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    Introduction
    Introduction
    ZHANG Tongwei, TANG Xuan, CHEN Honghan
    2023, 30(3): 0-00. 
    Abstract ( 118 )   PDF (518KB) ( 95 )   PDF (English) (129KB) ( 36 )  
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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
    2023, 30(3): 1-13. 
    DOI: 10.13745/j.esf.sf.2022.5.31

    Abstract ( 1977 )   HTML ( 42 )   PDF (5509KB) ( 268 )  

    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
    2023, 30(3): 14-27. 
    DOI: 10.13745/j.esf.sf.2022.5.41

    Abstract ( 1645 )   HTML ( 14 )   PDF (7452KB) ( 137 )  

    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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    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
    2023, 30(3): 28-43. 
    DOI: 10.13745/j.esf.sf.2022.5.32

    Abstract ( 1564 )   HTML ( 12 )   PDF (5910KB) ( 172 )  

    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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    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
    2023, 30(3): 44-65. 
    DOI: 10.13745/j.esf.sf.2022.5.33

    Abstract ( 1514 )   HTML ( 12 )   PDF (21976KB) ( 132 )  

    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
    2023, 30(3): 66-82. 
    DOI: 10.13745/j.esf.sf.2022.5.34

    Abstract ( 1514 )   HTML ( 5 )   PDF (8991KB) ( 138 )  

    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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    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
    2023, 30(3): 83-100. 
    DOI: 10.13745/j.esf.sf.2022.5.40

    Abstract ( 1407 )   HTML ( 7 )   PDF (19344KB) ( 150 )  

    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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    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
    2023, 30(3): 101-109. 
    DOI: 10.13745/j.esf.sf.2022.5.35

    Abstract ( 1414 )   HTML ( 6 )   PDF (5052KB) ( 140 )  

    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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    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
    2023, 30(3): 110-123. 
    DOI: 10.13745/j.esf.sf.2022.5.36

    Abstract ( 1536 )   HTML ( 9 )   PDF (8826KB) ( 163 )  

    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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    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
    2023, 30(3): 124-137. 
    DOI: 10.13745/j.esf.sf.2022.5.37

    Abstract ( 1426 )   HTML ( 5 )   PDF (17083KB) ( 155 )  

    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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    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
    2023, 30(3): 138-150. 
    DOI: 10.13745/j.esf.sf.2022.5.38

    Abstract ( 1511 )   HTML ( 4 )   PDF (7446KB) ( 116 )  

    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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    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
    2023, 30(3): 151-164. 
    DOI: 10.13745/j.esf.sf.2022.5.39

    Abstract ( 1402 )   HTML ( 4 )   PDF (10293KB) ( 115 )  

    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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    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
    2023, 30(3): 165-180. 
    DOI: 10.13745/j.esf.sf.2022.5.42

    Abstract ( 1436 )   HTML ( 9 )   PDF (16723KB) ( 151 )  

    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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    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
    2023, 30(3): 181-194. 
    DOI: 10.13745/j.esf.sf.2022.5.43

    Abstract ( 1330 )   HTML ( 3 )   PDF (18517KB) ( 111 )  

    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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    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
    2023, 30(3): 195-207. 
    DOI: 10.13745/j.esf.sf.2022.5.44

    Abstract ( 1350 )   HTML ( 11 )   PDF (6990KB) ( 156 )  

    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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    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
    2023, 30(3): 208-220. 
    DOI: 10.13745/j.esf.sf.2022.5.45

    Abstract ( 1633 )   HTML ( 24 )   PDF (6749KB) ( 212 )  

    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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    Special Section on The India-Eurasia Collision and Its Long-Range Effect (Part 7)
    Deep crustal structure of the southeastern Lhasa Terrane
    XU Xiao, YU Jiahao, XIANG Bo, GUO Xiaoyu, LI Chunsen, LUO Xucong, TONG Xiaofei, YUAN Zizhao, LIN Yanqi, SHI Hongcheng
    2023, 30(3): 221-232. 
    DOI: 10.13745/j.esf.sf.2023.2.10

    Abstract ( 169 )   HTML ( 11 )   PDF (13497KB) ( 173 )  

    The southern Lhasa Terrane is the foremost area to investigate plate boundary interactions in the India-Eurasia collision zone. Prior to Cenozoic continental collision, the southeastern Lhasa Terrane underwent multi-stage tectonic evolution under Mesozoic subduction of the Tethyan oceanic lithosphere, which resulted in a complex regional crustal structure that renders inconsistent observations of its deep structure using different geophysical methods with different resolving power. Presently, broadband seismic observation and deep seismic reflection data disagree on the location of the subduction front at the Indian plate. To gain further insight, a short-period dense array survey line was laid parallel to the previous survey line. By using high-resolution P-wave receiver functions, we discovered that the previously defined northward extension of the Indian lower crust based on broadband data is actually not continuous. In fact, the Indian crust exists only beneath the Yarlung Zangbo suture zone, which is consistent with deep seismic reflection results. The middle and upper crust of the Lhasa Terrane develops a thrust fault under south-north compression, while lower crustal eclogitization observed in the broadband seismic data is mainly located in the north of the Yarlung Zangbo suture zone in the middle Lhasa Terrane and southern part of the North Lhasa Terrane.

