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2024, Volume 31 Issue 5
25 September 2024 Previous Issue   

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Tectonic fracturing and fracture initiation in shale reservoirs—research progress and outlooks DING Wenlong, WANG Yao, ZHANG Ziyou, LIU Tianshun, CHENG Xiaoyun, GOU Tong, WANG Shenghui, LIU Tingfeng 2024, 31(5): 1-16.  DOI: 10.13745/j.esf.sf.2024.6.11 Abstract ( 1032 )   HTML ( 27 )   PDF (7879KB) ( 180 )   Shale oil and gas development has gained significant progress in China with the contineous research and technological advancements in unconventional oil and gas. Extensive production practices demonstrate that natural fracture development in shale reservoirs is a crucial factor influencing oil and gas enrichment, high production and stable yield, for fractures not only improve reservoir properties but also facilitate subsequent reservoir modification during hydrollic fracturing. The formation stages of tectonic fractures and fracture initiation holds significant importance for revealing the oil and gas enrichment patterns and preservation condition. Early studies on shale reservoir fractures mainly focused on fracture classification/identification/characterization, main controlling factors of fracture development, and fracture distribution prediction and modelling, with less attention to the determination of fracture formation stages, main controlling factors of fracture initiation, mechanism of fracture opening and closing, and quantitative characterization of fracture openness—this restricts the efficient exploration and development of shale oil and gas in China. This paper highlights research progress addressing the above gaps, particularly the delineation of fracturing stages, dating of filling veins, and quantification of fracture openness based on comprehensive literature review. The classification methods for fracturing stages can be divided into two categories: qualitative geological analysis and geochemical tracing of fracture fillings. These methods however have practical limitations, where only the relative sequence of fracturing stages can be obtained, and the results are affected by the accuracy of basic geological data such as burial/thermal histories. Most of the fissure veins in fractures are carbonate minerals and quartz. With the advent of high precision in situ U-Pb microprobe dating technology, it is possible to determine the absolute ages of different veins while avoiding the problem of multiple solutions to fluid activity periods due to differences in interpreting thermal/burial histories. The initiation of tectonic fractures is controlled by many factors, not only by rocks’ intrinsic properties but also the current crustal stress and formation-fluid pressure. The petrological characteristics of fibrous fillings widely distributed in fractures record the crystal growth process. Such special crystal morphology provides the evidence of multi-stage tectonic fracturing, revealing a fracture evolutionary process with multiple fracture opening and closing. At present, fracture openness is characterized using the fracture apature parameter or inferred by the fracture initiation pressure obtained by calculation and experiment. Based on the analysis of the above results, this paper points out the key problems and development trends in the study of the fracturing stages and initiation of tectonic fractures in shale reservoirs, aiming to enrich and improve the theory and research methodology for shale oil and gas reservoirs, and provide an important scientific basis for the study of structural preservation conditions and enrichment mechanism of shale oil and gas in China. Figures and Tables | References | Related Articles | Metrics

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Nature fractures in shales of the Lianggaoshan Formation in northern Sichuan Basin: Fracture development characteristics and fracture formation and evolution model HE Jianhua, CAO Hongxiu, DENG Hucheng, YIN Changhai, ZHU Yanping, LI Chang, LI Yong, YIN Shuai 2024, 31(5): 17-34.  DOI: 10.13745/j.esf.sf.2024.6.16 Abstract ( 610 )   HTML ( 12 )   PDF (18178KB) ( 137 )   The Jurassic Lianggaoshan Formation in Yingshan-Pingchang region, northeastern Sichuan has great exploration potential for shale oil and gas resources. To better understand the control of natural fractures on oil and gas enrichment and reservoir production, this paper utilizes multi-scale fracture characterization methods such as field observation, imaging, core logging, thin section, and CT scanning, combined with vein inclusion analysis, carbon-oxygen isotope analysis, acoustic emission testing, and U-Pb dating of veins. The characteristics of the multiscale fracture system in the study area are revealed; the formation history of natural fractures of different origins are clarified; and the fracture evolution model is constructed. The Jurassic shale reservoir mainly develops mechanical types of fractures, such as shear fractures, bedding slip fractures, bedding fractures, and fluid overpressure fractures. The tectonic deformation zone is dominated by NW-/NE-striking, high-angle parallel shear fractures and reticulate tension-shear hybride fractures, with high fracture density and large-scale vertical interlayers. The stable zone is characterized by bedding fractures, fluid overpressure fractures, and a small number of near-EW-/NNE-striking planar shear fractures, with low fracture density and high fracture filling. The Jurassic mainly experienced four episodes of fracture development : the early-middle Yanshanian (170-140 Ma), with the formation of near SN/NE-striking hydraulic overpressure fractures fully filled with fibrous calcite; the late Yanshanian (100-80 Ma), with the development of NNE-/near-EW-striking conjugate planar shear fractures, NE-striking expansion fractures, and hydrocarbon generation overpressure fractures; the early-middle Himalayan (67-32 Ma), with the continued development of planar shear fractures, NW-treding parallel shear fractures, longitudinal tension fractures, and bedding fractures; and the late Himalayan (15-6 Ma) with the activation of pre-existing fractures and formation of small number of NNE-striking tensile fractures. This study provides important support for quantitative prediction of effective fractures under multistage tectonic superposition in the Jurassic and identification of favorable areas for shale oil and gas enrichment. Figures and Tables | References | Related Articles | Metrics

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Fracture formation mechanism in shale oil reservoirs, Qintong Depression, North Jiangsu Basin and its influence on hydrocarbon occurrence GAO Yuqiao, HUA Caixia, CAI Xiao, BAI Luanxi, LU Jia 2024, 31(5): 35-45.  DOI: 10.13745/j.esf.sf.2024.6.30 Abstract ( 851 )   HTML ( 4 )   PDF (9620KB) ( 80 )   The second member of the Funing Formation in Qitong Depression, Subei Basin is a favorable section for shale oil exploration and development, where natural fractures provide important reservoir spaces and seepage channels for shale oil and have an important impact on shale oil exploitation and production increase. Based on core observation, imaging logging, thin section observation and experimental analysis, combined with advanced environmental scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM), this paper investigates into the genetic type, development characteristics, formation mechanism and distribution and occurrence state of natural fractures and their influence on oil and gas occurrence in the study area. The natural fractures are mainly tectonic fractures-including cross-bedding shear fractures, in-bedding shear fractures and in-bedding tensile fractures-and sedimentary rock fractures-mainly bedding fractures with a few sutures. The characteristics of fracture development in the sublayers of the second member vary greatly in the vertical direction. From sublayers Ⅰ to Ⅴ the fracture development intensity decreases and fracture types reduce to a single type; the fracture density is gradually reduced; the fracture extension length is gradually increased; and the fracture filling is gradually weakened. A complex fracture network is formed under various influencing factors including sedimentary rocks, structural setting and abnormal high pressure. The fracture network communicates with the reservoir matrix pores to form a three-dimensional pore-fracture system, providing a good space for shale oil storage and greatly improves reservoir permeability. As SEM/LSCM images show shale oil in the study area occur mainly as free or adsorbed oils in the forms of liquid oil droplets in pores and microfractures or oil films wrapping around mineral particles. The crude oil in fractures is mainly composed of light components, and the pores contain both light and heavy components. The development of natural fractures obviously improves the physical properties of shale reservoir rocks and is conducive to the later fracture network reconstructure—an important guarantee of shale oil enrichment and high-production in the study area. Figures and Tables | References | Related Articles | Metrics

