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2020 Vol. 42, No. 3

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2020, 42(3): 封二-封二.
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Effect of hydrocarbon expulsion efficiency on shale gas formation and enrichment
BORJIGIN Tenger, TAO Cheng, HU Guang, SHEN Baojian, MA Zhongliang, PAN Anyang, WANG Jie, WANG Xianghua, XU Ershe
2020, 42(3): 325-334. doi: 10.11781/sysydz202003325
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Shale gas enrichment is not only controlled by sufficient gas source but also by the intensity of later structural transformation.Gas volume is related to source rock quality and hydrocarbon expulsion efficiency, and the amount of shale gas generation depends on remaining hydrocarbon.Based on the identification and statistics of solid bitumen and the analysis of helium and carbon isotopes, the hydrocarbon expulsion efficiency, the in situ gas production of the Upper Ordovician Wufeng and Lower Silurian Longmaxi shales in Jiaoshiba and Pengshui exploration areas and its influence on shale gas enrichment were investigated. The hydrocarbon expulsion efficiency, residual oil volume and in situ gas volume for Jiaoshiba were 23%, 27.67 kg/t and 21.23 m3/t, while those values for Pengshui were 65%, 11.0 kg/t, and 18.99 m3/t, respectively, indicating differential hydrocarbon generation and expulsion related to various influence of the Indosinian Movement.According to the results of 4He isotope dating, Fuling shale gas began to be stored and accumulated at the initial stage (231 Ma) of the oil generation peak period with sufficient gas sources, while the closed system for Pengshui shale gas was formed at the later stage (183 Ma) than the gas generation peak with insufficient gas source. The δ13C2 and δ13C1-δ13C2 for Fuling shale gas were -35.8‰ and 4.8‰, while those values for Pengshui shale gas were -33.0‰ and 3.3‰, respectively.The different δ13C fractionation effects might be attributed to the distinct state of the hydrocarbon generation system. Therefore, the matching of the hydrocarbon generation peak period with the key structural transformation period in the process of hydrocarbon generation and evolution, the hydrocarbon retention amount in the maximum burial period and the structural transformation intensity in the process of uplift and denudation jointly controlled the generation, enrichment and preservation of shale gas.
Exploration potential of marine-continental transitional and deep-water shelf shale gas in Upper Permian, Sichuan Basin
ZHAO Peirong, GAO Bo, GUO Zhanfeng, WEI Zhihong
2020, 42(3): 335-344. doi: 10.11781/sysydz202003335
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Two types of organic-rich shale, i.e. marine-continental transitional shale and deep-water shelf shale, are well developed in the Upper Permian of the Sichuan Basin. The coal-bearing shale of the marine-continental transitional facies is primarily distributed in the Longtan Formation to the south of Chengdu-Nanchong-Guang'an-Shizhu-Fuling, which has thin individual layers but is cumulatively thick. It is dominated by type Ⅲ kerogen in the high-maturity to over-mature stage. The shale reservoir is primarily composed of clay-mineral-hosted pores, intergranular pores and micro fractures, with a small amount of organic porosity. However, the high clay mineral content and low brittle mineral content result in poor fracturing performance, requiring further engineering work. The organic-rich shale of the deep-water shelf facies is mainly distributed in the Wujiaping and Dalong formations along Guangyuan-Bazhong-Xuanhan-Yunyang-Shizhu in the northern part of the Sichuan Basin, where the Wujiaping and Dalong shales are 20-80 m and 10-40 m thick, respectively. These two shales are rich in organic matter with TOC contents higher than 2%, which are dominated by type Ⅱ1 kerogen at the high-maturity to over-mature stage. The storage space is mainly contributed by organic pores and intragranular dissolution porosity with good storage performance. High siliceous mineral content and low clay mineral content give rise to good fracturing performance, indicating for a good shale gas exploration potential in the Sichuan Basin.