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    Attributes and evolution of the eastern massif in the Qinghai-Tibetan Plateau
    LIU Xiaoyu, YANG Wencai, CHEN Zhaoxi, QU Chen, YU Changqing
    2023, 30(3): 233-241. 
    DOI: 10.13745/j.esf.sf.2023.2.5

    Abstract ( 207 )   HTML ( 3 )   PDF (12127KB) ( 223 )  

    Using high-quality seismic data collected from a large number of local seismic stations, we have achieved high-resolution imaging of the three-dimensional velocity structure of the crust and upper mantle in the Qinghai-Tibetan Plateau with an accuracy of 0.5°×0.5°×10 km, which revealed in great detail the crustal and upper mantle structure of the Qinghai-Tibetan Plateau and provided new evidence for understanding the dynamics of continental collision and plateau evolution. Then, based on the 3D seismic P-wave velocity data, a 3D depth map of the lithosphere-asthenosphere boundary beneath the Qinghai-Tibetan Plateau is obtained. It is found that there are fundamental differences in geological attributes between the eastern and western parts of the plateau: The eastern part is dominated by thick lithosphere with high wave velocity, high resistivity and density, and a thickness range of 150-180 km; whereas the west is dominated by thin lithosphere with low wave velocity, low resistivity and density, and thickness ranging between 130-155 km. Furthermore, there is no large-scale asthenospheric upwelling in the eastern part whereas ~20-30 km upwelling has occurred in the west. The coordinates of the two endpoints of the east-west boundary are (20°N, 85°E) and (40°N, 98°E) respectively. According to the paleomagnetic data, the eastern massif in the Qinghai-Tibetan Plateau is a stress transition zone situated between the western continent-continent subduction zone and Southeast Asia ocean-continent subduction zone since 40 Ma.

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    Structure, composition and evolution of the Indosinian South Qiantang accretionary complex
    WANG Genhou, LI Dian, LIANG Xiao
    2023, 30(3): 242-261. 
    DOI: 10.13745/j.esf.sf.2023.1.12

    Abstract ( 149 )   HTML ( 6 )   PDF (12232KB) ( 192 )  

    Orogenic belts can be divided into three types: accretionary, collisional and intraplate-type. Accretionary orogenic belts are of great significance for understanding the subduction and closure history of the ancient oceanic basin, as they can preserve the most geological information during oceanic subduction. Whereas to clarify the tectonic significance of accretionary orogens, it is critical to identify the basic units of the ancient subduction zone, which include trenches, accretionary complexes, forearc basins, magmatic arcs, and back-arc basins, where information on the accretionary complexes, the main product of oceanic subduction, such as their composition and structure as well as their recorded structural deformation and metamorphic history, can directly reveal the dynamic evolutionary process and accretionary orogenesis during plate convergence. The South Qiangtang accretionary complex is a recently identified geological unit of central Tibet. It has the potential to answer such scientific questions as the formation and evolution of the Paleo-Tethys Ocean and the origin of the Central Qiangtang uplift, which is of importance for resource and energy exploration. However, there are many unknowns about the South Qiangtang accretionary complex, including its tectonic attribute, dynamic process, subduction zone structure, and deep geological process. In this paper, we aim to clarify the composition, structure, and geological evolution of the South Qiangtang accretionary complex through detailed studies on the ancient-subduction-zone identification, composition and structure of accretionary complexes, and exhumation mechanism of high-pressure metamorphic rocks, in order to provide a theoretical basis for understanding the regional metallogenic evolution and carbon mineralization pathway and guiding resources prospecting. In the central Qiangtang, we identified a relatively complete trench-arc-basin system consisting of continental margin arc, fore-arc basin, trench slope basin, and high-pressure metamorphic rock-bearing accretionary complex, evidencing an ancient subduction zone. On the plane, the South Qiangtang accretionary orogenic belt presents “mylonitic structures” at different scales, reflecting the extensive and strong ductile shear rheology during plate subduction; vertically, it exhibits an obvious double-layer structure, where the upper layer is a fold-thrust belt formed from deformation of the South Qiangtang terrane, while the lower layer comprises multi-stage deformations, reflecting the important role of subduction-at-continental-margin in the orogenic process. In addition, differential metamorphic evolution of high-pressure metamorphic rocks is obvious across the orogenic belt, and the exhumation mechanism is complex and diverse, reflecting the complex deep process of the subduction zone. Therefore, according to our research and previous data, the South Qiangtang accretionary orogenic belt is the product of Permian-Early Triassic subduction and accretion of the Longmu Co-Shuanghu Paleo-Tethys Ocean that expanded during the Late Devonian-Early Carboniferous period, and formed in the Middle-Late Triassic during the collision of the North and South Qiangtang terranes.

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    Tectonic evolution and Cenozoic deformation history of the Qilian orogen
    WU Chen, CHEN Xuanhua, DING Lin
    2023, 30(3): 262-281. 
    DOI: 10.13745/j.esf.sf.2022.12.20

    Abstract ( 646 )   HTML ( 41 )   PDF (14534KB) ( 573 )  

