and decreases with ductile deformation.The total specific surface area of the pores is loosely connected with brittle deformation, but has a positive relationship with ductile deformation.Based on these results, the paper discusses the controlling factors and evolution models of micro-nano shale pores and fractures under structural deformation, and concludes that the structural stress on shale results in the modification of shale compositions, the evolution of pores and fractures in shale, and the storage and migration of shale gas.

    Impact of volcanic ash on the formation of organic-rich shale: A case study on the Mesozoic Bazhenov Formation, West Siberian Basin
    Xinping Liang, Zhijun Jin, Quanyou Liu, Alexander Shpilman, Peng Li, Vladimir Morozov, Boris Uspensky
    2021, 42(1):  201-211.  doi:10.11743/ogg20210117
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    The siliceous and calcareous organic-rich shale in the Upper Jurassic-Lower Cretaceous Bazhenov Formation in West Siberian Basin is characterized by high TOC and organic matters of moderate maturity and therefore has been the preferred shale oil exploration target in Russia.Recent study reveals several sets of centimeter to millimeter thick volcanic ash interlayers with yellow fluorescence in this organic-rich shale interval.A large number of volcanic ash-altered minerals such as zeolite are also detected through thin section observation, thus triggering an interest in the investigation of the relationship between volcanic ash and organic matter in this area.This study takes the Bazhenov Formation in the basin as an example to illustrate the influence of volcanic ash on the formation of the organic-rich shale.The composite geological-geochemical analysis on the volcanic ash in the formation shows that the organic-rich shale deposited synchronously with volcanic ash is rich in both nutrients such as silicon and phosphorus and catalytic elements for algae multiplication such as barium, manganese, molybdenum and uranium.It is therefore speculated that materials carried by volcanic ash promoted the mass reproduction or even facilitated a boom of palaeobios during the deposition of the Bazhenov, laying a material foundation for the formation of organic-rich matters.At the same time, a large number of strawberry-like pyrite layers were also developed in the shale overlying the volcanic ash interval, against a strong reducing enviroment of oxyen deficiency after volcanic ash eruption with Co/Ni ratio less than 1 and high S/Fe ratio.The fact that samples from the area with well-developed volcanic ash layers were measured to have higher TOC (over 7%) and maturity (Ro ranging between 0.7% and 1.1%) as well as higher hydrocarbon-generation potentials (almost reaching the peak) than those from adjacent areas without the layers indicates that the elements such as molybdenum and nickel carried by the volcanic ash served to promote the hydrocarbon generation of kerogens during thermocatalysis.The algal laminae interbedded with carbonate laminae in the shale could create plenty of interlayer pores for the migration of oil and gas.The formation of organic-rich shale in the Bazhenov Formation is, to some extent, inherently related to volcanic ash, which probably served to improve the paleo-productivity and preservation conditions for organic matter during the deposition period, promote the early hydrocarbon generation of organic matters, and facilitate the formation of organic-rich interlayers that respond quickly to stimulation operations.

    Petroleum Development
    Geological characteristics of gas condensate reservoirs and their exploration and development prospect in the Jurassic continental shale of the Dongyuemiao Member of Ziliujing Formation, Fuxing area, eastern Sichuan Basin
    Zhiguo Shu, Lin Zhou, Xiong Li, Haotian Liu, Yong Zeng, Hongguang Xie, Mingjun Yao, Yanchun Wang
    2021, 42(1):  212-223.  doi:10.11743/ogg20210118
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    Gas condensate reservoirs were recently discovered by Sinopec Jianghan Oilfield Company through Well Fuye10HF in the continental shale of the Dongyuemiao Member, Fuxing area, eastern Sichuan Basin.The well was tested with a daily gas rate of 55 800 cubic meters and liquid rate of 17.6 cubic meters, indicating a great breakthrough made in shale gas exploration in the area.An integrated analysis of core, drilling, logging and testing data was then combined with an assessment of rock quality, resource potential, preservation and surface conditions to characterize the reservoirs in terms of geology and development prospect.Applicable fracturing and production technologies to the reservoirs were also reviewed to provide a reference for the exploration and development of other shale reservoirs of similar geological context.The results show that the shale of high quality and extensive distribution in the member is physically sufficient for the generation of large-scale gas reservoirs.The member, conveniently buried in a moderate depth with gentle strata and undeveloped faults in the low-relief Fuxing area, is relatively friendly to a large-scale gas development.Despite a high clay mineral content and poor fracturability, the member responds sensitively to fracturing treatment featuring a "dense-fracture volume stimulation".The results also suggest that the reservoirs are of high pressure coefficient and produce in a way that requires further studies to formulate a suitable and efficient development scheme.