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Fracture pore characteristics and gas accumulation model of marine shales in the northwestern Ordos Basin: A case study of the Ordovician Wulalike Formation CHEN Rubiao, WANG Yuman, HUANG Zhengliang, LI Weiling, YAN Wei, LIANG Feng, GUO Wei 2024, 31(5): 46-60.  DOI: 10.13745/j.esf.sf.2023.6.14 Abstract ( 841 )   HTML ( 6 )   PDF (16069KB) ( 88 )   The Ordovician Wulalike Formation in the northwestern margin of the Ordos Basin is a new prospect for exploring shale gas in the North China plate. Its accumulation conditions and enrichment characteristics are quite different from that of marine shale gas fields in South China. Based on geological data such as drilling cores, logging, laboratory analysis and testing, focusing on the fracture characteristics of the Wulalike Formation, this article explores the accumulation model of marine shale gas in the northwestern margin and obtain four conclusions: (1) The Wulalike Formation, under the control of various sedimentary microfacies, develops siliceous shale, calcareous-siliceous and argillaceous-siliceous mixed shale, localized carbonate rocks (mainly brecciated limestone and marl) and calcareous shale, with large variations in lithofacies combinations in different blocks. However, the lower interval is generally rich in siliceous and exhibits similar brittleness to the lower interval of the Longmaxi Formation, which is beneficial for reservoir fracturing in the foreland thrust belt distribution zone. (2) The Wulalike Formation develops two generations of fractures: high-angle filled fractures and low-angle bedding fractures. Whilst the predominant type is the open bedding microfractures, which is more developed in the southern block compared to the northern block. (3) The lower interval of the Wulalike Formation generally has a total porosity between 2.46%-7.08%, similar to the Longmaxi Formation in the Sichuan basin, with matrix porosity accounting for 34.0%-90.0% (average 61.1%) and fracture porosity 10.0%-66.0% (average 38.9%). (4) Natural gas is mainly stored as free gas (accounting for over 64%), where primary and secondary intralayer migration occurs in inorganic pores and bedding fracture system. According to comprehensive analysis, the Majiatan-Shanghaimiao exploration area is mainly a fractured shale gas accumulation area controlled by structures. The fracture development period is synchronized with the peak gas generation period, which is conducive to the efficient accumulation of natural gas in the Wulalike Formation. The Tiekesumiao Block is a mixed shale gas reservoir. Figures and Tables | References | Related Articles | Metrics

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Characteristics of natural fractures and its influence on shale oil enrichment and preservation in Member 2 of Funing Formation in Gaoyou sag, Subei Basin SUN Yaxiong, LIANG Bing, QIU Xuming, DUAN Hongliang, FU Qian, ZHOU Jinfeng, LIU Shili, QIU Yongfeng, HU Huiting, GONG Lei 2024, 31(5): 61-74.  DOI: 10.13745/j.esf.sf.2024.6.17 Abstract ( 622 )   HTML ( 3 )   PDF (15365KB) ( 75 )   Significant breakthrough has been made in shale oil exploration in Member 2 of the Funing Formation in Gaoyou sag, Subei Basin, a key area for increasing petroleum reserve and production. The advancement of shale oil exploration in the study area, however, is hindered by a lack of understanding of the complex fracture system and its impact on shale oil enrichment and preservation. In this contribution, we systematically analyze the characteristics of nature fractures using core logging, thin section, scanning electron microscopy, and Maps and Qemscan methods; the impact of fracture types on shale oil enrichment and preservation are further revealed through organogeochemical analysis of frozen cores, combined with well production data. Structural fractures such as layer-perpendicular and layer-controlled fractures are widely developed in shales of Mmember 2 of the Funing Formation, as well as nonstructural fractures such as bedding fractures and compression fractures. The development of layer-perpendicular fractures is controlled by lithology and structural location, while layer-controlled fractures mainly develop in brittle layers such as dolomite bands, sand/dolomite mixed bands, and bedding calcite veins. The bedding fractures are mainly caused by abnormal high pressure and are concentrated in the middle and lower parts of the study area. The bedding factures and the layer-controlled fractures and caves are conducive to shale oil enrichment, as they are not only favorable storage spaces but also efficient flow channels. Layer-perpendicular fractures show higher effectiveness of shale oil (S1) enrichment compared to shale matrix; however, they can also be efficient oil seepage channels due to vertical and horizontal connectivity. This causes unfavorable preservation condition at the sweet spot “lower Ⅴ” in subsegment Ⅴ, but has less impact on subsegment Ⅳ. In general, the smaller the planar distance from the fault and the larger the fault scale is, the more developed and unfavorable the structural fractures are for shale oil preservation. Figures and Tables | References | Related Articles | Metrics

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Structural differences of shale laminae and their controlling mechanisms in the Wufeng-Longmaxi Formations in Tiangongtang, southwestern Sichuan ZHAO Shengxian, LI Bo, CHEN Xin, LIU Wenping, ZHANG Chenglin, JI Chunhai, LIU Yongyang, LIU Dongchen, CAO lieyan, CHEN Yulong, LI Jiajun, LEI Yue, TAN Jingqiang 2024, 31(5): 75-88.  DOI: 10.13745/j.esf.sf.2024.6.31 Abstract ( 791 )   HTML ( 9 )   PDF (23084KB) ( 95 )   The Tiangongtang area in southern Sichuan Basin is a shale gas exploration and development hotspot, famous for the development of large-scale marine deposits with good sedimentary stability and continuity. Focusing on the laminated organic-rich shale of the Wufeng-Longmaxi Formations, this paper clarifies the shale lamination types based on core description and microscopic observation of large and thin sections. By means of geochemical, mineralogical, petrograpic and pore structural analyses and field emission scanning electron microscope observation, the pore structure and reservoir physical properties are revealed. Accordingly, the internal structural differences and control mechanisms of different lamination combinations are revealed. Shales of the Wufeng-Long1-1 submember mainly develops three lamination combinations: graded, massive and sand-mud interbedded. Graded laminated and interlaminated shale assemblages have much higher horizontal permeability than the massive while the latter has the highest vertical permeability; the graded has the best reservoir physical properties. Moreover, the pore volume and specific surface area of fractures show a decreasing trend from the graded to massive to interlaminated. Under the control of sedimentary environment and diagenesis, graded laminated shales with higher TOC and organic-clay laminae have larger effective pore space, higher pore connectivity and larger pore volume than the interlaminated with lower TOC and thick silty and clay laminae. With relatively high organic content and disordered porous media massive shales have better quality effective storage spaces than sand-mud interbedded shales but worse quality than graded laminated shales. Figures and Tables | References | Related Articles | Metrics

