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    Main factors controlling the shale gas content of Cambrian shales of southern China—a discussion
    ZHANG Tongwei, LUO Huan, MENG Kang
    Earth Science Frontiers    2023, 30 (3): 1-13.   DOI: 10.13745/j.esf.sf.2022.5.31
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    The Cambrian shale gas resource potential of southern China is expected to be enormous with its widely distributed high-maturity organic-rich shales; however, success in shale gas exploration is uneven across the region. In this paper, we analyze the current research progress in the study of Cambrian shale gas geology in southern China, and discuss the key factors that determine the shale gas content, including reservoir characteristics, total organic carbon (TOC) content, porosity as well as tectonic conditions for gas preservation. The organic-rich Cambrian shales of the middle-upper Yangtze regions are mainly distributed in the intracratonic troughs in the Mianyang-Changning area and western Hubei, and in the deepwater shelf-slope area bordering Hunan, Guizhou and Chongqing. The porosity of Cambrian shales commonly ranges from 1% to 6%, and the pores are dominated by micropores and mesopores, mainly isolated, spongelike organic pores, which are very different from the widely-developed bobble-like organic pores in the Silurian Longmaxi shale. Organic pores are well developed in the Cambrian Qiongzhusi/Shuijingtuo shales of the troughs, with average organic porosity ranging between 20%-50%, and inorganic pores are also abundant and contribute to the total porosity. The shale gas content ranges from 1.5 to 5.5 m3/t, which is 10%-30% (in some samples as high as ~50%) of the estimated total gas generation, indicating favorable tectonic preservation conditions in the troughs. In contrast, organic pores are poorly developed in the Cambrian Niutitang shale of the deepwater shelf-slope area, with average organic porosity less than 20%. The shale gas content is below 1.0 m3/t, or less than ~10% of the estimated total gas generation, indicating poor preservation conditions and large-scale gas loss in the area. The gas loss might be caused by regional unconformities developed at the bottom of the organic-rich Cambrian shale formation as well as severe post-depositional tectonic uplift in the middle-upper Yangtze. However, interbedded carbonate-rich thin layers in troughs might act as a barrier to oil migration and gas loss. As a result, the two troughs should be the key target for Cambrian shale gas exploration, which is consistent with the current success in Cambrian shale gas exploration in China.

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    Geochemical characteristics of residual gas released from crushed shale from the Shuijingtuo Formation in Yichang, western Hubei—indication for gas-bearing capacity of shale
    MENG Kang, SHAO Deyong, ZHANG Liuliu, LI Liwu, ZHANG Yu, LUO Huan, SONG Hui, ZHANG Tongwei
    Earth Science Frontiers    2023, 30 (3): 14-27.   DOI: 10.13745/j.esf.sf.2022.5.41
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    In this paper, 13 shale samples from the Lower Cambrian Shuijingtuo Formation of well Liuxi ZK003, Yichang area, western Hubei Province were collected for the degassing experiment where residual shale gas released from the crushed shale samples were analyzed. Combined with results on TOC content and mineral composition in shale samples as well as characteristics of desorbed nitrogen-rich shale gas from adjacent area (well ZD1), the origin and source of nitrogen in the Cambrian shale gas were discussed. The total residual gas content in shale samples ranged between 1.45-14.75 μL.STP/g, varying greatly according to the abundance of carbonate minerals. The residual gas was dominated by nitrogen and methane with relative respective abundances of 7.92%-86.27% and 10.10%-90.88%, where the nitrogen abundance was obviously controlled by TOC and clay mineral contents in shale and negatively correlated with residual gas content. The residual gas was mainly hosted in the closed pores of calcite, dolomite, clay minerals and organic matter; whilst nitrogen released from organic matter and amino-clay minerals, under increasing thermal maturation, was the main source of nitrogen in the organic-rich shale formation. In addition, geological processes such as continuous tectonic uplift and denudation might not only cause shale gas leakage, but also lead to nitrogen enrichment in shale. The comparison of nitrogen contents and nitrogen isotopic compositions in Lower Paleozoic marine shale gas from different regions of southern China revealed that the nitrogen content in Cambrian shale gas (2.69%-98.92%) was much higher compared to the Lower Silurian Longmaxi Formation (0.01%-9.30%), and the 15NN2 value in the Shuijingtuo shale gas was obviously higher compared to other areas (15NN2=1.5‰-12.7‰). This enrichment of nitrogen might be related to nitrogen released from high temperature pyrolysis of amino-clay minerals.

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    Identification and comparison of organic matter-hosted pores in shale by SEM image analysis—a deep learning-based approach
    CHEN Zongming, TANG Xuan, LIANG Guodong, GUAN Ziheng
    Earth Science Frontiers    2023, 30 (3): 208-220.   DOI: 10.13745/j.esf.sf.2022.5.45
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    The introduction of deep learning models can greatly improve the efficiency of geological image analysis and thus increase the level of quantitative research. As an example, the Ar-ion polishing scanning electron microscope (SEM) images of shale samples from the Lower Cambrian Niutitang Formation in western Hubei, Upper Yangtze were analyzed using three deep learning models, Mask-RCNN, FCN and U-Net, to identify the minerals, organic matter and pores (basic tasks) after image pretreatment (binarization, etc.) We compared the running time and identification accuracy between the three models, and discussed the model applicability and model differences in geological image recognition and processing. In addition, we compared the best performance model, U-Net model, with the general image processing softwares (JmicroVision, Adobe Photoshop, etc.) in pore recognition. The FCN model performed well in the basic tasks, but could not distinguish the mineral components and fractures with similar colors; whereas the Mask-RCNN model could identify the main minerals with strong segmentation but not low-resolution pores and fractures. In comparison, the U-Net model greatly improved the efficiency of shale geological image recognition with an 300-fold increase in image recognition speed over the general image processing softwares. Applying the U-Net model, the pore structural types of the Niutitang shale of the study area can be divided into circular intra-granular mineral pores, random irregular inter-granular mineral pores, angular organic matter-hosted pores and dense organic matter-hosted micropores. The pore structural parameters obtained based on SEM image analysis of large enough sample size may be used for reservoir classification and evaluation. The example provided in this study may help improving the efficiency of geological image research as well as promoting artificial intelligence application in oil and gas research.