Main controlling factors and types of continental shale oil and gas enrichment in Sichuan Basin
ZHU Tong
2020, 42(3): 345-354. doi: 10.11781/sysydz202003345
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Continental shale gas has achieved commercial production in the Lower Jurassic Daanzhai Member in the Fuling and Yuanba areas in the Sichuan Basin; however, the accumulation conditions are different from the marine facies Lower Silurian Longmaxi shale gas, and the exploration potential remains to be explored. The X-ray diffraction analysis of mineral composition, geochemical analysis, argon ion polishing-scanning electron microscopy, mercury intrusion-adsorption combined measurement, pulse permeability, nano-CT and gas content were applied to study three key factors controlling the enrichment of lacustrine shale gas, including lithologic combination, maturity and pressure conditions. The shales deposited on a storm beach of the carbonate lake slope of the lake facies are interbedded with the coquina limestone of unequal thickness, which is the most favorable lithologic combination with favorable configuration conditions for shale gas generation (organic matter abundance greater than 1.4%), reservoir (porosity greater than 3%), permeability (horizontal fracture developed) and fracturing (brittleness index greater than 0.6). Maturity controls the development of organic pores and the formation of shale oil and gas (Ro=1.3% as the maturity boundary). Overpressure (pressure coefficient greater than 1.2) is the key to the enrichment and high yield of continental shale oil and gas. The accumulation of continental shale oil and gas can be divided into two types: ultra-high pressure, high maturity shale interbedded with limestone and ultra-high pressure, low maturity shale interbedded with limestone. The most favorable target for shale gas exploration in the lacustrine facies of the Sichuan Basin includes the Daanzhai Member in Yuanba and the Dongyuemiao section in Fuling. The Daanzhai Member in the northern Fuling and Langzhong-Pingchang, the Dongyuemiao Section in Jiannan, and the second section of Qianfoya Formation in Yuanba are secondary favorable target for shale oil and gas exploration.
Disruptive effects of faulting on shale gas preservation in Upper Yangtze region
YU Guangchun, WEI Xiangfeng, LI Fei, LIU Zhujiang
2020, 42(3): 355-362. doi: 10.11781/sysydz202003355
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The fracturing of the Paleozoic marine shale in the Sichuan Basin and its periphery was studied. The disruptive effects of faulting on shale gas preservation were analyzed based on the comprehensive interpretation of seismic structure, the analysis of fluid inclusions in calcite veins in fractures in shale layers, and the consideration of typical shale gas wells with different gas-bearing properties. Fault level, spatial stacking pattern, stage and duration have an important influence on the preservation conditions of shale gas. (1) The higher the fault level is, the larger the impact. The preservation conditions for shale gas are destroyed in the areas close to the first-level faults (within about 10 km), the second-level faults (within about 5 km), and the third-level faults (within about 2-3 km), but not influenced in the area close to the fourth-level faults. (2) Under the premise of similar fracture density in the well area, when different levels of faults intersect from parallel or small angles to large angles, the damage to shale gas preservation gradually increases. (3) The more fault activity periods and the longer their duration, the greater is the damage to shale gas preservation in the later period. Fault period and activity duration can be divided into 4 types: (1) short-term activity in the late Yanshan period, whose impact on preservation conditions was small; (2) intermittent activity in the Yanshan and early Himalayan periods where preservation conditions were affected to some extent; (3) continuous activity since the late Yanshan period, which caused serious damage to preservation conditions; and (4) continuous activity since the Himalayan period, which had a greater impact on preservation conditions.