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Characterization and 3D modeling of multiscale natural fractures in shale gas reservoir: A case study in the Pingqiao structural belt, Sichuan Basin QIAO Hui, ZHANG Yonggui, NIE Haikuan, PENG Yongmin, ZHANG Ke, SU Haikun 2024, 31(5): 89-102.  DOI: 10.13745/j.esf.sf.2023.6.13 Abstract ( 960 )   HTML ( 1 )   PDF (10680KB) ( 66 )   Natural fractures in shale reservoirs are important reservoir spaces and seepage channels. Identifying the types and spatial distribution of natural fractures is essential for shale gas exploration and development. This paper, based on seismic/outcrop data, core observation, well logging and micro-test analysis, mainly considering the effect of fractures on shale gas enrichment and high production, divides natural fractures of shale reservoirs into three scale levels: large, medium-small and micro, and clarifies the methodologies for fracture characterization and modeling at each scale level and application results. In summary: (1) large fractures were mainly characterized using stacked 3D seismic data; medium-small fractures using a combination of core, image log and seismic attributes data; and microfractures using microanalysis such as core description, high-resolution scanning electron microscope and maps analysis. Through fracture characterization, the fracture density, crack opening, dip, orientation and filling status at each scale level were determined. (2) The DFN model of large fractures was established via deterministic modeling, using the characterization parameters of post-stack seismic attributes as the input. For medium-small fractures, single-well image logs were used as prior information; a fracture probability model of multi-information fusion was established as the spatial trend; and the DFN model was established via stochastic modelling. Microfracture modeling was based on microfracture parameters obtained from micro-test analysis; microfracture density model was established by combining well data with TOC and other main control factors; and the DFN model was established via stochastic modeling. (3) Taking the shale gas reservoir in the Pingqiao tectonic zone, Sichuan Basin as an example, fracture characterization and fracture 3D geological modeling for different fracture types were carried out. The fracture initiation site, scale, orientation and occurrence characteristics were defined, and fracture attributes such as fracture location, dip angle, azimuth angle, geometric size, development density, porosity and permeability were described. The methodologies for the multiscale natural fracture characterization and modeling provide a basis for numerical modeling of shale gas reservoirs. The 3D geological model of shale reservoir in the Pingqiao tectonic belt and the simulation results are in good agreement with geological knowledge and production data, thus providing a reference for the devlopment of shale gas fields. Figures and Tables | References | Related Articles | Metrics

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Three-dimensional modeling of multiscale fractures in Chang 7 shale oil reservoir in Qingcheng oilfield, Ordos Basin LIU Yanxiang, LÜ Wenya, ZENG Lianbo, LI Ruiqi, DONG Shaoqun, WANG Zhaosheng, LI Yanlu, WANG Leifei, JI Chunqiu 2024, 31(5): 103-116.  DOI: 10.13745/j.esf.sf.2024.6.29 Abstract ( 484 )   HTML ( 4 )   PDF (11816KB) ( 65 )   Shale oil reservoirs generally develop multiscale natural fractures which are the main reservoir storage spaces and seepage channels. The development of multiscale natural fractures restricts efficient reservoir exploration and development. In this article, taking Chang 7 shale oil reservoir in Qingcheng oilfield, Ordos Basin as an example, combined with field surface outcrop, core, thin section, well log, and seismic data, the characteristics of multiscale fracture development are clarified on the basis of fracture classification; a multiscale fracture modeling method is developed. A three-dimensional discrete network model of multiscale fractures in well X in typical drilling area is established. Based on ant-tracking seismic attributes after stacking, the distribution characteristics of macroscale fractures are clarified, and a geological model of macroscale fractures is established using deterministic methods. Through geomechanical analysis, combined with fracture interpretation results from conventional well logging, intensity constraint models for mesosccale/small-scale fracture developments are established. Using the intensity constratint models, combined with relavant fracture parameters, geological models of mesoscale and small-scale fractures are established, respectively, using random modeling technique. Finally, fracture network models of all scales are fused to form a multiscale fracture network model, and an equivalent fracture attribute model is established. It was found that macroscale fractures of Chang 71 in area X are developed in the northeast while mesoscale and small-scale fractures are developed mostly in the west, southwest, and northeast of the study area. The established multiscale fracture model is consistent with the pattern of fracture development in single well and actual production performance data, thereby providing a geological basis for increasing shale oil and gas storage and production in well X drilling area in Qingcheng oilfield. Figures and Tables | References | Related Articles | Metrics

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Mechanical behavior of calcite vein-bearing shale of the Niutitang Formation in Fenggang area, northern Guizhou based on CT tests WU Zhonghu, MENG Xiangrui, LAN Baofeng, LIU Jingshou, GONG Lei, YANG Yuhan 2024, 31(5): 117-129.  DOI: 10.13745/j.esf.sf.2024.6.15 Abstract ( 934 )   HTML ( 1 )   PDF (10828KB) ( 66 )   Core observations of shales from the Niutitang Formation in the northern Qianbei region show that calcite veins often act as natural fracture fillers and largely influence the shale damage patterns. The damage characteristics of calcite vein-bearing shales is important for the prediction of fracture initiation and extension behavior during hydraulic fracturing and for the engineering design. In order to reveal the influence of calcite veins on the mechanical properties and fracture characteristics of shale, uniaxial compression and acoustic emission tests are conducted at seven inclination angles of 0° to 90° in 15° increment. Combined with CT scanning technology and finite element calculations, a three-dimensional (3D) microscopic numerical model is constructed. The effects of calcite vein angle on the fine-scale shale damage process as well as shale mechanical properties are discussed, and the spatiotemporal evolution of shale microcracks are analyzed. The results show that (1) under different calcite vein angles the shale acoustic emission and stress-strain curves show similar curve shape changes, in four stages, namely compression-density, elasticity, yielding, and post-peak damage, with obvious distinctions between stages. The change curve of the characteristic intensity is “U”-shaped with a local minimum at θ of 75°. (2) Calcite veins significantly affect the damage mode: As the vein angle decreases, the damage mode changes from cleavage to cleavage-type shear, to shear-slip, and finally to cleavage-tension. (3) The reconstructed 3D model is largely consistent with physical testing data, providing insights into the process of crack expansion and penetration at shale’s interior and surface. The spatial distribution of acoustic emission reflects the compression, tension, and shear damage types at different stages, revealing the fracture mechanism of calcite-containing shale from the microscopic point of view. (4) Calcite and matrix simulatineously and anisotropically affect the macroscopic mechanical properties of shale: The higher the calcite vein angle, the stronger its slip guidance effect, and the weaker the mechanical properties of the specimens. Figures and Tables | References | Related Articles | Metrics