    Production characteristics and optimized development technologies for normal-pressure shale gas in the structurally complex areas of southeastern Chongqing
    Xipeng He, Bi Lu, Guisong He, Jianhua Ren, Wei Wang, Zuhua Chen, Yuqiao Gao, Dazhi Fang
    2021, 42(1):  224-240.  doi:10.11743/ogg20210119
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    The normal-pressure shale gas reservoirs in some structurally complex areas of southeastern Chongqing is characterized by low pressure coefficient, high proportion of adsorbed gas and complex in-situ stress.Difficulties in forming complex fracture networks in these reservoirs are also frequently experienced during frac operations.Based on the analysis of drilling results and production characteristics of more than 60 horizontal shale gas wells in the areas, we summarized the production law, delineated the production stages, clarified the main factors controlling production capacity, and proposed an optimized technology combination for the reservoir development.It shows that the reservoirs produce mainly fracturing fluids with only a small amount of gas during the initial stage after fracturing and start to yield more gas as the flowback rate of frac fluids increases.At the later stage, gas production gradually stabilizes (depletes slowly) but features relatively high gas production per MPa of pressure drop and limited reserve covered by single wells.The whole production process can be divided into four stages: the liquid (flowback fluid only) stage, transition stage, stable production stage and low-pressure gas drainage stage.The formation pressure coefficient plays a critical role in all of the stages.During the liquid stage, higher pressure coefficient helps to shorten the liquid producing time and to initiate gas production when the flowback rate is still low.In the transition stage, it maintains a longer gas-liquid equilibrium time at lower flowback rate.In the stable production stage, it ensures a stronger gas production capacity that yields more gas with each unit of pressure drop and recover more reserves from a single well.Apart from formation pressure coefficient, the productivity of the reservoirs after fracturing operations is also conditioned by the stimulated reservoir volume (SRV) of frac operations.SRV is in turn controlled mainly by optimal hit ratios of target window, length and orientation of laterals and stimulated areas.Higher formation pressure coefficient and optimal target window drilling rate, longer lateral length, and wider stimulated areas generally signify higher single-well production and more commercially recoverable reserves.Given the above-mentioned understandings, we put forward an optimized technology combination and related fracturing parameters for the normal-pressure shale gas development in the complex areas of southeastern Chongqing.

    Shale gas exploration and development in the Lower Paleozoic Jiangdong block of Fuling gas field, Sichuan Basin
    Yaqiu Lu, Bang Liang, Chao Wang, Chao Liu, Jing Ji
    2021, 42(1):  241-250.  doi:10.11743/ogg20210120
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    Shale gas exploration and development in China has turned to shales buried in more than 3 500 m deep, posing development challenges with their high-pressure, poor physical properties and reluctance to form complex fracture networks after stimulations. The Jiangdong block, one of the major blocks in the phase-Ⅱ productivity construction of the Fuling shale gas field in the Sichuan Basin, contains shale gas at or deeper than such depth in an area of 57.8 km2, accounting for about half of the total block area. The main targets are the high-quality thin black shale layers (① to ⑤) in the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations that are rich in carbon, silicon, lamellation and pyrite. By applying an appraisal index system for deep shale gas blocks and an optimized horizontal well location selection method to the shales, we come to the conclusion that the major geological factors influencing the performance of deep shale gas wells are the burial depth, in-situ stress, fracture growth characteristics and dip angle of shale layers. It is therefore recommended that, during horizontal well drilling design, the angle between the wellbore and the minimum principal stress be less than 40° with a level difference between two designed target points (A and B) being kept as less than 300 m as possible. The exploration and development practices in the deep shale in the Jiangdong block have achieved relatively higher test production and recoverable reserves from single wells, which is of realistic guidance and reference for other similar deep shale gas exploration and development in the basin.