    The Qilian orogen—formed along the northern margin of the eastern Tethys as results of pre-Cenozoic multi-phase subduction, continental collision and punctuated orogeny involving the North China craton and the Qaidam paleocontinent—develops widespread ophiolitic mélange belts and (ultra-) high pressure metamorphic and arc igneous rocks. The present Qilian Mountains, a key tectonic zone undergoing plateau uplift/expansion along the northern margin of the Tibetan Plateau, with complex intracontinental deformation and deep structures, records the histories of tectonic deformation and basin-mountain evolution during different stages of plateau growth in the Cenozoic. This paper, on the basis of comprehensive analysis of regional geological data, discusses the nature of Proterozoic metamorphic basement, paleo-oceanic evolution during the Neoproterozoic-Paleozoic, and Mesozoic-Cenozoic structural deformation, and explores the tectonic evolution of the Qilian orogen and the intracontinental deformation history of the Qilian Mountains. The Early-Neoproterozoic and Early-Paleozoic arcs represent respectively subduction-collision events took place in the Paleo-Qilian and (South/North) Qilian oceans. Basement structure beneath the North China craton suggests that the Qilian ocean is not the ocean separating the Gondwana and Laurasia continents, but rather a relatively small embayed sea along the southern margin of the Laurasia continent. The northeastern margin of the Tibetan Plateau experienced two-stage tectonic deformation and basin-mountain evolution in the Cenozoic, while transition from Early-Cenozoic thrust activity to joint action of strike-slip/thrust faults occurred in the Miocene, where, with rapid uplift of the Eastern Kunlun Range, a large Paleogene basin split into two basins—the current Qaidam Basin and the Hoh Xil Basin. Since the Middle-Late Miocene the tectonic framework along the margin has been mainly controlled by the development and clockwise rotation/lateral growth of two large near-parallel transpressional tectonic systems, of Eastern Kunlun and Haiyuan. The growth process and development mechanism of the large-scale strike-slip fault system in the Qilian orogen is a central issue of research on intracontinental deformation and requires in-depth quantitative examination.

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    Genesis of the Baikal Rift and the Fenwei Graben and the remote effects of the Indo-Eurasian collision
    LIANG Guanghe
    2023, 30(3): 282-293. 
    DOI: 10.13745/j.esf.sf.2023.1.30

    Abstract ( 212 )   HTML ( 37 )   PDF (14938KB) ( 352 )  

    The Baikal Rift and the Fenwei Graben are two famous rift systems located in the eastern Eurasian plate, with similar topographic features and geomorphic characteristics. Their tectonic evolution histories are also similar. Many studies have shown that their formation is closely related to the Indo-Eurasian collision, however, the mechanism for this long-range effect is not clear. In this paper, basing on the evolution of extensional tectonics caused by mantle upwelling in the continental margin during the continental drift and continental margin splitting, combined with the Cenozoic tectonic evolution of Eurasia, we studied the evolution of microcontinental fragments and extensional tectonics under the long-range effect. The results show that the origin of the two rift systems is closely related to the Cenozoic large-scale fragmentation and drift of microcontinents in the eastern margin of the Eurasian. Briefly, the Qinghai-Tibet Plateau uplift results in large-scale mantle upwelling and continental breakup in the Cenozoic, where the breakup and drift of the proto-Japan/proto-Kamchatka blocks form a differential sinistral strike-slip environment, and lead to the formation of the two rift systems in two seismic zones on the southeast side of the Siberian and Ordos Basins. These two seismic zones, which have similar genetic mechanisms to the Türkiye earthquake in February 2023, are formed by strike slip faults, and their genetic mechanisms can be reasonably explained by the new continent drift model.

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    Quantum Science and Big Data: Two powerful tools that drive rapid advancements in geology
    JIAO Shoutao, ZHANG Qi, TANG Jun, YUAN Jie, WANG Zhen, CHEN Wanfeng, CAI Hongming, WANG Yue
    2023, 30(3): 294-307. 
    DOI: 10.13745/j.esf.sf.2022.12.53

    Abstract ( 255 )   HTML ( 23 )   PDF (1760KB) ( 267 )  

    Among the five branches of basic science (mathematics, physics, chemistry, biology and geology), quantum physics, quantum mathematics (including quantum computing and quantum astronomy) and quantum chemistry are currently driving the rapid development of modern science, and remarkable progress has been made in quantum biology. By comparison, quantum geology is lagging behind. To address this issue, we suggest that both quantum science and Big Data should be emphasized in geological research. However, the scientific community has yet to reach a consensus on the following two questions: (1) Is the “quantum geology” idea reasonable? (2) Can quantum approach be adopted in geological research? Here we summarize the development, application and future of quantum technology, discuss the urgency and feasibility of conducting quantum geology research in China, and suggest several potential research areas. As Big Data technology has been widely used in geological research, we point out several challenging issues, which include Big Data and paradigm revolution, Big Data thinking, Big Data and artificial intelligence, Big Data and pragmatism, as well as the prospect of and challenges in geological Big Data research. Finally, we suggest that a forward-looking approach should be taken in promoting quantum geology research, and that geological Big Data research should be greatly strengthened and the data-sharing problem in geological research should be addressed.

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    Nanogeology brings quantum science to geology
    LIU Rui, ZUO Lei, ZHANG Peng, ZHAO Zongchao, TAO Gang, WANG Kun, TAO Dongping
    2023, 30(3): 308-312. 
    DOI: 10.13745/j.esf.sf.2022.4.65

    Abstract ( 180 )   HTML ( 12 )   PDF (3164KB) ( 209 )  

    Nanogeology is the study of nanoscale (1-100 nm) geological phenomena and geological nanomaterials. Nanoscale material is the smallest indivisible unit of macro material, and already it has shown excellent quantum properties. Due to this unique quality of nanomaterials, nanogeology currently is the most plausible discipline for the geological application of quantum science, as studies have shown that natural nanomaterials (such as mineral nanoparticles) occur in various geological bodies, and some nanoscale geological bodies have shown quantum phenomena such as quantum entanglement, which has unique advantages in the information transmission and data processing. These results provided a solid foundation for integrating quantum science, especially quantum entanglement, into geology, and also provided a possibility for solving geological problems by using quantum properties. Presently, however, geological applications of quantum science as well as nanogeology are still in the early stage of research, and there are still many unknown nanomaterials and nanoscale phenomena to be discovered and studied, only then quantum science can truly enter geology.