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Development and distribution pattern of fault-controlled fractures in complex structural deformation zones JU Wei, YANG Hui, HOU Guiting, NING Weike, LI Yongkang, LIANG Xiaobai 2024, 31(5): 130-138.  DOI: 10.13745/j.esf.sf.2024.6.20 Abstract ( 509 )   HTML ( 2 )   PDF (9544KB) ( 99 )   Natural fractures are important reservoir spaces and seepage channels for oil and gas. In complex structural deformation zones, fractures are obviously controlled by faults, but the laws and modes of fracture formation are not yet clear. This study investigates natural fractures developed in complex fault zones in the Kuqa depression. Based on geological observation, fracture interpretation from imaging logging, and theoretical analysis, a “fault-controlled fracture coefficient (K)” is defined and calculated to reveal the development and distribution pattern of fault-controlled fractures in the study area. The results indicate (1) the thrust faults obviously control both the occurrence and development of natural fractures, where the fracture density is exponentially inversely proportional to the distance from the fault, and the fault-controlled fracture zones can be divided into strongly-controlled, weakly-controlled, and regional fracture zones with increasing distance to the fault. (2) Among the strike-slip faults, high-angle oblique faults develop fault-controlled fracture zones where fracture development is significantly affected by the scale of the faults; while fractures associated with oblique thrust faults are mainly developed within the fault zone, and the width of the fault zone varies along the strike direction. (3) The fault-controlled fracture coefficient (K) is defined as the ratio of the width of the strongly-controlled fracture zone to the fault displacement (slip displacement). According to the analysis, the K values of thrust faults in the Kuqa depression ranged between 1.50-1.80, and that of strike-slip faults ranged between 0.125-0.150. The results have both theoretical and practical significance for guiding oil and gas exploration and development in complex structural deformation zones. Figures and Tables | References | Related Articles | Metrics

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Fracture development mode in fan delta front unconventional tight oil reservoirs: A case study of Paleogene He-3 in southeastern Biyang Depression YIN Shuai, ZHANG Ziyang, ZHANG Xingxing, WANG Jingchen, HU Wei, DING Wenlong, LI Hu 2024, 31(5): 139-155.  DOI: 10.13745/j.esf.sf.2023.6.21 Abstract ( 662 )   HTML ( 1 )   PDF (13259KB) ( 59 )   The southeastern Biyang Depression is located on the steep slope of the short axis of a paleolake basin, with a complex sedimentary system of fan delta front driven by traction flow in the Paleogene. Quantitative characterization of fractures in different fabric sand bodies plays an important role in guiding unconventional tight oil exploration and development. The third member of the Paleogene Hetaoyuan Formation (He-3) in the southeastern Biyang Depression belongs to fan delta front deposit under an “uplift in depression” setting, and natural fractures have a direct influence on the adjustment of infill well pattern and the scientific formulation of enhanced oil recovery scheme. In this paper, taking the shallow middle-layer (Ⅱ-Ⅵ oil formation) continental tight sandstones of He-3 as an example, the fracture characteristics and distribution law are systematically studied using core, physical property, conventional and image logging data and numerical simulation method; the pattern of fracture development in unconventional tight oil reservoirs at the front edge of the fan delta of the study area is proposed. Lithology, sand body thickness, sedimentation and structure have significant control on fracture distribution in shallow, middle-layer tight sandstone reservoirs. Fractures are more likely to develop in single or composite thin sand bodies. Usually fractures are relatively underdeveloped when the single sand body thickness is greater than 6 m. Shallow, middle-layer fractures are mainly developed at the front of underwater distributary channel, single wing of lateral accretion channel, estuary bar and far sand bar. The river channel types are divided into homogeneous and heterogeneous. The superimposed thickness is larger in asymmetrical heterogeneous channels formed by rapid lateral accumulation, which can exceed 18 m, and the fractures are relatively developed. The homogeneous channel front, that is, the microfacies area near the fan delta front of the estuary bar, usually develops vertically in the sedimentary sequence near the top of the estuary bar, indicating strong hydrodynamic conditions and relatively developed fractures. Fractures are closely related to oil and gas accumulation. Fractures in He-3 are mainly formed in the Neogene depression period. The three-dimensional distribution of the main small-layer fractures is restored through DFN simulation. The results show that two groups of conjugate fractures are mainly developed in the E-W and NE directions, and one group is mainly developed locally. These fractures are mostly distributed parallel or perpendicular to the uplift structure. Natural fractures are more developed in the lower parts and wings of an anticline compared to its crest. In addition, the intersection of multiple conjugated fractures can form a “fracture chimeric zone,” which may be the direct cause of long-distance water channeling in the pressure flooding process, so the “fracture chimeric zone” should be avoided in the flooding process. Figures and Tables | References | Related Articles | Metrics

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Fracture development characteristics and main controlling factors of natural fracture in the Upper Triassic Xujiahe Formation in Yuanba area, northeastern Sichuan Basin PAN Lei, DU Hongquan, LI Leitao, LONG Tao, YIN Xuefeng 2024, 31(5): 156-165.  DOI: 10.13745/j.esf.sf.2024.6.12 Abstract ( 758 )   HTML ( 5 )   PDF (7877KB) ( 49 )   The Upper Triassic Xujiahe Formation in Yuanba area, northeastern Sichuan Basin is a typical tight reservoir with poor physical properties, whilst the development of natural fractures improves the reservoir properties. Natural fractures provide the main reservoir spaces and seepage channels thus have an important impact on the natural gas migration, accumulation and high yield. Based on outcrop, core, thin section and image log data, this paper investigates the genetic origin of natural fractures and analyzes the characteristics and main controlling factors of natural fracture development in the study area. Natural fractures are mainly low- and high-angle shear fractures of tectonic origin and oriented NW-SE (300°±10°) and NEE-SWW (70°±5°), with low fracture filling. Fracture development is mainly influenced by structural location, lithology and rock thickness. The favorable sites condusive to high natural gas production have the nose-like structure, within 400 m from the upthrown side of the fault, with vertical fault length between 60-120 m, in the vicinity of the inflection point of the NW fault extension line. Fractures are most developed in medium-fine quartz sandstone and fine-grained feldspar lithic sandstone, with the high-energy, large-scale sand body (high content of quartz and low content of shale) more conducive to fracture development. There is a significant negative correlation between the degree of fracture development and the thickness of the rock layer, where high fracture density and high fracture development are achieved when the rock layer is less than 1 m thick. Figures and Tables | References | Related Articles | Metrics