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    Fossil assemblages and their stratigraphic distribution in the Shuijingtuo Formation (Cambrian Series 2, Stage 3) in Yichang area, Hubei Province, China
    PAN Xiaoqiang, HUA Hong, DAI Qiaokun, LUO Jinzhou, LIU Ziwei
    Earth Science Frontiers    2023, 30 (3): 28-43.   DOI: 10.13745/j.esf.sf.2022.5.32
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    In recent years, with the breakthrough of well Eyiye 1, the Cambrian Shuijingtuo Formation (Series 2, Stage 3) has become a new target for shale gas exploration and development in the Middle Yangtze area. Based on previous studies, this paper presents a brief summary of the main fossil types and palaeobiological assemblages and their temporospatial distribution in the Shuijingtuo Formation to provide a reference for shale gas exploration in western Hubei. Well-preserved fossil assemblages occur abundantly in the Early Cambrian sedimentary succession in the Yichang, Zigui, Changyang and Xingshan areas, which provides an excellent geological record of the Cambrian explosion, with rapid metazoan diversification events spanning the Ediacaran-Cambrian transition, making this region an ideal place for studying Cambrian strata and paleontology. Many fossil Lagerstätten, namely the Yanjiahe biota, Shuijingtuo biota (including the Qingjiang biota), Shipai biota and Jingshan Burgess Shale-type biota, were recovered from the bottom part of the Cambrian Terreneuvian upward to Series 2. Among them, the Shuijingtuo biota is comparable with the Niutitang fauna in Guizhou and the Xihaoping biota in southern Shaanxi, and represented by fossils of more than 10 phyla, such as trilobites, radiolarians, brachiopods, bradoriids, hyoliths, archaeocyatha, monoplacophora, lobopodium, protoconodonts, sponges, gastropods, chancelloriids and possibly cnidarians, as well as some unidentified taxa and microflora. Whilst the Qingjiang biota found in Changyang is of typical Burgess Shale-type fossil Lagerstätten and distinguished by soft-bodied taxa, which opens up a new window on the Cambrian explosion in the deep water habitat far away from the coast. The Cambrian Shuijingtuo Formation in the Yichang area developed a black rock system with distinct lithology, palaeobiological assemblages and tectonic movement, which is dominated by black shale or calcareous shale mixed with carbon limestone and contains high abundance of diverse sponge fossils, indicating a deep-water, low-oxygen sedimentary environment at that time. Meanwhile, the flourishing of microbial organisms indicates the ocean at that time possessed high primary productivity that laid a good material foundation for the burial and enrichment of organic matter and formation of shale gas; and the hydrocarbon reservoir did not experience deformation and destruction owing to the Huangling anticline rigid substrate. Therefore, the black rock system of the Shuijingtuo Formation in the Yichang area has high exploration and development potential for shale gas.

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    Significance of water absorption characteristics and difference of pore structures in the Cambrian shale intervals, Yichang area for shale reservoir evaluation
    ZHANG Liuliu, CHEN Gengxin, LE Xingfu, ZHANG Yu, SHAO Deyong, YAN Jianping, MENG Kang, ZHANG Tongwei
    Earth Science Frontiers    2023, 30 (3): 138-150.   DOI: 10.13745/j.esf.sf.2022.5.38
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    The Cambrian Shuijingtuo shale formation of Well ZK003, Liuxi, Yichang area is selected to study its pore development characteristics, where different formation layers are compared through water absorption experiment, helium porosity test, organic carbon content determination, and mineral composition analysis. The results of the study show that the Cambrian shale formation can be divided into members 1-4 from bottom to top, and different members have very different pore characteristics. In general, the higher the TOC content, the greater the water saturation limit (per gram of shale weight); and the higher the initial water absorption rate, the greater the porosity of shale. In shales from the organic-rich members 1 and 2, the TOC content ranges from 1.8% to 11.5%, averaging 4.6%; the helium porosity ranges from 1.6% to 5.8%, averaging 3.9%; and the water absorption saturation limit is between 6.1-21.4 mg/g shale weight, or on average 15.3 mg/g shale weight. Between the two members, shale from member 1 has higher TOC content but significantly lower porosity, water absorption value, and water absorption rate. The carbonate content in member 1 shows a good negative correlation with the water absorption value; whereas a positive correlation is observed for the clay content in member 2. It is believed that in member 1 carbonate cementation hinders the development of micropores and nanopores; while in member 2, development of micropores, which have large specific surface area and autogenic to porous clay, lead to strong water absorption. Overall, member 2 of the organic-rich shale formation has high porosity and good roof/floor isolation by overlaying marl and carbonate, which is conducive to the enrichment and preservation of shale gas, therefore making it a high-quality shale gas reservoir in the study area. The research results can be used for screening high-quality Cambrian shale reservoirs and predicting shale gas exploration “sweet spots”.