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    Carbon sequestration, transport, transfer, and degassing: Insights into the deep carbon cycle
    CHEN Xueqian, ZHANG Lifei
    2023, 30(3): 313-339. 
    DOI: 10.13745/j.esf.sf.2022.12.51

    Abstract ( 513 )   HTML ( 33 )   PDF (7116KB) ( 430 )  

    Carbon plays a fundamental role in subduction zones in melting enhancement, magma genesis and evolution, and petrological/thermodynamic processes in the deep Earth. The occurrence state of carbon in the deep Earth is controlled by temperature, depth (pressure), oxygen fugacity, and fluid property. When carbon of various occurrence states is transported to the deep Earth via subducting slab and then returns to the atmosphere through degassing, the so-called ‘deep carbon cycle’ is realized. Carbonation/decarbonation reactions are the main mechanisms affecting carbon transfer between the solid Earth, the atmosphere, and the oceans. Carbonation processes include silicate weathering, hydrothermal alteration, trench outer-rise serpentinization, organic carbon burial, and reverse weathering; while carbon transport is achieved by subduction of depositional and metasomatic sediments. Surface carbon, when transported to the Earth’s interior, may be retained within the subducting slab, transferred into the upper mantle wedge, or recycled into the deep Earth depending on the depth and redox state under specific tectonic settings; that carbon is then returned to the atmosphere via decarbonation mechanism through volcanic degassing, diffuse degassing in the forearc, dissolution, metamorphism, and melting to maintain a carbon balance at subduction zones. This systematic review summarizes the carbon occurrence states, carbon movements and change of carbon-bearing phases during carbon sequestration, transport, transfer, and degassing relevant to deep carbon cycling and the related carbon fluxes, analyzes the reasons for the inconsistencies in carbon-flux estimates, and discusses future research directions. Since the industrial revolution anthropogenic CO2 emission has contributed greatly to global warming, exerting extra pressure on Earth as a self-regulating system. In the context of transition to a low-carbon economy, China adheres to energy conservation, carbon emission reduction, and forest growth, and aims to peak CO2 emissions by 2030 and achieve carbon neutrality by 2060 to address the world climate crisis.

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    Processes and mechanism of lithospheric thinning in the eastern North China Craton during the Early Cretaceous: Evidence from the Beidashan pluton, Liaoning Province
    LUO Niangang, GAO Lianfeng, ZHANG Zhenguo, YIN Zhigang, CUI Jianyu, WU Junfei, XING Jie, DING Kai, GAO Chenyang, WANG Yue
    2023, 30(3): 340-365. 
    DOI: 10.13745/j.esf.sf.2022.9.2

    Abstract ( 153 )   HTML ( 11 )   PDF (15047KB) ( 149 )  

    The North China Craton (NCC) experienced pervasive lithospheric thinning in the Early Cretaceous, but the thinning process and thinning mechanism are still controversial. To shine a new light on the thinning events this paper focuses on the Beidashan Pluton of Liaoning Province, eastern NCC to conduct detailed field geological survey, combined with laboratory research including petrographical, geochemical, U-Pb zircon chronological and Lu-Hf isotopic analyses, so as to explore the formation age, source area and petrogenesis of the Beidashan pluton. The lithology of the Beidashan pluton is porphyritic biotite monzogranite which has a weighted mean age of 117.1±1.4 Ma by zircon U-Pb dating. The quasi peraluminous porphyritic biotite monzogranite is rich in silicon (69.15%-69.87%) and alkali (8.46%-8.66%) and poor in magnesium (0.80%-0.90%) and calcium (2.03%-2.04%), and is enriched in large ion lithophile elements (LILEs) (Rb, Th, K, Ba) and depleted in high field strength elements (HFSEs) (Nb, Ta, Ti). The rare earth element (REE) pattern is right-leaning, showing a steep linear slope for light REE (LREE) and flattening of heavy REE (HREE). The zircon saturation temperature of granite melt (763.14-768.87℃) is clearly lower than that of A-type granite (> 800 ℃). The above characteristics generally represent high-k, calc-alkaline I-type granitoids. Zircon εHf(t) values range from -21.49 to -19.04, and the two-stage Hf model ages (tDM2) range between 2536-2383 Ma, indicating the monzogranite is mainly derived from partial melting of Paleproterozoic ancient basaltic crust. According to the chronological and petrogeochemical analyses, the Beidashan pluton was formed under regional extensional environment caused by slab roll-back of the paleo-Pacific plate during the Early Cretaceous, and the upwelling asthenosphere caused large-scale delamination of the deep lithosphere of the North China Craton resulting in lithospheric thinning.

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    Formation and evolution of strike-slip fault zones in the eastern Sichuan Basin and identification and characterization of the fault zones: A case study of the Fuling area
    ZENG Tao, FAN Rui, XIA Wenqian, ZOU Yutao, SHI Siyu
    2023, 30(3): 366-385. 
    DOI: 10.13745/j.esf.sf.2022.12.58

    Abstract ( 323 )   HTML ( 16 )   PDF (26363KB) ( 192 )  