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A deep kernel method for fracture identification in ultra-deep tight sandstones using well logs DONG Shaoqun, ZENG Lianbo, JI Chunqiu, ZHANG Yanbing, HAO Jingru, XU Xiaotong, HAN Gaosong, XU Hui, LI Haiming, LI Xinqi 2024, 31(5): 166-176.  DOI: 10.13745/j.esf.sf.2024.6.22 Abstract ( 396 )   HTML ( 1 )   PDF (6674KB) ( 61 )   Fractures are the main seepage channels for oil and gas migration in ultra-deep tight reservoirs, and are crucial for ultra-deep oil and gas exploration and development. Ultra-deep tight reservoirs have highly complex petrophysical characteristics under high-temperature, high-pressure environments, resulting in ambiguous and multi-solution well log responses pertaining to fractures. To solve this problem, we proposes a deep kernel method (DKM) for fracture identification in ultra-deep tight sandstones. This method employs kernel principal component analysis to extract non-linear log features associated with fractures. It utilizes a deep learning cascade structure to extensively explore the log response characteristics across various scales for accurate fracture identification. Furthermore, it employs gradient-free optimization algorithms to automatically determine the optimal model structure and parameters. We conducted a case study in the ultra-deep tight sandstone reservoirs of the Lower Cretaceous Bashijiqike Formation in the Keshen gas field, Tarim Basin, and the proposed method was applied and verified. Through sensitivity analysis of logging responses to fractures, six specific logging curves were chosen for fracture identification. The first three variables, DEN, RD, and RM, correspond to direct measurements from well logging, whereas the latter three, RSD, nT1, and nT2, are reconstructed curves specifically developed to enhance the detection of fracture-related information. This distinction effectively clarifies the differences in logging parameters between fractured and non-fractured zones. A comparative analysis between the fracture identification results and the core fracture descriptions demonstrated the accuracy of the deep kernel method in identifying fractures within ultra-deep tight sandstone formations. This method achieved an accuracy improvement of over 5% compared to the conventional multi-kernel support vector machine method, thus exhibiting robust applicability for single-well fracture identification. Figures and Tables | References | Related Articles | Metrics

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Characteristics of stress state transitions and its geological and mechanical response in the Kuqa Depression ZHANG Hui, ZHANG Guanjie, XU Ke, YIN Guoqing, WANG Zhimin, LUO Yang, WANG Haiying, ZHANG Binxin, LIANG Jingrui, YUAN Fang, ZHAO Wei, ZHANG Wei, LU Xing 2024, 31(5): 177-194.  DOI: 10.13745/j.esf.sf.2023.6.25 Abstract ( 338 )   HTML ( 1 )   PDF (22422KB) ( 66 )   Combined with the latest drilling, logging and seismic data we carry out numerical simulation to better understand the characteristics of the late Himalayan paleostress evolution in the Kuqa Depression, Tarim Basin. Using finite element numerical simulation method we obtained the three-dimensional distribution of the late Himalayan stress field in the Bashijiqike Formation in the Kelasu tectonic belt whereby accurately determined the paleostress state. By analyzing the relationship between the paleostress state and macrostructural style, fracture occurrence and reservoir physical properties, the geological and mechanical response to the paleostress state transition was systematically revealed. Finally, the influece mechanism between the paleostress state transition and reservoir physical properties was revealed by means of discrete element numerical simulation. During the late Himalayan, the stress field gradually transformed from a thrust-type stress field to a strike-slip type stress field at a depth range of 6500-7500 m. Above 6500 m depth, under thrust stress field, the compaction hole reduction increased with increasing burial depth, and the stress state and rock strength were not conducive to the development of fractures. Below 7500 m depth, the strike-slip stress field was conducive to the preservation of reservoir pores and rock fracturing. The above results deepen the understanding of the tectonic settings, reservoir properties and rock mechanical properties of the Kuqa Depression, suggesting the paleostress state transition provided the mechanical basis for the formation of large and medium-sized oil and gas field in the ultradeep reservoir of the Kelasu tectonic belt. This discovery has guiding significance for the next stage oil and gas exploration and development in the Kelasu tectonic belt. Figures and Tables | References | Related Articles | Metrics

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Geological and engineering applications of full-stratum geomechanical modeling in complex structural areas XU Ke, LIU Jingshou, ZHANG Hui, ZHANG Guanjie, ZHANG Binxin, WANG Haiying, ZHANG Yu, LAI Shujun, QIAN Ziwei, QIANG Jianli 2024, 31(5): 195-208.  DOI: 10.13745/j.esf.sf.2024.6.28 Abstract ( 323 )   HTML ( 1 )   PDF (7698KB) ( 64 )   Areas with complex geological features have poorly defined geostress distribution patterns with strong geostress heterogeneity and anisotropy, which seriously restrict the progress of oil and gas exploration and development. To solve the problems of high modeling accuracy requirements for complex structures and lithology and inaccuracies in current 3D geomechanical modeling techniques for complex structures, this paper, taking the Bozi-Dabei area in Kuqa depression as an example, proposes a full-stratum inverse finite element geomechanical modeling technique for complex structure areas. By iterative scanning of long and large-scale strip-connected anticlines, the modeling accuracy is improved, and the complex intersection relationship between faults and rock masses is accurately established—thus, the modeling of geostress mesh and error tracing in areas with complex structures is achieved. Using this technique, the current distribution characteristics of geostresses in Bozi-Dabei were clarified, and the main controlling factors of strong geostress heterogeneity and anisotropy in deep reservoirs were revealed. Furthermore, the range of stress disturbance during drilling was clearly defined. Results showed that (1) the modeling technique was effective for full-stratum modeling of complex deformation areas, with high accuracy meeting the needs of exploration, development, and production. (2) The key factors affecting the strong geostress heterogeneity and anisotropy in reservoirs of the study area were the mechanical-layer structure of salt layers and the attitude of shallow high, steep strata. (3) The modeling technique was effective for determining the range of stress disturbance during drilling, thus it can be an important evaluation tool for efficient oil and gas exploration and development. Figures and Tables | References | Related Articles | Metrics

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Characteristics and main controlling factors of fracture development in the Ordovician carbonate reservoir, Tahe oilfield HAN Pengyuan, DING Wenlong, YANG Debin, DENG Guangxiao, WANG Zhen, MA Hailong, LÜ Jing, GENG Tian 2024, 31(5): 209-226.  DOI: 10.13745/j.esf.sf.2024.6.18 Abstract ( 397 )   HTML ( 7 )   PDF (13649KB) ( 60 )   The characteristics of natural fractures in Ordovician carbonate reservoirs, Tahe oilfield are systematically studied and the main controlling factors of fracture development are analysed using different experimental methods. Natural fractures can be divided into four types and seven subtypes: structural (tensile, shear), superficial (weathering), diagenetic (stylolites, bedding), and composite genetic (structural-weathering, structural-diagenetic), with structural being the dominating type. Fractures are mainly oriented NE-SW, NW-SE and NNE-SSW and dip toward NNW, NE and SEE, with dip angles mostly greater than 70° and fracture density ranging from 1.5 to 3.5 m-1. According to filling characteristics and fracture cross-cutting relationships fracture filling can be divided into three stages. Fractures with stage I filling show strong fluorescence reaction under the microscope, demonstrating filled fractures can still be effective due to internal microfractures. The formation and development of fractures in carbonate reservoirs are controlled by four factors: tectonic stress field, fault, lithology and karstification. The magnitude and direction of the tectonic stress field during the fracture formation period determine the fracture size and distribution mode. Under the significant influence of strike-slip faults a large number of fault-associated fractures are developed, and their fracturing pattern is consistent with that of strike-slip faults. The radius of influence of a single strike-slip fault zone is 2048.56 m, and the development of fault controlled fractures is strong within 0-308.25 m from the fault zone and significantly reduced beyond this range. The tension-shear section of the strike-slip fault and the end of the fault zone show good fracture development, and the active plate has high fracture density with more abundant oil/gas filled fractures. The level of fracture development is positively correlated with the content of brittle minerals and significantly influenced by Young’s modulus and Poisson’s ratio. The highest level of fracture development is found in dolomite, and the most effective fractures are in sandy limestone. Large-scale fracture development are seen on mounds under karst platform in shallow areas and along the vertical direction, with an average fracture density of 2.67 m-1, strongly enhancing reservoir performance. Figures and Tables | References | Related Articles | Metrics