Early diagenesis characteristics of biogenic opal and its influence on porosity and pore network evolution of siliceous shale
LU Longfei, LIU Weixin, YU Lingjie, ZHANG Wentao, SHEN Baojian, BORJIGIN Tenger
2020, 42(3): 363-370. doi: 10.11781/sysydz202003363
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Opaline siliceous shale from the Nenjiang Formation in the Songliao Basin and siliceous shale from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the eastern Sichuan Basin were selected to study the diagenetic evolution of biogenic siliceous shale and the characteristics of shale physical properties and pore structure changes during this process. X-ray diffraction, helium porosity, nitrogen adsorption, and high-pressure mercury intrusion were used to analyze mineral composition, total porosity, and pore structure characteristics. The dehydration and recrystallization of the biogenic opal occurred early, and the transition to quasi-crystalline opal CT and crystalline quartz was completed at the early diagenetic stage. During the conversion of opal-A to opal-CT, the total shale porosity decreased rapidly from more than 75% to around 30%. During the conversion to quartz, the rate of pore loss decreased rapidly, and the decrease was only about 5%, showing two stages. At the same time, the pore volume distribution of different types of pores also changed significantly. The loss of macropores was larger than the loss of micropores. The composition of pores gradually changed from macropores and mesopores to mesopores and micropores. In the early diagenetic stage of siliceous shale, the mechanical compaction and pressure solution occurred synchronously and had a strong effect on shale transformation, which reduced shale porosity, increased hardness, and enhanced the support and resistance to compaction, reducing the transformation and destruction of the early to middle and subsequent diagenesis. The characteristics of rapid diagenesis of siliceous shale in the early stage of biogenesis are the important controls for maintaining high porosity in the middle and late stages of diagenesis.
Shale petrofacies division of Wufeng-Longmaxi formations in Sichuan Basin and its periphery
YUAN Tao, WEI Xiangfeng, ZHANG Hanrong, LI Chunyan, WEI Fubin, LU Longfei, WANG Qiang
2020, 42(3): 371-377. doi: 10.11781/sysydz202003371
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The research of shale petrofacies is significant for the exploration of shale gas "sweet spots". Different petrographic division schemes have been proposed but no consensus has been reached. This paper has studied the criteria and boundary values of the petrographic classification reference parameters of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formations in the Sichuan Basin and its periphery by considering shale quality and compressibility. (1) There are three petrofacies: marlstone, silicolites and claystones with >50% of calcium content, >75% of siliceous content and >50% of clay content, respectively. (2) Four organic petrofacies have been classified as low-carbonaceous shale, medium-carbonaceous shale, high-carbonaceous shale and enriched carbonaceous shale with 1%, 2% and 4% of organic matter as boundaries, respectively. There are five inorganic petrofacies, such as low-siliceous shale, medium-siliceous shale, high-siliceous shale, low-calcareous shale and medium-calcareous shale with 25% and 50% of silicon or calcium mineral components as boundaries, respectively. This set of petrographic comprehensive division schemes based on organic carbon-mineral components and combined with siliceous genetic analysis has strong applicability and popularization in scientific research and practical applications, which is significant for the study of the lithofacies distribution law in the Wufeng-Longmaxi formations.
Microstructure characteristics of Wufeng-Longmaxi shale gas reservoirs with different depth, southeastern Sichuan Basin
LIU Weixin, LU Longfei, WEI Zhihong, YU Lingjie, ZHANG Wentao, XU Chenjie, YE Deliao, SHEN Baojian, FAN Ming
2020, 42(3): 378-386. doi: 10.11781/sysydz202003378
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Thin section analysis, X-ray diffraction, conventional scanning electron microscopy (SEM), high-resolution Ar+ ion polishing SEM, mercury intrusion, nitrogen adsorption, total organic carbon (TOC) content and porosity analyses were used to determine the mineral composition, microstructure and micro-pore volume of shale reservoirs from different depths in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in southeastern Sichuan Basin. There are slight differences in the content of siliceous, clay, carbonate and other minerals in shale reservoirs of well D1 in the deep layer and well J1 in the shallow layer, and the longitudinal changes are similar. Only high-quality shale layers with a silica content greater than 40% have different thicknesses. The shale reservoirs of the Wufeng-Longmaxi formations in the deep and shallow layers are dominated by micro-pores in organic matter disseminated in the shale matrix, and the microfractures around the boundaries of silt particles or organic grains and micro-foliation seams parallel to bedding plane are also common, while and intergranular pores are not developed. The high quality shale reservoir at the bottom is structurally isotropic, and the vertical and horizontal structural anisotropy becomes stronger upwards, with thinner micro-lamina and more foliation seams. The TOC content is high at the bottom, and diminishes upwards. The porosity at the bottom shows the same changing trend, indicating a large contribution of organic pores; however, the decrease in porosity is significantly smaller than that of TOC, which is inferred to be related to the existence of more inorganic pores in the shale. The mineral composition, microstructure, TOC and porosity of the Wufeng-Longmaxi shales in the deep and shallow layers have similar changes with depth. Compared with well J1 in the shallow layer, the high-quality reservoir in well D1 in the deep layer has a larger number of micro-pores, meso-pores, macro-pores and total pore volume, which might be explained by more inorganic pores such as grain boundary pores.The shale reservoirs of the Wufeng-Longmaxi formations in the southeastern Sichuan Basin maintain a good porosity under deep burial conditions and have good storage space.