    Basement strike-slip fault zones of petroliferous basins are newly recognized high-yield hydrocarbon enrichment zones with broad prospects for oil/gas exploration and development. Increasingly more attention has been paid to the identification and geometrical/kinematical characterization of basement strike-slip faults as well as the role of “reservoir/pool controls” in this type of fault systems. This paper reports the new discovery of large-scale NW and NE-trending basement strike-slip fault zones in Fuling area, eastern Sichuan Basin, and establishes the identification criteria for strike-slip faults in this area based on characteristic fault patterns. The characteristic fault patterns include vertical faults and flower-like structures on the profile; planar linear extension of fault with staggered geological boundaries; and obvious segmentation of master faults in “ribbon-like” or “dolphin-like” strike-slip fault patterns and with changing mechanical properties. A new method is proposed for determining the strike-slip directions based on seismic analysis to measure the change of vertical displacements and the difference in tectonic-activity intensities between the two sides of the fault. It is found that the NW-trending basement strike-slip fault zone generally presents left-lateral slip but with variable strike-slip deformations in different evolutionary stages. Since its formation, the NW-trending faults zone has experienced at least 5 left-lateral and 3 right-lateral slip events which can be characterized by left-lateral slip in the deep crust, right-lateral slip in the shallow crust, high activity-intensity in the north and low activity-intensity in the south, with a cumulative left-lateral strike-slip offset reaching up to 1400-3400 m. In comparison, the NE-trending basement strike-slip fault zone is characterized by right-lateral strike-slip, and the cumulative strike-slip offset is about 1930 m. It suggests that there are significant differences in the formation and evolution of the two fault zones controlled mainly by regional tectonic movements, evolution of stress field environment in different evolutionary stages as well as stress release of regional detachment layers where the NW-trending strike-slip fault zone is affected by Lower-Triassic gypsum-salt detachment layer and the NE-trending strike-slip zone is affected by Middle-Lower-Silurian shale detachment layer.

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    Zircon and apatite fission track chronology of intrusive rocks in the Bainaimiao Cu-Au deposit, Inner Mongolia
    LÜ Zhao, XU Zhan, PANG Jianzhang, WANG Jichun, WANG Jianping, YUAN Shuopu
    2023, 30(3): 386-398. 
    DOI: 10.13745/j.esf.sf.2022.12.57

    Abstract ( 179 )   HTML ( 1 )   PDF (8469KB) ( 89 )  

    Porphyry copper deposits often experience late-stage uplifting, denudation, destruction and transformation after deposition. The Bainaimiao Cu-Au deposit in Inner Mongolia is one of the few Early-Paleozoic porphyry copper deposits in the orogenic belt of northern China, therefore, it is of both theoretical and practical (in mineral prospecting) significance to study the mineral alteration/preservation processes. According to thermochronological analysis of zircon fission tracks and apatite fission tracks, the total denudation of the Cu-Au mining area since 150 Ma is between 3.11-3.25 km, and the late-stage uplifting and cooling of intrusive rocks in the study area can be characterized by a three-stage process. During the rapid exhumation stage between 150-90 Ma, the tectonic stress is mainly compressional, and the average denudation rate is 0.0457 mm/a; in the stable denudation stage from 90 to 20 Ma, extensional stress dominates, and the average denudation rate is 0.0033 mm/a; and during the rapid exhumation stage from 20 Ma to present, compressional stress again dominates, and the average denudation rate is 0.0388 mm/a. Thus, from the Caledonian to Yanshanian, affected by subduction and collision of the ancient Asian Ocean and intracontinental orogenic activities, the mining area is under tectonic stresses alternating between compressional and extensional, which provides favorable geological conditions for mineral alteration/preservation in the Bainaimiao Cu-Au deposit.

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    Application of magnesium stable isotopes for studying important geological processes—a review
    LIU Jiawen, TIAN Shihong, WANG Ling
    2023, 30(3): 399-424. 
    DOI: 10.13745/j.esf.sf.2022.10.46

    Abstract ( 306 )   HTML ( 14 )   PDF (8777KB) ( 284 )  

    Magnesium (Mg) is the fourth most abundant major rock-forming element on Earth (after O, Fe and Si). It has three naturally occurring stable isotopes, 24Mg, 25Mg and 26Mg, among them the relative atomic mass difference between 26Mg and 24Mg is up to 8.33%. Such a large relative atomic mass difference can result in variable degrees of Mg isotope fractionation in different geological processes, thus making Mg stable isotopes effective tracers for studying various geological evolution processes. In recent years, improvements of isotope analytical methods and the development of isotope-ratio mass spectrometry have greatly expanded the use of Mg stable isotopes in geological research, and the related publications have been comprehensively reviewed. Here, following a brief introduction to the reference materials and analytical methods for Mg isotope analysis, this paper discusses in detail the application of Mg isotope in four geological research fields, including applications 1) in ore genesis studies as effective tracers for probing mineralization processes and source of ore-forming materials; 2) in lamprophyre genesis studies for compositional analysis of source materials; 3) in geothermometer research, where four common mineral-pair Mg isotope geothermometers are reviewed and their applicability analyzed; and 4) in the study of Mg isotopic fractionation during metamorphic evolution, peritectic reaction and magmatic processes involving garnet-bearing rocks, and its implications. This systematic review is aimed to deepen the understanding of Mg stable isotopes and further demonstrate their broad application prospects.