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    Fractal characterization of pore structure in Cambrian Niutitang shale in northern Guizhou, southwestern China
    TANG Xuan, ZHENG Fengzan, LIANG Guodong, MA Zijie, ZHANG Jiazheng, WANG Yufang, ZHANG Tongwei
    Earth Science Frontiers    2023, 30 (3): 110-123.   DOI: 10.13745/j.esf.sf.2022.5.36
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    Pore structure is the key factor affecting the distribution and flow of shale gas, and fractal analysis can be used to quantitatively characterize the complex pore structures in shale. To better understand the porous structure of the organic-rich shale from the Lower Cambrian Niutitang Formation in northern Guizhou Province, southeastern China, shale samples from Well ZK, Songtao area, are analyzed to determine the pore parameters using high-pressure mercury injection and low-temperature nitrogen adsorption methods, combined with scanning electron microscope observation, shale geochemistry and mineral composition analysis. The pore fractal dimensions are calculated by FHH model, and the influencing factors for pore structure are discussed. To summarize: (1) the quartz content in the organic-rich shale ranges between 39%-68.4%; clay content, 11.5%-28.2%; organic carbon content, 2.77%-5.81% (average 3.81%); while kerogen is mainly of type I, with high thermal maturity. (2) The BET specific surface area ranges between 11.954-21.744 m2/g (average 14.572 m2/g); total pore volume, 0.0186-0.0259 cm3/g (average 0.0214 cm3/g); and average pore size, 4.773-7.025 nm (average 5.967 nm). Micropores contribute largely to the total specific surface area; while mesopores and macropores account for a large proportion of pore volume. (3) The organic-rich shale has complex pore structure dominated by micropores, with high heterogeneity and multifractal characteristics. The pore fractal dimensions D1 and D2 obtained from low-temperature nitrogen adsorption data have narrow pore-size distributions (D1, 2.65-2.71; D2, 2.79-2.85), while mercury injection data yield a wider pore-size distribution for macropores (DHg between 2.21-2.81). (4) D2 is positively correlated with TOC content and micropore volume, and mineral composition has no significant effect on pore fractal dimensions. The fractal dimension of the Niutitang shale in the study area is similar to that of the gas-producing Longmaxi shale, indicating shales of this area have good pore structure.

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    Differential shale gas generation in the Lower Cambrian Qiongzhusi stage in the Middle-Upper Yangtze region
    HE Chencheng, CHEN Honghan, XIAO Xuewei, LIU Xiuyan, SU Ao
    Earth Science Frontiers    2023, 30 (3): 44-65.   DOI: 10.13745/j.esf.sf.2022.5.33
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    The Lower Cambrian Qiongzhusi black shale of the Middle-Upper Yangtze region is considered the best candidate for shale gas exploration after the breakthrough of shale gas development in the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations because of its high total organic carbon (TOC) content and high hydrocarbon-generating potential. The high-quality argillaceous source rock is mainly distributed in the Mianyang-Changning trough in Sichuan, the Xiang’exi trough in western Hunan and western Hubei, and the deep-water shelf and slope basins in eastern Sichuan and northern Guizhou, with source-rock intervals mainly developed at the bottom of the troughs and in the middle and upper parts of the deep-water shelf and slope basins, as indicated by TOC profile studies. In this paper, fluid inclusion analysis and super-low-concentration U-Pb dating were performed on collected shale specimens, in combination with comprehensive literature research, to investigate the hydrocarbon generation history in the Lower Cambrian Qiongzhusi stage. In addition, the shale gas composition and isotopic characteristics are discussed. There are two possible modes of shale gas generation and accumulation: (1) Source rock developed in western Hunan, western Hubei and eastern Sichuan experienced a single-stage oil/gas generation process, where oil generation started in the late Caledonian (430 Ma), at temperatures of 95.7-105.2 ℃, followed by crude oil cracking in the late Hercynian (270 Ma), at ~144.7℃, wet gas secondary cracking in the early Yanshanian (270-130 Ma), at 215.3 ℃, and shale gas leakage from the late Yanshanian to present (130-0 Ma). (2) Source rock from southern Shaanxi and southern, central and southwestern Sichuan experienced a two-stage oil/gas generation process, where the 1st and 2nd stage oil generation occurred at 421.5 Ma and 262.4-256.4 Ma, at temperatures of 115.8-128.9 ℃ and 137.1-150.0 ℃, respectively; crude oil and/or kerogen cracking in the Mianyang-Changning trough started in the Late Hercynian (259.4±3.0 Ma), at ~140 ℃, on average, followed by wet gas secondary cracking in the middle-late Yanshanian (175-133 Ma), at ~220 ℃, with ongoing shale gas accumulation and reservoir adjustment due to favorable preservation conditions. The differential hydrocarbon generation is controlled by regional tectonic and depositional evolution. Shale gas in the study area is mainly oil-type gas of organic origin, generated from kerogen and crude oil cracking. Due to the development of faults in northern Guizhou and southeastern Chongqing, the early-stage shale gas was washed by surface and atmospheric water, causing changes in its elemental and isotopic compositions and resulting in nitrogen enrichment and decrease of δ13C1 value.