Characteristics of organic matter-hosted pores in Lower Silurian Longmaxi shale with different maturities, Sichuan Basin
YANG Yunfeng, BAO Fang, BORJIGIN Tenger, Pan Anyang, SHEN Baojian
2020, 42(3): 387-397. doi: 10.11781/sysydz202003387
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Organic matter-hosted pores provide reservoir space and migration pathways for shale gas. The evolution of organic matter-hosted pores (OM pores) of different macerals from Longmaxi shale with a wide variety of thermal maturities has been investigated using field emission scanning electron microscopy (FE-SEM). The Longmaxi shale contains rare graptolites which are the main component of structured organic matter. No OM pores occur in graptolite fragments, irrespective of thermal maturity. OM pores locally developed in graptolite fragments are formed from hydrocarbon generation of organic matter which was replaced by macromolecular material from surrounding sediment or in situ polymerized by lipids from the organism itself. Solid bitumen is not only the major organic component in the Longmaxi shale, but also the main host of OM pore development. The diagenesis of fine-grained sediments and thermal evolution of organic matter have been combined with the morphology of solid bitumen to distinguish pre-oil solid bitumen and post-oil solid bitumen. The post-oil solid bitumen is dominant. The evolution of OM pores within solid bitumen is closely related to thermal maturity. Generally, OM pores within solid bitumen become greater as thermal maturity increases. During the mature to early postmature (GRo < 2.3%) stages, OM pores within solid bitumen are not well developed probably due to the masking by oil and bitumen generated from organic matter. OM pores within solid bitumen are well developed during the late postmature to early overmature (2.3% < GRo < 4.5%) stages, with two main types being spongy and bubble-shaped. For organic-rich Longmaxi shale, the contribution of OM porosity to total porosity is more than 50%. During the late overmature (GRo>4.5%) stage, organic matter carbonization will cause intense damage to shale pores so that the exploration risk of shale gas increases.
Lithofacies of Upper Ordovician Wufeng Formation in Sichuan Basin and its periphery
DU Wei, HU Zongquan, LIU Guangxiang, ZHU Tong, NIE Haikuan, YAN Caina, WANG Guanping
2020, 42(3): 398-404. doi: 10.11781/sysydz202003398
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The Upper Ordovician Wufeng Formation was divided into the shale and Guanyinqiao sections. Based on the comparative analysis of shale gas drilling cores and the whole rock mineral composition of the Wufeng Formation in the Sichuan Basin and its periphery, three end members including siliceous minerals (quartz), clay minerals and carbonate minerals were chosen for lithofacies classification. Four lithofacies types were identified in the shale section including siliceous shale, clayey siliceous shale, gray siliceous shale and gray clayey shale, while in the Guanyinqiao section, shell marl developed. The shale gas drilling showed that the four lithofacies types vary in TOC, brittle mineral and gas contents. The quality of siliceous shale is the best, and the quality of gray clayey shale is the worst. The lithofacies types of the shale section and the distribution characteristics of the Guanyinqiao section were controlled by the sedimentary environment. The shale thickness in Yongchuan area and other relatively shallow water areas in the southern Sichuan Basin is relatively large, but the thickness of siliceous shale is small, and the overall quality of the shale section is poor, so it is not recommended as the target window of horizontal wells. The thickness of siliceous shale in Weiyuan area in the southern Sichuan is small due to the influence of ancient land and topography, and it also is not recommended as the target window of horizontal wells. The thickness of siliceous shale in Jiaoshiba, Wulong, Dingshan and Nanchuan areas in the southeastern Sichuan Basin is more than 4 m, which can be regarded as a target window of horizontal wells.