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    The Upper-Lower Cretaceous boundary in the southern Songliao Basin: A case study of ICDP borehole SK-3
    GAO Hang, WANG Pujun, GAO Youfeng, WAN Xiaoqiao, YANG Guang, HU Jingsong, WU Huaichun
    2023, 30(3): 425-440. 
    DOI: 10.13745/j.esf.sf.2023.1.28

    Abstract ( 203 )   HTML ( 15 )   PDF (10416KB) ( 180 )  

    The internationally recognized absolute isotopic age of the Upper-Lower Cretaceous stratigraphic boundary is 100.5 Ma, and the Global Boundary Stratotype Section and Points (GSSP) for the boundary is placed in marine strata; however, there have been no reports of GSSP (“golden spikes”) for terrestrial boundaries anywhere in the world. The terrestrial Upper-Lower Cretaceous stratigraphic boundary is of great value in understanding the evolution of paleogeography, paleoenvironment and paleoclimate on land under the greenhouse conditions in the Middle Cretaceous, and the ideal geological records for studying this boundary may be found in the Songliao Basin, northeastern China, where a complete set of Cretaceous continental strata are developed. Here, we investigated the rock types, lithologic sequence and sedimentary facies in the core section of International Continental Scientific Drilling Program (ICDP) borehole SK-3, southern Songliao Basin. Based on the core description, gray and gray-green rhyolitic lithic crystalline tuffite deposit is found in the coastal-shallow lacustrine sedimentary sequences of the upper part of the second member of the Quantou Formation at 1191.6 m depth. The tuffite deposit, according to zircon U-Pb dating of its syndepositional magmatic components, has an zircon age of (96.8±2.9) Ma and belongs to the Middle-Cenomanian period. Combined with previous data, the deposition rate for the Quantou-Denglouku Formations in the study area is 90.54-110 m/Ma, and we conclude accordingly that the Upper-Lower Cretaceous stratigraphic boundary is located in the middle of the first member of the Quantou Formation in borehole SK-3 (at 1526.6-1598.6 m depth). Considering analytical errors in the age-dating results, the Upper-Lower Cretaceous stratigraphic boundary in borehole SK-3 is independently calculated to be between the middle of the second member of the Quantou Formation and the upper part of the fourth member of the Denglouku Formation (at 1279.6-1917.6 m depth). Other researchers have tentatively placed the boundary in the Denglouku Formation based on the chronological data of ICDP borehole SK-1 and SK-2, northern Songliao Basin. The continuous coring data reveal continuous fluvial-lacustrine sedimentary sequences in the Denglouku-Quantou Formations in borehole SK-3; while results of seismic horizon matching further suggest the Denglouku-Quantou Formations are widely developed across the Songliao Basin and sedimentary records of continuous deposition are widespread. Therefore, the Denglouku-Quantou Formations across the basin should provide the geological record of continuous deposition of the Upper-Lower Cretaceous stratigraphic boundary. And this paper provides the basic dataset and exploration direction to find the “golden spikes” for the Upper and Lower Cretaceous continental stratigraphic boundary in the Songliao Basin and adjacent areas.

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    Cyclostratigraphic study of the Neoproterozoic Browne-Hussar formations in western Australia
    YANG Kunkun, LI Haiyan, ZHAO Hanqing, CHU Runjian, LIU Guanghong, WU Huaichun, ZHANG Shihong
    2023, 30(3): 441-451. 
    DOI: 10.13745/j.esf.sf.2022.9.9

    Abstract ( 146 )   HTML ( 5 )   PDF (5484KB) ( 94 )  

    Current cyclostratigraphic researches mainly focus on the Phanerozoic strata whilst the Precambrian era—the longest span of geological time in Earth’s history—is less studied. The Officer Basin in western Australia possesses well-preserved mid-Neoproterozoic (~800 Ma) successions. The Lancer 1 well, located in the western Officer Basin, has recovered ~750 m of continuous sedimentary strata of the Neoproterozoic Browne-Hussar formations, mainly interbeded with sandstone and mudstone and occasional dolomite, showing great promise for the cyclostratigraphic analysis. In this study, the gamma-ray logging data of the Browne-Hussar formations are used as paleoclimate proxy. Spectral analysis shows that the wavelength ratios of the cycles in the 940-1 200 m mudstone interval of the Hussar Formation correspond to the hierarchies of the Milankovitch cycles, where sedimentary cycles, defined as 34 m, 8-10 m, 1.8-2.7 m or 1.2-1.4 m thick cycles, can be interpreted to correspond respectively to the Milankovitch long-eccentricity, short-eccentricity, obliquity and precession cycles. After calibrating to the 405-ka long-eccentricity cycle, a floating astronomical timescale is established for the mudstone interval of the Hussar Formation and the periods of the four Milankovitch cycles are estimated to be 405 ka, 127 ka, 27-34 ka and 15.2-18.1 ka, respectively. Based on the obliquity period of (27±0.7) ka, the Earth-Moon distance and the length of day (LOD) are calculated to be (362 935±1 495) km and (20.56±0.2) h, respectively. These results represent the first report on climate cycles in the Early-Neoproterozoic (~800 Ma) in response to orbital forcing, which provides a key reference to elucidating the evolution of the Earth-Moon system.

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    Microfacies analysis of sand formation in Shengyi District, Shengtuo Oilfield, Dongying Sag
    CUI Wenfu, SHEN Zhihan, JIANG Zaixing, HE Lirong, WANG Cheng, ZHANG Jiazhi, LIU Tong
    2023, 30(3): 452-464. 
    DOI: 10.13745/j.esf.sf.2022.12.55

    Abstract ( 138 )   HTML ( 10 )   PDF (14837KB) ( 90 )  

    The Shengyi District is located in the western Shengtuo Oilfield, where rich oil/gas deposits are found in reverse drag anticline traps formed under the control of the Shengbei Fault. In this study, sedimentary microfacies of the 1st sand group, 2nd member, Shahejie Formation (or ‘Sha’er 1 sand formation’, for short), Shengyi District, was investigated through core analysis, grain size analysis, and sedimentary facies analysis. The sand formation is mainly made up of shallow lake beach-bar deposits, with five sedimentary microfacies: main bar; bar edge; beach ridge; beach sheet; and inter-beach depression. Facies characteristics and depositional environments of the Sha’er 1 sand formation, revealed by facies analysis on typical cores from single and continuous wells, show that the deposition of the sand formation is mainly controlled by provenance, landform, wind field, and water depth. This research can provide strong technical support for sustainable and stable oil field development in the later stage.