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    Origin of excess barium in the Cambrian shale of Yichang area, western Hubei, and its implication for organic matter accumulation
    LUO Huan, SHAO Deyong, MENG Kang, ZHANG Yu, SONG Hui, YAN Jianping, ZHANG Tongwei
    Earth Science Frontiers    2023, 30 (3): 66-82.   DOI: 10.13745/j.esf.sf.2022.5.34
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    We analyzed the total organic and inorganic carbon, trace elements, and rare earth elements in shale samples from the Lower Cambrian Yanjiahe and lower Shuijingtuo Formations of well EYY1, Yichang area, western Hubei Province. Combining with already published studies and data on other 7 Cambrian sections in the middle-upper Yangtze region, we discussed the origin and source of excess barium (Baex) in the organic-rich Cambrian Shuijingtuo shale and their implications for paleo-productivity. According to our analysis, there is a general barium enrichment in the organic-rich Cambrian shale of the middle-upper Yangtze region. The Baex level increases from the shallow-water inner shelf to the deepwater outer shelf and slope then to the hydrothermal depositional area, which implies that the Baex level increase is controlled by the distance from terrigenous source and by the paleo-geographic background. The excess Ba is of hydrothermal or biogenic origins, and the Baex-Eu anomaly plot can be utilized to infer its main source. The Baex levels are extremely high (>10000 μg/g) in the hydrothermal depositional regions across northern Guizhou to northwest Hunan, with obvious positive Eu anomalies in these regions. The anomaly values are positively correlated with Baex levels, indicating that Baex is mainly of hydrothermal origin and change in Baex reflects the intensity of hydrothermal activity. In contrast, the Baex levels are not as high (5000-10000 μg/g) in western Hubei and southern Guizhou and relatively low (<1000 μg/g) in the western Sichuan Basin, where no positive Eu anomalies are present in these regions, indicating Baex is of biogenic origin and change in Baex is related to paleo-productivity level. By comparing TOC contents, U/Th ratios, and Baex levels in Cambrian shales from different areas of middle-upper Yangtze, we found the Yichang area has a relatively high paleo-productivity level and strong reducing depositional environment, which provides rich organic source and favorable preservation conditions for organic matter accumulation in the Cambrian Shuijingtuo shale formation.

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    Mesozoic intraplate metallogenesis in South China
    HU Ruizhong, GAO Wei, FU Shanling, SU Wenchao, PENG Jiantang, BI Xianwu
    Earth Science Frontiers    2024, 31 (1): 226-238.   DOI: 10.13745/j.esf.sf.2024.1.9
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    Intraplate metallogenesis is a significant global scientific issue. The South China block is renowned for its large-scale mineralization occurring far away from active continental margins during the Mesozoic. It formed a low-temperature metallogenic province of gold, antimony, lead, and zinc deposits in the west (Yangtze block) and a high-temperature metallogenic province of tungsten-tin polymetallic deposits in the east (Cathaysia block), making it an ideal natural laboratory for intraplate metallogenesis studies. The two metallogenic provinces of South China have long been considered distinct systems due to their spatial separation. However, our research revealed that the low-temperature mineralization in the west (230-200 Ma and 160-130 Ma) occurred simultaneously with the high-temperature mineralization in the east, and has similar geochemical fingerprints to the latter. Both types of mineralization were probably driven by the Indosinian intracontinental orogeny and Yanshanian asthenosphere upwelling beneath South China. Therefore, there is a genetic linkage between the two metallogenic provinces, and together they constitute a giant polymetallic domain spreading planarly under intraplate setting. The current distribution status of the low- and high-temperature mineralization types in the west and east is controlled by the eastward increase of denudation degree in South China after ore formation. It is predicted that there may exist high-temperature W-Sn polymetallic deposits beneath the eastern region of the low-temperature metallogenic province. A new intraplate metallogenesis model for South China was established. The significant features of the new model include metallogenesis occurring within preexisting weakness zones of lithosphere, continental crust serving as sources for metallogenic elements, and coexistence of high- and low-temperature mineralization exhibiting a distinct planar distribution. The model differs from the metallogenic mechanism in continental plate margins.

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    Organic matter-hosted pores in the Cambrian Niutitang shales of the Upper Yangtze region: Pore development characteristics and main controlling factors
    MA Zijie, TANG Xuan, ZHANG Jinchuan, ZHAI Gangyi, WANG Yufang, LIANG Guodong, LUO Huan
    Earth Science Frontiers    2023, 30 (3): 124-137.   DOI: 10.13745/j.esf.sf.2022.5.37
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    Shale gas exploration and development is relatively low level in the Lower Cambrian Niutitang Formation in the Upper Yangtze where organic matter-hosted pores (OMPs) developed unevenly across the region. In this paper, shale samples from three wells with different depositional settings—namely well EYY1 in the western Hubei rift trough of eastern Upper Yangtze, well W001-4 in the shallow-water shelf of northern Yangtze, and well SNY1 in the Hannan ancient land of northern Yangtze—were collected and analyzed, and shales were classified according to mineral compositions. Organic matter types and OMP development characteristics were investigated by scanning electron microscope method, and the main controlling factors for OMP development were discussed. The results show that (1) mineral compositions and types of the Niutitang shales vary between different regions. Shale samples from EYY1 and W001-4 were classified into siliceous, calcareous and mixed shales; while in SNY1 only siliceous shale was found. (2) Organic matters in the Niutitang shales were divided into depositional and migrated organic matters. In the three wells, the carbon-to-oxygen elemental ratio (C∶O) in the depositional organic matter (averaging 6.74) was higher than that in the migrated organic matter (averaging 2.71). (3) OMPs were well developed in EYY1 and SNY1, with relatively high surface porosities of 6%-28%; while in W001-4 they were poorly developed, with surface porosities ranging between 3%-10%. The OMP diameter in the three wells showed a unimodal distribution, centering between 2-50 nm in EYY1 and SNY1 and generally between 5-25 nm in W001-4. Organic matter in siliceous shales had the highest surface porosity, higher than in calcareous shales. (4) Migrated organic matter showed higher level pore development compared to depositional organic matter. OMP development was affected by counteraction between pore production from hydrocarbon generation from organic matter and pore reduction from compaction. Clay mineral is another important factor hindering micro-scale OMP development in shales, and siliceous shales rich in clay minerals may be an important clue for the determination of exploration “sweet spots” for Cambrian shale gas resource.