Shale gas resource evaluation based on "pressure coefficient": a case study of Upper Ordovician Wufeng-Lower Silurian Longmaxi formations in southeastern Sichuan Basin
CHEN Feiran, DUAN Jinbao, ZHANG Hanrong, WEI Xiangfeng, LIU Zhujiang, WANG Qiang, YU Guangchun
2020, 42(3): 405-414. doi: 10.11781/sysydz202003405
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With the continuing shale gas exploration and development in China, it is necessary to establish a scientific method to evaluate shale gas resources. This paper focuses on considering the heterogeneity of shale gas resource distribution and the impact of preservation conditions. On the basis of identifying the main influencing factors of shale gas "pressure coefficient", the study area is divided into A-type area (pressure coefficient >1.2), B-type area (pressure coefficient 1.0-1.2) and C-type area (pressure coefficient 0.8-1.0). By dissecting the shale gas calibrated area of Upper Ordovician Wufeng-Lower Silurian Longmaxi formations in Fuling, the resource volume is obtained as a key parameter for analog calculation, and the shale gas resources are evaluated by analogy. Comprehensive calculation of the total shale gas resources in the southeastern Sichuan area is 5.52×1012 m3, 4.04×1012 m3 of which is accounted for between 4 500-6 000 m depth, indicating that shale gas with a burial depth greater than 4 500 m in this area has a greater shale gas exploration potential.
A method for analyzing nanopore connectivity of shale using a fluid suction experiment
ZHANG Wentao, HU Wenxuan, BAO Fang, YU Lingjie, FAN Ming, ZHANG Qingzhen
2020, 42(3): 415-421. doi: 10.11781/sysydz202003415
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A method for analyzing the pore connectivity of shale based on a suction experiment of tracer fluid is presented. Sodium chloraurate solution is absorbed into shale pores which have been previously evacuated, and then the solute in the pores is converted to gold by heating. After the suction experiment, the sample surface is polished and analyzed with scanning electron microscopy, in order to get the characteristics of connected pores at the nano-scale. The results of two core samples from the Wufeng-Longmaxi formations in the Sichuan Basin show the connected network in shale could be divided into three parts. The interior connectivity of pores in organic matter is related to the density of pores, and organic matter normally has low flow efficiency because of narrow throat radius. Fractures distributed at grain boundaries such as minerals, organic matter and clay sheets are advantageous pathways, and organic pores are connected by grain boundaries. The development of micro-cracks/fractures, which have high permeability, could significantly improve shale connectivity. Shale pore connectivity has great anisotropy, which means connectivity parallel to bedding is much better than that vertical to bedding.