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    Two-phase oil/water seepage in fractured granite rock mass: Insight from seepage visualization experiment and numerical simulation
    SUN Zhe, ZHANG Bin, CHEN Dawei, LI Yutao, WANG Hanxun
    2023, 30(3): 465-475. 
    DOI: 10.13745/j.esf.sf.2022.12.54

    Abstract ( 225 )   HTML ( 18 )   PDF (7923KB) ( 106 )  

    Hard rock caverns used for underground oil storage are mostly constructed in crystalline rock mass and water sealed, thus the seepage characteristics of the rock mass directly affect the safety of oil storage. The challenge of water sealing in underground caverns can be simplified to the problem of two-phase oil/water seepage through fractures in the surrounding rock mass, so it is necessary to consider both oil and water displacements to addressing this challenge. In this study, seepage visualization experiment and numerical simulation were carried out on fractured natural granite. The fracture roughness was quantified by fractal analysis, and transparent fractures with different roughness values were prepared by 3D printing. Physical model test was carried out by using the home-built single-fracture multiphase-seepage visualization device, and fluid displacement in rough fractures was studied by numerical simulation. Difference in the mobility ratio (water to oil) leads to different two-phase (oil-water) flow dynamics, where the oil-flooding front moves as piston flow while water-flooding front as fingered flow. In oil displacement, the increase of fluid pressure or fracture width can significantly improve oil displacement efficiency, whereas increasing fracture roughness hinders oil flow and impedes oil displacement. Considering the relevance of fracture roughness and oil viscosity, safety requirements for water sealing are not adequate if only fluid pressure is considered. Due to Jamin effect, it is difficult to remove oil completely from rock fractures by water flooding, or to recover leaked oil. The research results provide a reference for understanding two-phase seepage in rough fractures in rocks and water sealing mechanism in underground caverns.

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    Characteristics and mitigation of coseismic geohazards associated with the Luding MS 6.8 earthquake
    SUN Dong, YANG Tao, CAO Nan, QIN Liang, HU Xiao, WEI Meng, MENG Minghui, ZHANG Wei
    2023, 30(3): 476-493. 
    DOI: 10.13745/j.esf.sf.2022.12.50

    Abstract ( 429 )   HTML ( 17 )   PDF (29682KB) ( 168 )  

    Understanding the characteristics of geohazards and the post-seismic effect in the meizoseismal area is the key to supporting post-disaster reconstruction and disaster mitigation. In this report, the number of coseismic geohazard events following the MS 6.8 earthquake of 2022 in Luding, Sichuan, China, and their spatial distribution and controlling factors are studied based on UAV high-resolution remote sensing image interpretation, post-seismic emergency damage assessments and detailed focus-area investigation. In addition, three important risk factors for post-seismic geohazard mitigation are analyzed, and five recommendations emphasizing post-seismic reconstruction are proposed. The main findings are: (1) The coseismic geohazard events are predominantly small-scaled events and followed by medium-sized events, while large-scaled events are relatively few. These events are densely distributed along the Dadu river and its right tributary, roads, seismogenic fault and other regional faults. (2) The main controlling factors for secondary disasters are, in the order of decreasing dominance, ground motion, fault zone, terrain slope, terrain lithology and low fortification standard in the meizoseismal area. (3) The main issues in the prevention and control of post-seismic geohazards are: rainfall induced aggravation of existing structural deformation and onset of new disasters; the shattered mountains becoming source areas to trigger a chain of remote disasters; and the massive debris on channel slopes becoming a material source to cause mudslide. Accordingly, we suggest to strengthen the hazard identification system combining technology with human intervention, construct high-tech monitoring and early warning systems to monitor both hazard spots and entire slope areas, implement science-based comprehensive avoidance-and-relocation plans, raise anti-earthquake construction engineering standards in earthquake zones, and implement technical support mechanisms in building hillside houses in rural areas.

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    Effect of bedrock slope angle on deformation and failure of overlying rock-soil mixture: Insight into the evolution of landslides
    SUN Yongshuai, HU Ruilin
    2023, 30(3): 494-504. 
    DOI: 10.13745/j.esf.sf.2022.9.5

    Abstract ( 110 )   HTML ( 3 )   PDF (7696KB) ( 56 )  

    The slope angle of bedrock surface plays an important role in deformation and failure of the overlying rock-soil mixture (debris). Therefore, it is of great importance to quantify the effect of bedrock slope on the evolution of landslides in order to obtain a comprehensive evaluation of the mechanical behavior and stability of landslides; however, such quantitative studies are currently lacking. To address this gap we carried out an experimental study on deformation and failure of rock-soil mixture under different slope angles of the underlying bedrock using a home-built large size push-shear apparatus. The results show that with increasing slope angle the included angle between the shearing and bedrock surfaces increases, and so does the distance between the two surfaces. At 25° inclination the overall movement of the rock-soil mixture in the shear zone is most stable; whereas at 45° inclination occurs the most serious debris rupture in the shear zone while the overall movement of debris is most unstable, and the slip zone is most difficult to manage. With increasing slope angle, the maximum shear stress first increases and then starts to decrease at 45° inclination—at this point, the maximum shear stress reaches maximum in the deformation process, and the peaking time of pore-water/soil pressure changes (from their initial values) reach maximum as well. The results and parameters obtained in this study provide a reference basis for evaluating landslide stability and other geological hazards based on the determination of bedrock slope.