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    Characterization of the shale gas formation process based on fluid inclusion evidence: An example of the Lower Cambrian Niutitang shale formation, Xiushan section, southeastern Chongqing
    LIU Xiuyan, CHEN Honghan, XIAO Xuewei, LI Peijun, WANG Baozhong
    Earth Science Frontiers    2023, 30 (3): 165-180.   DOI: 10.13745/j.esf.sf.2022.5.42
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    Direct geological evidences for the shale gas formation process are scarce. However, a clear understanding of the shale gas formation process in terms of oil/gas generation stages and the relevant temperatures and thermal maturity indicators is essential for shale gas exploration. Fluid inclusions can record the geological information on fluid activities during geological time therefore can provide evidence to reconstructing the fluid evolution history. In this study, shale samples are collected from the Lower Cambrian Liutitang Formation, Xiushan section, southeastern Chongqing to investigate the fluid inclusions entrapped in veins, using integrated fluid inclusion analysis method. Three types of hydrocarbon inclusions are detected, which are bitumen + gas, bitumen and gas inclusions indicating different stages of the shale gas forming process. The shale gas forming process can be divided into four stages: primary oil generation, oil cracking into wet gas, wet gas cracking into dry gas, and shale gas loss, where abnormal high temperatures associated with acid fluids activity play a key role in the process of transforming oil into gas. These findings can be applied to advancing shale gas exploration in areas that have a history of abnormal high temperature associated with thermal fluid activity and experienced relatively weak late-stage tectonic deformation.

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    Mechanism of organic matter enrichment and organic pore development in the Lower Cambrian Niutitang shales in northern Guizhou
    WU Chenjun, LIU Xinshe, WEN Zhigang, TUO Jincai
    Earth Science Frontiers    2023, 30 (3): 101-109.   DOI: 10.13745/j.esf.sf.2022.5.35
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    Based on the Lower Cambrian Niutitang Formation profile from a cored well in the northern Guizhou area of the Upper Yangtze region, the organic matter characteristics, sedimentary environment and pore development mechanism of the Lower Cambrian Niutitang organic-rich shales were systematically studied by measuring organic carbon contents, mineral compositions, major and trace elements in and pore characteristics of typical shale samples. Organic-rich shales are well developed in the middle and lower parts of the Niutitang Formation, with the total organic carbon content (TOC abundance) ranging from 0.36% to 6.67%, or 2.53% on average. The organic-rich shale layer with TOC abundance greater than 2.0% has an overall thickness of about 80 m, and are formed in a strong reductive sedimentary environment according to the U/Th, Mo/Al and U/Al ratios. The pore volume and pore specific surface area in clay-rich shales with medium level TOC in the middle Niutitang Formation are higher than those in clay-poor shales with high TOC content in the bottom section. Intraparticle pores within clay aggregates are well-developed in clay-rich shales with medium level TOC in the middle Niutitang Formation, providing reservoir space for crude oil or asphalt migration during hydrocarbon generation. Abundant organic matter pores are formed by secondary oil cracking under higher temperature conditions during thermal evolution. The clay-rich shale layer with medium level TOC in the middle Niutitang Formation is more favorable for shale gas enrichment compared to the bottom high-TOC shale layer.

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    Biogenic silica of the Lower Cambrian Shuijingtuo Formation in Yichang, western Hubei Province—features and influence on shale gas accumulation
    ZHANG Yu, HUANG Dejiang, ZHANG Liuliu, WAN Chuanhui, LUO Huan, SHAO Deyong, MENG Kang, YAN Jianping, ZHANG Tongwei
    Earth Science Frontiers    2023, 30 (3): 83-100.   DOI: 10.13745/j.esf.sf.2022.5.40
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    Shale samples from the lower Cambrian Yanjiahe-Shuijingtuo formations in Yichang, western Hubei were collected by high-resolution sampling method to investigate the vertical variation of biogenic silica and its significance for shale gas enrichment, by combining quantitative analyses of total organic carbon (TOC), major elements and helium porosity with qualitative microscopic observation using transmission and cathodoluminescence microscope. According to the results, the excess silicon content is low in the organic-poor Lower Cambrian Yanjiahe-upper Shuijingtuo formations containing no visible trace of bioclastic debries and very little or barely no authigenic quartz. While the organic-rich layer of the Shuijingtuo formation has larger quantity of biogenic silicon and is divided into members 1 and 2 as the silicon sources are obviously different. The bottom part (member 1) has high excess silicon content ranging between 4.44%-24.97% (average 16.76%), with significant fluctuation under the influence of calcareous intercalation; while biogenic silicon, mainly siliceous sponge spicules and a few flocculent siliceous aggregates, accounts for a high proportion with little influence from terrigenous clasts; and organic matter could be seen in the spicule cavity and interparticle pores of siliceous aggregates. In contrast, the middle-upper part (member 2), influenced by both biogenic silicon and terrigenous clasts, has significantly lower excess silicon content ranging between 7.13%-20.47% (average 13.66%), with radiolarian, fusiform siliceous aggregates and sponge spicules as the main bioclastic types and mixed with organic matter. TOC is positively correlated with excess silicon in the organic-rich strata, and large numbers of fine granular chalcedony are observed under microscope in lumps of organic matter developed in an algal cyst-like structure, which reflects that the development of high hardness crystalline biogenic silicon, transformed from opal in the organic-rich layer is conducive to the enrichment of organic matter. Compared to member 2, the TOC content in member 1 is significantly higher and porosity relatively low, which may correspond to the difference in biogenic silicon contents in the two members. The close symbiosis between biogenic silicon and terrigenous clasts may play a protective and supporting role in the development and preservation of pores. The quartz origin may be the main factor influencing the gas-bearing property of shales from the organic-rich layers of the Cambrian Shuijingtuo formation in Yichang area.