Segmented sealing of cores and collection and test of escaped light hydrocarbons and its preliminary application
BAO Yunjie, LI Zhiming, LI Maowen, QIAN Menhui, LIU Peng, XU Ershe, TAO Guoliang, YANG Zhenheng
2020, 42(3): 422-427. doi: 10.11781/sysydz202003422
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Light hydrocarbons easily escape from hydrocarbon-bearing cores when they are recovered from wellbores, which makes it necessary to improve the methods for collecting light hydrocarbons from cores and characterizing the differences of hydrocarbon losses. A new method for the segmented sealing of cores and the collection and test of light hydrocarbons was developed based on the patent "a segmented sealing device for cores and its instructions(2013101681829)". The device has a cylindrical structure with several annular seals built in, which can non-destructively seal the core inserted therein to form several closed spaces that are not interconnected to store light hydrocarbons escaping from the core. The device is matched with auxiliary equipment such as gas sampling pumps, portable methane detectors, and gas chromatographs, which are used to sample light hydrocarbons in different confined spaces to determine the concentration and composition of mobile light hydrocarbons. The concentration of escaped light hydrocarbons in confined spaces can be converted to escape quantity. Preliminary applications show that the device and method can be used to obtain light hydrocarbons that escape after the cores are recovered from the wellbores, and then to characterize the differences in the amount and components of light hydrocarbons escaping from different parts of the core in an axial direction. Combined with lithology and physical properties, it provides a basis for the evaluation of shale oil and gas formation heterogeneity and its main controlling factors.
Origin and carbon isotope reversal of shale gas in Wufeng-Longmaxi formations, Sichuan Basin: implication from pyrolysis experiments
MA Zhongliang, SHEN Baojian, PAN Anyang, BORJIGIN Tenger, NING Chuanxiang, ZHENG Lunju
2020, 42(3): 428-433. doi: 10.11781/sysydz202003428
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The Wufeng-Longmaxi shale in the Sichuan Basin is the only stratum in China to realize commercial development of shale gas. There are some differences in shale gas generation and a lack of direct experimental evidence of the reasons for these differences. Hydrocarbon generation experiments using pyrolysis in a gold tube with low and medium maturity shale and graptolite of O3-S1 were carried out. Gas from remaining oil produced by hydrogen-rich and lipid-rich organic matter such as planktonic algae is the main contributor of shale gas in the Wufeng-Longmaxi formations. The oil generation ability of graptolite is poor, and it can produce gas during the high-maturity and over-mature stages, up to about 20% of the hydrogen-rich and lipid-rich organic matter such as algae. The lower layer of the Wufeng-Longmaxi shale in the Sichuan Basin is superior to the upper layer, which is mainly composed of graptolite, because of abundant hydrogen-rich and lipid-rich organic matter such as phytoplankton. Pure thermal evolution fractionation and the mixture of primary (from kerogen) and secondary (from oil) gas do not cause hydrocarbon gas isotopes to "reverse". Formation uplift, formation water, minerals, metals and other late transformation of hydrocarbons in shale gas may be an important cause of shale gas isotopic "inversion". Carbon isotope "inversion" may be more of a reflection of the later shale gas preservation process.
Thermal-pressure simulation experiment of pore evolution of Upper Ordovician shale in Baltic Basin
LI Chuxiong, SHEN Baojian, PAN Anyang, ZHANG Wentao, LI Ang, DING Jianghui
2020, 42(3): 434-442. doi: 10.11781/sysydz202003434
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Most of the marine shale in South China is highly over mature and the pore evolution history cannot be determined. The Upper Ordovician shale in the Baltic Basin in Europe was selected to conduct a laboratory thermal-pressure simulation experiment to emulate geological conditions in order to reveal the evolution and distribution of marine shale pores. The evolution characteristics and formation mechanism of the experimental shale pores during the maturation of organic matter were systematically explained based on the organic petrological characteristics of the raw shale samples, the quantitative statistics of the simulated products, and the scanning electron microscopy analysis. The development of the overall shale pores increases with the increase of the thermal evolution of organic matter. The pores tend to communicate with each other, and gradually evolve from the initial undeveloped state to a complex and interlaced pore network. Organic and inorganic pores are subdivided into 8 categories according to their morphology and origin: spongy organic matter (OM), shrinkage OM, bubble OM, mold, mineral dissolution, intergranular, clay mineral interlayer and modified mineral pores. The transformation degree of organic matter and the primary migration of oil and gas are influenced by the difference of organic macerals. The distribution of organic matter pores showed a strong heterogeneity, and the development of inorganic mineral pores occurred in stages. The effective preservation of pores needs special attention during the shale gas exploration in the high evolution stage.