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Coupling relationship between strike-slip fault and paleokarst in Tahe Oilfield and its influence on the development of Ordovician reservoirs MA Hailong, YANG Debin, WANG Zhen, ZHANG Juan, WU Bo, ZHANG Shiliang, YUAN Feiyu 2024, 31(5): 227-246.  DOI: 10.13745/j.esf.sf.2024.6.24 Abstract ( 434 )   HTML ( 4 )   PDF (24983KB) ( 58 )   The coupling relationship between strike-slip faults and ancient karst processes has not been well studied. This paper examines strike-slip fault TH12518 in the Tahe Oilfield aiming to clarify the coupling relationship between the fault’s internal structure and ancient landform and water systems, by studying the characteristics of spatiotemporal structural superposition and transformation in the Ordovician Yijianfang Formation as well as karst activities during different tectonic phases. TH12518 experienced multistructural superposition under multistage tectonic activities, yielding five structural styles: pull-apart + translational + pull-apart (I); split + translational + translational (II); translational + compressive + pull-apart (III); translational + compressive + translational (IV); reverse thrust + overthrust (V). Stuctural styles I, II mainly occur in the middle Calendonian period, while structural supperposition between various structrual styles (I-V) occurs in the late Caledonian and early Haixi periods, under “bending effect.” There are significant differences in karst landforms between different karst periods. The water systems are underdeveloped during episode I of the middle Caledonian; whilst during episodes II, III the northern part uplifts to form a quasi karst plain while the southern karst slope area develops near N-S trending surface water systems and underground rivers. In the late Caledonian, multiple sets of water systems are developed in the karst slope area, forming a new NE-E trending underground river and surface water system along the fault. And in the early Haixi, large meandering rivers are developed on karst slopes. The karst processes in the Yijianfang Formation differ between different karst periods, regions, and segmentation styles. Episode I of the middle Caledonian develops exposed karst, overall, with infiltration and dissolution along the fault and strong dissolution in the N-NE trending fault segments; episodes II, III develop exposed karst in the north and semi-open overpressure karst in the south; with the segmented dissolution further enhanced, a sinking, buried fault-controlled karst is also developed in episode II. In the late Caledonian, exposed karst, semi-open overpressure karst, and sinking, buried fault-controlled karst are developed. The north, controlled by the karst baseline, exhibits limited downward karst process at the Cambrian top-dolomite interface; the south, not controlled by the reference plane, shows extensive downward dissolution at least to the top of the Cambrian Avatag Formation. Vertically, the karst fissures and caves are clearly layered. Whilst in the early Haixi, semi-open overpressure karst is developed, with weak karstification except for the bending effect and segmentation. This study provides a geological basis for large-scale karst development in the region. Figures and Tables | References | Related Articles | Metrics

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Fracture stages and distribution patterns in Cambrian fractured dolomite reservoirs, Sandaoqiao gas field MENG Qingxiu, CAO Zicheng, DING Wenlong, YANG Debin, MA Hailong, DIAO Xindong, WANG Ming, HAN Pengyuan, WANG Huanhuan 2024, 31(5): 247-262.  DOI: 10.13745/j.esf.sf.2024.6.23 Abstract ( 621 )   HTML ( 1 )   PDF (17505KB) ( 49 )   The Sandaoqiao gas field in northern Tarim Basin, southern slope zone of Tabei Uplift, develops typical fractured dolomite reservoirs in deep Cambrian. The dolomite reservoirs show weak dissolution where structural fractures are the main controlling factors of reservoir storage space and physical properties. Through comprehensive analysis of fracture intersections using core logs, microscope, and micro CT, and analysis of the strength of ancient structures through acoustic emission experiments, combined with regional diagenesis and burial history, the characteristics of differential development of two main fracture stages are clarified. The first stage, mostly between the late Caledonian to early Hercynian, forms a wide, high-angle echelon fracture, with rough fracture surface and severe filling and cementation. The second stage, considered between the late Hercynian and Indosinian, forms finer, medium- to high-angle fractures, with straight fracture surface, mostly semi-filled. The intersection between the two stages results in significant local displacement. Through statistical analyses of core fracture densities and single-well fracture development parameters the degree of fracture development in the dolomite reservoirs at different structural locations is quantitatively characterized. The top parts of the limbs of anticlines in the Cambrian dolomite buried hill have the highest degree of fracture development while the lower parts show relatively low fracture development. A fracture development model for asymmetrical anticlines in the research area is established. The understanding of the differences and distribution pattern of fracture stages in this study can provide a reference for the regional fracture development stages, identification/characterization of effective fractures, quantitative prediction of fracture distribution, and evaluation of favorable fracture zones in the study area. Figures and Tables | References | Related Articles | Metrics

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Fractures in Ordovician carbonate rocks in strike-slip fault zone, Shunbei area: Fracture distribution prediction and fracture controlling factors LI Yuntao, DING Wenlong, HAN Jun, HUANG Cheng, WANG Laiyuan, MENG Qingxiu 2024, 31(5): 263-287.  DOI: 10.13745/j.esf.sf.2024.6.27 Abstract ( 715 )   HTML ( 2 )   PDF (23272KB) ( 64 )   Tectonic fractures are one of main reservoir spaces in carbonate rocks, which can provide a good conduit for oil and gas transportation and reservoir space in tight limestone. The development of tectonic fractures is affected by various factors such as tectonic location, lithology, reservoir thickness, temperature, peripheral pressure and tectonic faulting, among which tectonic faulting caused by local tectonic stress in the regional tectonic stress field is an important factor controlling the development of tectonic fractures. In view of the characteristics of carbonate reservoirs and fracture development, we use the inversion function of rock mechanical parameters based on 3D seismic data volume, calibrated using dynamic rock mechanical parameters for a single well, to obtain a non-homogeneous rock mechanical model to improve the authenticity and accuracy of mechanical parameters in the model in the simulation of the stress field. Using the method of self-adaptive boundary condition constraint the optimal boundary conditions are automatically obtained when the error between the simulation and measured results is minimized, significantly improving the accuracy and reliability of the stress field simulation. On this basis, the fracture development characteristics and fracture activity in reserviors in the SHB16 fault zone and adjacent areas are quantitatively characterized using parameters including reservoir tensile rupture rate, shear rupture rate, comprehensive rupture rate, horizontal stress difference, stress difference coefficient, and sliding trend coefficient for the fault plane. We carried out qualitative and quantitative investigation into the effect of controlling parameters, such as horizontal stress difference, distance from faults and fault activity intensity in the vertical direction, on the fracture development characteristics; the correlations between variables were quatified using Spearman’s rank correlation coefficient. On the basis of clarifying the controlling factors of reservoir fracture development, we constructed the reservior development indexes for Ordovician carbonate reservoirs to classify the Ordovician carbonate reservoirs into categories I-IV from the best to the worst, and clarified the correlation between the deformation modes of the strike-slip faults and the degree of fracture development in sizable reserviors, further establishing the geologic model under different reservoir categories. The above results not only improve the accuracy and reliability of quantitative prediction of fracture development characteristics and multiparameter distribution rules based on stress field simulation, but also have significant importance for speeding up the exploration and development process of carbonate reservoirs. Figures and Tables | References | Related Articles | Metrics