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    Geochemical characteristics of rare earth elements in high-fluoride groundwater in the Guide Basin and its implications
    WANG Zhen, GUO Huaming, LIU Haiyan, XING Shiping
    2023, 30(3): 505-514. 
    DOI: 10.13745/j.esf.sf.2022.9.3

    Abstract ( 149 )   HTML ( 3 )   PDF (6833KB) ( 108 )  

    High-fluoride groundwater is one of the most serious environmental geological problems in China and abroad. Although the formation mechanism of high-fluoride groundwater has been studied extensively by many researchers, it is not completely clear whether chemical speciation and migration characteristics of rare earth elements (REEs) in high-fluoride groundwater are indicative of the fluoride enrichment process. This study focused on the Guide Basin where high fluoride content groundwater is widely distributed, and explored the distribution and migration processes of fluoride and REEs in aquifer by field sampling, laboratory testing and comprehensive geochemical analysis, combined with hydrogeochemical simulation. It was found that the average fluoride concentration in groundwater was 2.67 mg·L-1—with 75% of samples higher than 1.5 mg·L-1—and showed an upward trend along the groundwater flow path. PHREEQC calculation results showed that fluoride mainly occurred as free F- in groundwater (99.5%). XRD and SEM-EDS results showed that the main minerals in aquifer sediments were quartz (52.9%-56.5%) and plagioclase (19.8%-21.8%) that has undergone chemical weathering. REE contents in groundwater samples were low (0.052-0.267 μg·L-1), and REEs mainly occurred as LnCO3+ and Ln(CO3)2- (>99%)(Ln represents REEs). NASC-normalized REE patterns showed that LREEs were enriched relative to HREEs, with slight negative Ce anomaly and obvious positive Eu anomaly. Migration of fluoride and REEs in groundwater was affected by reductive dissolution of iron oxide minerals and inhomogeneous hydrolysis of feldspar minerals, and, to certain extent, REE enrichment process was indicative of fluoride enrichment in groundwater. This research expanded REE applications in the study of high-fluoride groundwater and provided a reference for identifying high-fluoride groundwater distribution and revealing fluoride enrichment mechanisms.

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    CMIP6 multi-model prediction of future climate change in the Hotan River Basin
    HE Chaofei, LUO Chengyan, CHEN Fulong, LONG Aihua, TANG Hao
    2023, 30(3): 515-528. 
    DOI: 10.13745/j.esf.sf.2022.12.60

    Abstract ( 209 )   HTML ( 9 )   PDF (8368KB) ( 82 )  

    Climate system models are the most extensive and effective tool for simulating historical and future climate, but they cannot be used directly to predict future climate change due to certain deficiencies and limitations of the models. In this study, taking 1971-2000 as base period, six climate models were evaluated using quantile-based daily bias correction (DBC), multi-model ensemble (MME) and correlation coefficient-based weighted multi-model ensemble (r-MME) methods to assess their applicability to predicting future climate change in the Hotan River Basin. In addition, r-MME was used to aggregate deviation-correction factors for the six models under three futuristic scenarios, SSP1-2.6, SSP2-4.5 and SSP5-8.5, to predict the near future (2021-2050) and long-term (2061-2090) spatial/temporal trends in daily maximum and minimum temperatures and precipitation. Results show that combining r-MME and DBC can comprehensively utilize each model’s advantages and greatly improve simulation accuracy, as correlation coefficients of 0.918, 0.821 and 0.878 were achieved, respectively, in prediction of maximum/minimum annual average temperatures and annual average precipitation. These three climate indices all showed an upward trend, under the three scenarios, where the low-forcing SSP1-2.6 had the smallest long-term increments of 2.830 ℃, 2.523 ℃ and 46.412 mm, respectively, and the high-forcing SSP5-8.5 had the largest increments of 5.697 ℃, 6.452 ℃ and 93.206 mm, respectively; besides, the decrease of temperature difference between maximum and minimum temperatures in the future will result in more obvious warming and humidification in the basin. The research results can provide an important theoretical basis for the economic and heathy-ecosystem developments in as well as water resources prediction for the Hotan River basin under future climate change.

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    Attribute modeling constrained by multi-source data-based 3D geological structural model: A case study in Tongzhou District, Beijing
    XUE Tao, BAO Xunshuan, ZHU Xiaodi, HUANG Xiao
    2023, 30(3): 529-536. 
    DOI: 10.13745/j.esf.sf.2022.9.6

    Abstract ( 272 )   HTML ( 20 )   PDF (4130KB) ( 229 )  

    Method development supporting multi-source heterogeneous data has always been one of the key and challenging issues in 3D geological modeling. Taking Tongzhou District, Beijing as an example, we designed an automated method of building 3D geological model using multi-source data. Firstly, in the data preprocessing stage, the virtual drilling is generated based on section, seismic section interpretation and isosurface (line) data so as to uniformly convert the heterogeneous multi-source data into isomorphic virtual drilling data, and then a 3D geological structural model is built by automated modeling of 3D geological structure using drilling and virtual drilling data. Secondly, the structural constraints of the attribute model are created based on the 3D geological structural model, and the attribute model grid is constructed using hexahedron or tetrahedron units. Thirdly, each attribute block in the attribute model grid is assigned an attribute value through 3D spatial interpolation. Finally, we map the attribute change onto color change to establish the corresponding attribute model, thus completing attribute modeling constrained by the geological structure model. This method has been used in engineering geological survey and urban underground space development, which contributes to the improvement and application of 3D geological model.

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