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    Influence of microscopic fabric on organic matter occurrence and pore development in mudrock: A case study of the Cretaceous Eagle Ford Shale
    SHAO Deyong, LI Yanfang, ZHANG Liuliu, LUO Huan, MENG Kang, ZHANG Yu, SONG Hui, ZHANG Tongwei
    Earth Science Frontiers    2023, 30 (3): 151-164.   DOI: 10.13745/j.esf.sf.2022.5.39
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    The characteristics of organic matter (OM) occurrence and pore development in the Cretaceous Eagle Ford Shale of South Texas, USA were studied at the micrometer scale through analyses of the original oil-window core and artificial gas-window cylinder samples by Ar ion milling/scanning electron micrography (SEM) method, combined with energy-dispersive spectroscopy mapping. Three typical rock fabric types—siliceous-argillaceous seams, foraminifera fossils, and coccolith-rich lenses—were observed under microscope which revealed significantly varying OM occurrences and pore development characteristics. Specifically, in siliceous-argillaceous seams, enriched amorphous kerogen developed few pores within the oil window due to oil sorption until nano spongy pores began to form in large numbers within the gas window as a result of sorbed-oil cracking; whereas structured kerogen, of small quantity, had little or no pore development through the entire petroleum-formation period due to its abnormally low hydrocarbon generation potential. In foraminifera fossil chamber, filling OM represented migrated bitumen that charged during the early stage and developed an abundance of bubbly pores and spongy nanopores within the oil window, and then, with coalesce of nanopores, micron scale large pores began to dominate within the gas window. And in coccolith-rich lenses, filling OM of migrated bitumen charged in the late stage was characterized by abundant spongy nanopores within the oil window, whereas within the gas window abundant mineral intergranular pores re-exposed due to high OM conversion rate. This study demonstrated that the formation and development of OM-hosted pores in marine mudrock were controlled by both OM type and thermal maturity. It provided an important approach to studying the process of hydrocarbon generation, expulsion, and retention as well as revealing the carrier medium of OM-hosted pores and pore preservation mechanisms in mature marine shales in the subsurface by investigating the spatial relationship between microscopic fabric, OM occurrence, and pore heterogeneity in organic-rich mudrock.

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    SEM image characteristics and paleoenvironmental significance of framboidal pyrite from the Lower Cambrian Shuijingtuo Formation in Yichang area, western Hubei Province, southern China: A case study of well EYY1
    SONG Hui, SHAO Deyong, LUO Huan, MENG Kang, ZHANG Yu, TANG Xuan, ZHANG Tongwei
    Earth Science Frontiers    2023, 30 (3): 195-207.   DOI: 10.13745/j.esf.sf.2022.5.44
    Abstract1341)   HTML10)    PDF(pc) (6990KB)(146)       Save

    Framboidal pyrite occurs widely in shales of all ages and its morphological and grain size characteristics can be used as a redox indicator in environmental research. In this study, micromorphology of pyrites in shale samples from the Lower Cambrian Shuijingtuo and Yanjiahe Formations in well EYY1, Yangtze area, was investigated in detail using Ar ion milling and scanning electron micrography (SEM) techniques, where relevant parameters for framboidal pyrite indicative of paleoenvironments were obtained through statistical analysis using image processing software (ImageJ). The results show that various types of pyrites including framboidal, lumpy, euhedral and anhedral pyrites are developed in EYY1. Among them, framboidal pyrite, as the most predominant type, shows obvious changes in gain size (D), microcrystalline grain shape and size (d) and D/d ratio in the vertical direction. The grain shape of microcrystalline framboidal pyrite in the Yanjiahe Formation is mainly octahedral; while in the Shuijingtuo Formation it is mainly dodecahedral or near-spherical in the lower part, octahedral or cubic in the middle part, and cubic or tetrahedral in the upper part, with decreasing microcrystalline grain roundness upward from the bottom part, indicating weaker reducing conditions in the same trend. The average grain sizes of framboidal pyrite in organic-rich shales and microcrystalline framboidal pyrite in the EYY1 core section are 4.43 and 0.338 μm, respectively, and gradually decrease from bottom to top, which is opposite to the upward reduction-to-oxidation trend revealed by geochemical parameters. The D/d ratio for framboidal pyrite is relatively small in the Yanjiahe Formation and largest at the bottom of the Shuijingtuo Formation then gradually decreases upward. These results suggest that framboidal pyrite developed at the boundary between the Shuijingtuo and Yanjiahe Formations may reflect the redox condition prior to the stratigraphical change. The small grain size of framboidal pyrite in the upper Shuijingtuo Formation may suggest an aqueous redox interface and reduced abundance of iron-bearing minerals in the source supply.