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Typical strike-slip fault zones in southeastern Sichuan: Fault characteristics and potential for fault-controlled fractured vuggy reservoirs SHI Siyu, LI Bisong, LI Rangbin, ZOU Yutao 2024, 31(5): 288-300.  DOI: 10.13745/j.esf.sf.2024.6.32 Abstract ( 661 )   HTML ( 2 )   PDF (15032KB) ( 53 )   Controlled by the physical fracturing of medium and small-scale basement strike-slip fault zones fault-controlled fractured vuggy reservoirs are developed in Shunbei, Tarim Basin, where exploration breakthroughs have been made in recent years. Recent research has found that the NW-trending basement fractures in the southeastern Sichuan Basin also have typical strike-slip characteristics, while further analysis is needed to determine whether the area has undergone similar reservior development. Based on 3D seismic data, we start with the geometric and kinematic characteristics of strike-slip faults, and analyze and compare the deformation characteristics of typical strike-slip fault zones between Shunbei and the Fuling/Qijiang areas, southeastern Sichuan. Meanwhile, based on the analyses of fault movement direction, fault activity, and structural background, the formation and evolutionary history of basement faults in the study area is restored and compared with that of Shunbei. Finally, based on the proven seismic reflection characteristics of Shubei reservoirs, we sort out the seismic anomalies near typical strike-slip fault zones in the study area, and comprehensively analyze the area’s potential for reservoir development. The analyses suggest that the geometric and kinematic characteristics of the NW-trending basement strike-slip fault zone in the study area are similar to those in Shunbei that has undergone multi-periods of parallel strike-slip activities, while Fuling has experienced three periods of fault activities of different types. In addition, the seismic reflection characteristics corresponding to typical reservoirs in northern Shunbei can also be identified in the study area. It is generally believed that typical strike-slip fault zones in Fuling have great potential for the development of fracture-controlled fracture-cave reservoirs, and Qijiang also has certain development potentials. Figures and Tables | References | Related Articles | Metrics

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Characterization of Connectivity in Ultra-Deep Fractured-Caveate Reservoirs Considering Fluid-Solid Coupling: A Case Study of the Manfen Block in the Fuman Oil Field of the Tar Basin CAI Zhenzhong, ZHAO Haitao, WANG Peng, LI Jing, XU Guojin 2024, 31(5): 301-312.  DOI: 10.13745/j.esf.sf.2024.6.33 Abstract ( 473 )   HTML ( 1 )   PDF (6790KB) ( 48 )   Ultra-deep fractured-cave carbonate reservoirs are buried at great depths, with high stresses, and possess complex and diverse storage spaces. Fluid flow and seepage coexist, making the characterization of reservoir connectivity challenging. Accurate characterization of reservoir connectivity is crucial for identifying oil and gas enrichment areas, predicting reserves precisely, optimizing well patterns, and planning well locations. Therefore, this study focuses on the ultra-deep fractured-cave carbonate reservoirs in the Fuman Oil Field of the Tarim Basin. Considering the effect of fluid-solid coupling, a coupled mathematical model of stress seepage and free flow in fractured-cave reservoirs was established. A systematic study on the connectivity of ultra-deep fractured-cave carbonate reservoirs was conducted using a combination of rock permeability evolution experiments and numerical simulations. The research results indicate that with the increase of confining stress and axial stress, rock permeability gradually decreases, leading to weakened rock connectivity. Under low confining stress conditions, axial stress significantly affects rock permeability and connectivity. As the fracture aperture and angle increase, reservoir connectivity gradually improves. Fracture-connected caves can significantly enhance reservoir connectivity. The fluid flow rate and equivalent permeability increase with the increase in cave volume, which greatly improves reservoir connectivity. However, simply increasing the number of caves weakens the free flow (Stokes) effect, resulting in a less significant improvement in reservoir connectivity. The research findings provide technical support for the precise evaluation of deep carbonate reservoirs and enhance the efficiency of oil and gas exploration and development. Figures and Tables | References | Related Articles | Metrics

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Lithological associations, deformation characteristics of the Lower-Middle Cambrian and their influence on oil and gas migration in the North-central Tarim Basin GU Yu, WU Jun, FAN Tailiang, LÜ Junling 2024, 31(5): 313-331.  DOI: 10.13745/j.esf.sf.2024.9.5 Abstract ( 507 )   HTML ( 3 )   PDF (27397KB) ( 61 )   Exploration and production practices in the North-central Tarim Basin have revealed that the distribution of oil and gas is strictly controlled by faults, with significant variability in oil and gas between different fault segments. Fault zones played an important role in their vertical migration. Therefore, revealing lithological associations of the Lower-Middle Cambrian in the North-central regions of this basin is of great significance for determining strata deformation and clarifying factors affecting migration efficiency of oil and gas. This study was based on cores, drilling/logging, 3D seismic and production dynamic datum from this study area. Firstly, the lithological types and associations analysis were carried out. Subsequently, the seismic response characteristics of lithological associations was clarified through well-seismic calibration and regional seismic profile tracking. Finally, the deformation characteristics of target layers under fault movements was identified, and lithological associations and their deformation influences on oil and gas migration was revealed. The results can be obtained as follows: (1) The lithological types consist of dolomites, gypsum-bearing rocks and argillaceous dolomites, which are classified into three types of lithological associations according to their stacking patterns: upper and lower segmented, symmetrical and interbedded types. (2) There are significant differences in seismic reflection characteristics among lithological associations. The upper and lower segmented lithological associations are corresponded to the weak amplitude reflection at the upper part, the strong amplitude reflection and sheet parallel to sub-parallel reflection at the lower part. The symmetric lithological associations show that the seismic reflections with medium-strong amplitude and good continuity are interspersed with sheet-parallel reflections of weak amplitude and medium continuity. The interbedded lithological associations are characterized by strong amplitude, high frequency and good continuity in seismic reflection. (3) Under different stress segmentation effects, the deformation characteristics of target layers are divided into three types: thickness reduction, thickness increase, and thickness stability. The combinations of lithological associations and deformation characteristics jointly controls the oil and gas migration. There are significant differences in the stacking styles of lithological associations in the east part and west part of the Shunbei block. The west part of this study block is a symmetrical stacking, while the east part is characterized by segmented to symmetrical stacking type. The thinning of the target layer thickness is conducive to the passage of oil and gas. The symmetrical lithological association has a higher degree of deformation and fragmentation at the interface and inside the formation, which is conducive to the oil and gas migration, the upper and lower segmented lithological associations are more favorable in tension sections. Figures and Tables | References | Related Articles | Metrics