Microbial diversity above a shale gas field using high-throughput sequencing
GU Lei, XU Kewei, TANG Yuping, YANG Fan, WANG Guojian, DU Wei, GAO Bo, BORJIGIN Tenger
2020, 42(3): 443-450. doi: 10.11781/sysydz202003443
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In recent years, microbial analysis has become a new detection technology in oil and gas surface exploration. The microbial diversity above a shale gas field was studied in order to further understand microbial information. High-throughput sequencing technology was used to study the abundance and diversity of hydrocarbon species in a shale gas field in the Dingshan area of the southeastern Sichuan Basin. Three main microorganisms were identified, including Methylophilales, Methylocystis, and Methylococcus, which are worthy of further biochemical research and can be used as potential oil and gas indicating bacteria. The developmental abundance of three kinds of microorganisms has a good matching relationship with the distribution of the fugitive area in Dingshan. The high-throughput analytical technology can be applied well in the study of oil and gas microorganisms in Dingshan area, and the distribution characteristics of microorganisms in bedrock samples were obtained. Furthermore, a preliminary analysis of the differences between oil and gas microbes in Dingshan and Jiaoshiba areas was made, indicating that the identified potential oil and gas indicating bacteria deserve further study.
Characteristics and main controls of nano-pores in the Lower Silurian Longmaxi shale, Sichuan Basin
LIANG Feng, QIU Xunxi, DAI Yun, ZHANG Qin, LU Bin, CHEN Peng, MA Chao, QI Lin, HU Xi
2020, 42(3): 451-458. doi: 10.11781/sysydz202003451
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The Upper Ordovician Wufeng-Lower Silurian Longmaxi shale is the only yield layer of commercial deve-lopment of shale gas in China at present. However, there are significant differences in the production of individual shale gas wells in different areas of the Sichuan Basin and its periphery. This paper investigates those differences from the perspective of reservoir space. The pore development of 36 shale samples from the Lower Rhuddanian in 5 typical wells with different reservoir pressure in different tectonic areas in the Sichuan Basin was compared by applying, petromi-neralogical, structural geological and reservoir geological methods. The reservoir space in these shales is dominated by organic pores. With the increase of TOC content, the volumes of micropores (< 10 nm), mesopores (10~50 nm), macropores (>50 nm) and total pores in different structural regions show different trends, reflecting different stages of pore evolution. A preliminary organic pore development model of shale has been established. The extent of pore development in shale samples located in the overpressure area in the basin is obviously better than that in the structural transformation area of the basin margin. It is confirmed by experiments that the shale pores may be compacted under high pressure conditions, and the degree of compaction is closely related to the organic matter pore size, the rock mineral composition, the organic matter content, the domain structure conditions and the pressure coefficient. The areas affected by the weak tectonic movement and having a high pressure coefficient are favorable areas for pore development.