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    Fluid inclusion evidence on the shale gas formation process in the Lower Cambrian Niutitang Formation in Jishou slope zone, western Hunan Province—a case study of well XJD 1
    XIAO Xuewei, CHEN Honghan, LIU Xiuyan, PENG Zhongqin, LI Peijun, WANG Baozhong
    Earth Science Frontiers    2023, 30 (3): 181-194.   DOI: 10.13745/j.esf.sf.2022.5.43
    Abstract1323)   HTML3)    PDF(pc) (18517KB)(107)       Save

    Southern China with frequent paleo-fluid activities possesses abundant natural resources. Based on the core data of the Lower Cambrian Niutitang Formation in Jishou slope zone, western Hunan, focusing on the black shale reservoir, we carried out a preliminary investigation of the regional fluid activities and hydrocarbon evolution through thin section, cathodoluminescence and fluid inclusion analyses and other experimental tests. The results show that dolomite, quartz and calcite minerals are deposited in the veins-both single and mixed veins-of the Niutitang Formation. Carbonate veins are mostly associated with mixed thermal fluids (upwelling fluid and seawater). The fluid inclusions are mainly bitumen, methane and saline inclusions. We identified four periods of fluid activities that are associated with hydrocarbon evolution: Early-Middle Ordovician, Early Silurian, Early-Middle Triassic and Palaeocene.

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    Interactions between clay minerals and microbes: Mechanisms and applications in environmental remediation
    DONG Hailiang, ZENG Qiang, LIU Deng, SHENG Yizhi, LIU Xiaolei, LIU Yuan, HU Jinglong, LI Yang, XIA Qingyin, LI Runjie, HU Dafu, ZHANG Donglei, ZHANG Wenhui, GUO Dongyi, ZHANG Xiaowen
    Earth Science Frontiers    2024, 31 (1): 467-485.   DOI: 10.13745/j.esf.sf.2024.1.19
    Abstract1097)   HTML10)    PDF(pc) (3834KB)(148)       Save

    Clay minerals and microbes co-exist in natural environments, and their interaction can influence energy flow and element cycling. Clay minerals provide microbes with physical/chemical protection against environmental stress, as well as nutrients boosting their metabolism. Structural iron in clay mineral is an important electron acceptor/donor for iron-reducing/oxidizing microbes, where in redox environment many iron-reducing/oxidizing bacteria can reduce/oxidize structural Fe(III)/Fe(II) in clay minerals as they gain energy from the redox process. During such process redox microbes can alter the atomic structure of clay minerals through dissolution, transformation and precipitation where secondary minerals are also produced. Clay mineral-microbe interaction plays important role in geochemical cycling of carbon, nitrogen, silicon and phosphorus. Clay mineral can reduce organic carbon bioavailability and mineralization rate through adsorption; whereas under fluctuating redox conditions it can activate molecular oxygen to produce reactive oxygen species to degrade organic matters thus increasing their bioavailability. Through adsorption clay mineral can also reduce extracellular enzyme activity in organic matter degradation. Microbes can affect nitrogen cycling in clays by coupling iron oxidation (reduction) with nitrate reduction (ammonia oxidation) in clay. Phosphorus adsorption on clays and silicon release during weathering can affect the metabolic activity of microbes. Clay mineral-microbe interaction can find a wide range of application in heavy metal stabilization, organic pollutant degradation and sterilization.

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    Developing structural control models for hydrothermal metallogenic systems: Theoretical and methodological principles and applications
    YANG Liqiang, YANG Wei, ZHANG Liang, GAO Xue, SHEN Shilong, WANG Sirui, XU Hantao, JIA Xiaochen, DENG Jun
    Earth Science Frontiers    2024, 31 (1): 239-266.   DOI: 10.13745/j.esf.sf.2024.1.40
    Abstract1093)   HTML7)    PDF(pc) (5137KB)(221)       Save

    A defining feature of a hydrothermal metallogenic system (HMS) is strong structural control on ore mineralization. A systematic analysis of the geometry, kinematics, thermodynamics, and rheology of multiscale ore control structures is crucial for understanding the genesis of HMSs and for ore prospecting. The main challenges include: transitioning from static to multiscale spatiotemporal analysis of the 4D dynamical system involving ore-control structural frameworks, permeability structures, ore-forming fluid pathways, and mineralization deformation networks; identifying key influencing factors of fluid pathways that control ore deposition; and unraveling the mechanism of structure-fluid coupling control of ore formation and localization. This study presents the theoretical and methodological principles and application for developing structural control models for HMSs in the following aspects. (1) The theoretical core. It states that fluid, not structure, is at the core of a structural control model. Fluid flow and ore formation within a hydrothermal system are influenced by the fault zone architecture and permeability structure, where permeability, in linking fluid flow and fluid pressure variation, is key to understanding ore control structures. (2) Stress and pressure dynamics. It considers that differential stress and fluid pressure difference result in diverse combinations of ore control structures, while differences in regional stress field and host rock strength result in variations in mineralization type. (3) Growth of fluid pathways. It considers that fluid pathways initiate from isolated microfractures within the upstream host rocks of overpressured fluid reservoirs which evolve along the direction of the steepest pressure gradient to form new extended fractures through growth and interconnection. These extended fractures eventually interconnect to form fluid pathways. As ore deposition takes place during brief periods of high fluid flux when repeated fault sliding induces rapid changes in fluid pressure, flow velocity, and stress, rapid pressure release—caused by a disruption of dynamic equilibrium in the fluid system due to fluid pathways growth—is a key factor driving metal precipitation. (4) Integrated research. Methodology involves integrating macro and microscopic examination of ore control structures, integrating geological history and stress analysis, combining local and regional analyses, adopting shallow and deep perspectives, and employing a multidisciplinary, multiscale approach to study various ore-controlling factors. (5) Geological mapping. Methodology involves using structure-alteration-mineralization network mapping to characterize alteration-mineralization rock blocks in terms of geometric parameters for ore control structures (such as type, shape, size, occurrence, spacing), and performing quantitative analyses (such as topological analysis of hydrothermal vein-fracture systems, 3D geometric analysis of ore bodies) to determine ore-control structural frameworks and permeability structures and reveal the connectivity of mineralization deformation networks and their ore-forming potential. (6) Numerical modeling. Methodology involves developing geological models, selecting appropriate thermodynamic parameters and dynamic boundary conditions, and utilizing methods such as HCh and COMSOL to perform quantitative simulation of spatiotemporal variations in fluid flow, heat-mass transfer, stress deformation, and chemical reactions during ore formation. This is an effective approach to unveil the mechanism of ore formation controlled by structure-fluid coupling and ore localization pattern, predict ore-forming centers, and identify mineral exploration targets. Based on the above principles, this paper proposes a research methodology for model building, focusing on deriving metallogenic models and ore deposition patterns based on structure-fluid coupling control. Briefly, hydrothermal veins-fracture systems and structure-alteration-mineralization networks are selected as primary research subjects. Research methods include geometric description, kinematic assessment, rheological/dynamic analyses, and thermodynamic synthesis, seeking to delineate ore-control structural frameworks, identify mineralization centers, trace the developments of ore-forming fluid pathways and various mineralization styles, and reveal the spatiotemporal evolution patterns of permeability structures. Additionally, the causal relationship between tectonic reactivation and ore localization is explored. Finally, a metallogenic model based on structure-fluid coupling is constructed to support strategic mineral exploration. This research methodology was applied for mineral prediction in the Jiaojia gold field, Jiaodong Peninsula; its validity and effectiveness were tested and approved.

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    Integrated chronostratigraphic framework for Cretaceous strata in the Songliao Basin
    WU Huaichun, LI Shan, WANG Chengshan, CHU Runjian, WANG Pujun, GAO Yuan, WAN Xiaoqiao, HE Huaiyu, DENG Chenglong, YANG Guang, HUANG Yongjian, GAO Youfeng, XI Dangpeng, WANG Tiantian, FANG Qiang, YANG Tianshui, ZHANG Shihong
    Earth Science Frontiers    2024, 31 (1): 431-445.   DOI: 10.13745/j.esf.sf.2024.1.22
    Abstract1092)   HTML5)    PDF(pc) (8617KB)(117)       Save

    The Cretaceous is a typical greenhouse period with global large scale formation of organic-rich source rocks. However, we have only limited understanding of the terrestrial environmental status, climate change and biosphere responses and evolution during the greenhouse period; the effect of climate and environmental changes on the terrestrial organic accumulation and large scale source rock formation remains unclear. To study these issues the Songliao Basin in northeastern China offers an unique opportunity. The Songliao Basin has widespread distribution of Cretaceous source rocks and possesses the most complete sedimentary record of the Cretaceous in the world. One challenges, however, was the lack of high-quality rock records due to low continuity and low completeness of the records, which has caused controversies over the stratigraphic division of the Songliao Basin. Recently, the 8197 m drill core obtained from ICDP (Continental Scientific Drilling Program) boreholes (SK-1, SK-2, SK-3) provided a virtually complete terrestrial stratigraphic record of Cretaceous strata of the basin. In this study, by evidence synthesis using results of lithostratigraphy, biostratigraphy, magnetostratigraphy, radioisotope geochronology, and cyclostratigraphy, we constructed a high-resolution, integrated Cretaceous chronostratigraphic framework for the Songliao Basin, which provided fine chronological reference for studying the evolution of deep-time Cretaceous climate and environment as well as promoting sustainable oil and gas development in the Songliao Basin.

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    Geothermal resources exploration and development technology: Current status and development directions
    SUN Huanquan, MAO Xiang, WU Chenbingjie, GUO Dianbin, WANG Haitao, SUN Shaochuan, ZHANG Ying, LUO Lu
    Earth Science Frontiers    2024, 31 (1): 400-411.   DOI: 10.13745/j.esf.sf.2023.9.25
    Abstract1090)   HTML9)    PDF(pc) (4567KB)(109)       Save

    Geothermal energy is an important non-carbon-based renewable energy with several advantages such as local availability, deployment stability/reliability, and low carbon emissions. It represents an unique resource for ensuring energy security and promoting green, low-carbon transition. Since the beginning of the 21st century China's geothermal industry has experienced rapid development, making it a world leader in direct geothermal use, particularly in direct utilization of medium and deep geothermal resources for heating. However, as the majority of China's land regions are within tectonic plates and the country lacks medium-high temperature geothermal resources in the energy-intensive regions in the east, the development of geothermal power generation in China has been slow. In this study, the main distribution characteristics and development status of geothermal resources in China are discussed, and the current geothermal exploration, development, and utilization technologies are summarize, which involve exploration techniques based on geothermal genesis models, site selection evaluation techniques, thermal storage description technologies, sustainable development techniques, and key engineering technologies related to “heat extraction without water consumption.” In order to increase the geothermal energy market size amid energy transition, it is necessary to tap into deep geothermal resources with higher quality and broader applications in the future. Recommendations from this study include continuing strengthening basic theoretical research and technical innovation, inventorying China's deep geothermal resources as early as possible, addressing key technology challenges (e.g., high-temperature drilling/completion, complex-structure well, deep thermal storage retrofitting, underground heat exchange, EGS), and promoting synergistic development of “geothermal plus” multisource energy. Efforts should also be made to increase construction of demonstration projects and foster application market development. At the same time, it is essential to establish a sound policy and regulatory system, increase policy support, strengthen management and supervision, and create a favorable environment for the healthy, standardized, and sustainable development of the geothermal industry.

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