Pyrite type and its effect on shale gas accumulation: a case study of Wufeng-Longmaxi shale in Sichuan Basin and its periphery
ZHANG Guangrong, NIE Haikuan, TANG Xuan, DU Wei, SUN Chuanxiang, CHEN Song
2020, 42(3): 459-466. doi: 10.11781/sysydz202003459
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Pyrite is one of the most common minerals in shale, but its type, genesis and effect on shale gas accumulation are still in the initial study stage. The shale in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Sichuan Basin and its periphery was studied. Strawberry-like, stratified, tuberculosis, euhedral and anhedral pyrites were identified through thin section observation, scanning electronic microscopy analyses. The pyrites in the Wufeng-Longmaxi formations are mainly developed during the syngenetic and early diagenetic stages. The strawberry-like pyrites are formed in the syngenetic stage, indicating a reducing environment, which is conducive for organic matter enrichment and preservation. The pyrite content has a positive correlation with the organic carbon content. The layers with a high content of pyrites are usually rich in organic matter. Strawberry-like pyrites, together with biogenic quartz and terrestrial detrital quartz, formed a rigid particle grid supporting the original intergranular pores, which was beneficial to hydrocarbon charging, preservation and organic matter-hosted pore development. The bottom of the Wufeng -Longmaxi shale with well developed strawberry-like pyrite in the Sichuan Basin and its periphery, indicate it not only has a good condition for organic matter enrichment, but also is conducive to the formation of high-quality shale gas reservoirs. The strawberry-like pyrite content can be used as a clue for identifying shale gas "sweet spots".
Reservoir characteristics of marine-continental shale gas in Upper Permian Longtan Formation, western Guizhou province
DENG Ende, YAN Zhihua, JIANG Bingren, WANG Ran
2020, 42(3): 467-476. doi: 10.11781/sysydz202003467
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The reservoir characteristics of marine-continental shale gas in the Upper Permian Longtan Formation in the western Guizhou province were studied using X-ray diffraction, organic geochemistry, scanning electron microscopy, nitrogen adsorption, gas content determination and isothermal adsorption analyses of shale samples from some characteristic wells. The shale in the Longtan Formation in the study area has a large overall thickness, but there are many layers and each layer is thin. The clay mineral content is the highest, followed by the quartz content. The vitrinite group is dominant in the kerogens, which are mainly of type Ⅲ. The TOC content is generally greater than 3.0%. The Ro value averages 1.01%, showing a relatively lower maturity, mainly at the gas generation peak. The reservoir has ultra-low porosity and ultra-low permeability. Intergranular, intragranular and organic pores as well as micro-fractures were observed. A large number of nano-scale pores and fractures were present. Meso-pores are dominant, shaped like narrow-necked and wide-body ink bottles or narrow pores. Large specific surface area, total pore volume, pore fractal dimension and correlation coefficient indicate that the shale has a rough surface, poor connectivity, and strong heterogeneity. The on-site analysis of shale finds high total gas content and strong adsorption, and the interbedded coal seams show high gas content, indicating a large resource potential. The geological conditions and enrichment space are favorable for shale gas in the study area, and effective reservoir reconstruction technology can be selected for coalbed mining.
Geological characteristics and shale gas potential of Lower Cambrian Hetang Formation in Lower Yangtze region: a case study of Jiangshan-Tonglu area, western Zhejiang province
ZHU Wenbo, ZHANG Xunhua, WANG Xiuqi, QU Zhongdang, HUANG Zhengqing, ZHOU Daorong
2020, 42(3): 477-488. doi: 10.11781/sysydz202003477
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Abstract:
The high degree of thermal evolution is a main factor restricting the development of the Lower Paleozoic shale gas in the Yangtze region. The latest field study shows that there is a region with relatively lower thermal evolution in the Jiangshan-Tonglu area in western Zhejiang province. Centering on the lower thermal evolution area, this paper analyzes the geological and geochemical characteristics of the Lower Cambrian Hetang Formation, based on the regional geological survey of the western Zhejiang province and the latest borehole data analysis. The study area is located in a good sedimentary facies belt (basin-shelf facies), with a large deposition thickness, and a high organic matter abundance. While the thermal evolution degree is relatively low, with high brittle mineral content and moderate clay mineral content. And micro-pores and micro-fractures are well developed. The study area has a good material basis for accumulation. The Changshan-Kaihua area has been identified as the most favorable area for shale gas exploration on the basis of considering a thick shale deposit with high organic matter abundance, relatively low thermal evolution, stable tectonic background and deeply buried area, while avoiding the areas with relatively strong igneous activity.
2020, 42(3): 488-488.
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Abstract:

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