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Multi-scale fracture development characteristics and fracture network patterns of buried-hill in metamorphic rocks: A case study of the Bozhong Z metamorphic buried-hill GONG Lei, QIN Xinnan, GAO Shuai, FU Xiaofei, SU Xiaocen, WANG Jie 2024, 31(5): 332-343.  DOI: 10.13745/j.esf.sf.2024.6.19 Abstract ( 729 )   HTML ( 3 )   PDF (7849KB) ( 51 )   Natural fractures are important reservoir spaces and effective seepage channels for metamorphic buried hill reservoirs. For metamorphic rock reservoirs an interconnected network of multi-scale fractures is key to forming high quality contiguous reservoirs and achieving high, stable reservoir productivity. In this study, the power law distribution of fractures is established based on detailed characterization of multi-scale fractures using image log, core, thin section and SEM data. The contribution of malti-scale fractures to reservior storage is clarified, and the spatial pattern of fracture network and its impact on productivity is analyzed. According to the results, fracture systems of different scales in the study area show similar change patterns, i.e., the increase of fracture intensity follows the power law with the decrease of fracture size. In tight reservoirs, large and well connected macroscopic fractures provide important permeability, but their contribution to porosity is limited due to low fracture density; whereas microfractures with high fracture density can provide important reservoir space, but they mainly play the role of connecting matrix pores due to their small aperture and limited connectivity. According to spatial combination of multi-scale fractures, five types of fracture networks are recognized. Among them, the multi-scale/high-density/multi-set combination type and large-scale/medium-density/multi-set type networks can form large-scale continuous high-quality reservoirs and achieve high, stable production, while small-scale/high-density/multi-set type and large-scale/low-density/multi-set type networks require hydraulic fracturing to achieve stable production; small-scale/low-density/single-set type fracture network can not improve tight reservoir due to difficulty of obtaining industrial oil flow. Figures and Tables | References | Related Articles | Metrics

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The control mechanism of deep coal rock microstructure on in situ stress TANG Zhitan, LIU Jingshou, YAN Xia, FENG Yanqing, JIANG Shu, ZHANG Binxin, ZHANG Guanjie, FU Yiming 2024, 31(5): 344-357.  DOI: 10.13745/j.esf.sf.2024.6.26 Abstract ( 619 )   HTML ( 1 )   PDF (7356KB) ( 52 )   The theory of shallow coalbed methane (CBM) exploration and development cannot be directly applied to deep CBM partly due to the effect of in situ stress. In situ stress in deep coal beds restricts the process of CBM adsorption/desorption and seepage, determines the effectiveness of coalbed fissures, and affects the design of horizontal well trajectory. Thus the pattern of in situ-stress change in deep coal seams is of great significance to the exploration and development of CBM. In this paper, taking the Daji block in Ordos Basin as an example, using data from array acoustic logging, microseismic monitoring, and core testing, considering the microstructure types and attitude, boundary stress conditions, and combination of mechanical properties of top and bottom slabs, we established a three-dimensional geomechanical microstructural model of deep coal beds by using ANSYS finite element software to comprehensively analyze the control mechanism of microstructure types and attitute on in situ stress in deep coal beds. Results show that with smoother microstructure the stress distributes more uniformly; conversely, the stress concentrates more easily. The influence of the attitute of microstructure on in situ stress in coal seams is mainly as follows: as the curvature of the microstructure increases, the differential horizontal stress at the bending point increases, and the minimum principal stress increasingly concentrates around the bending point. According to cross-simulation between microstructure type and mechanical properties of coal seam, lithology of top/bottom slabs, and boundary stress conditions, in situ stress under positive curvature is positively correlated with Poisson’s ratio and negatively correlated with Young’s modulus, whereas the opposite is true under negative curvature. Compared to with sandstone top slab, the magnitute of in situ stress in coal seams with limestone top slab is more significantly affected by the change of microstructure type. The magnitude of the regional stress has relatively small influence on in situ stress in deep coal seams. The research results provide an useful reference for the genetic analysis of in situ stress in deep coal beds, and for the efficient development of coalbed methane and the practice of geoengineering integration. Figures and Tables | References | Related Articles | Metrics

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The principle, process, and measurement of karst carbon sink CAO Jianhua, YANG Hui, HUANG Fen, ZHANG Chunlai, ZHANG Liankai, ZHU Tongbin, ZHOU Mengxia, YUAN Daoxian 2024, 31(5): 358-376.  DOI: 10.13745/j.esf.sf.2024.2.6 Abstract ( 664 )   HTML ( 7 )   PDF (5909KB) ( 76 )   The carbon cycle in karst ecosystems consists of two parts: biological carbon cycle (driven by plant photosynthesis) and karst carbon cycle (driven by carbonate dissolution and weathering). There is a synergistic effect between karst carbon cycle and terrestrial biological carbon cycle, which significantly impacts terrestrial freshwater ecosystems. Karst carbon sinks primarily occur in the surface karst zone, where plant roots, soil, and rocks intermingle. Their migration and transformation processes take place in both groundwater and surface water systems. There are at least three uncertainties in the measurement of karst carbon sinks within a watershed: changes in the proportion of carbon derived from carbonate rocks versus that from the atmosphere/soil across the entire karst watershed, differentiation of carbon sinks produced by the weathering and dissolution of carbonate versus silicate rocks in certain karst watersheds, and distinctions between endogenous organic carbon produced by aquatic plant photosynthesis and exogenous organic carbon from terrestrial ecosystems. It is recommended to use the watershed as a unit, define watershed boundaries, identify geological structures, analyze land cover configurations, reveal the main controlling factors of the karst carbon cycle and carbon sink effects, establish inversion and forward models, and address gaps in the service functions of karst carbon sinks. Figures and Tables | References | Related Articles | Metrics

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Hydrochemical characteristics and karst carbon sink effect of border polje river in subtropical monsoon region: A case study of the Qingbo River in Mashan County, Guangxi ZHANG Chunlai, YANG Hui, HUANG Fen, QIU Cheng, ZHU Tongbin 2024, 31(5): 377-386.  DOI: 10.13745/j.esf.sf.2024.2.14 Abstract ( 686 )   HTML ( 3 )   PDF (4202KB) ( 40 )   The concurrent occurrence of rainfall and heat in the subtropical monsoon climate promotes the formation of karst carbon sinks. Border poljes, recharged by allogenic water, are regions with strong carbonate rock dissolution in karst areas. Studying the hydrochemical characteristics of different water types in these regions helps deepen the understanding of karst carbon sequestration processes. This paper employs statistical methods, Piper diagrams, Gibbs diagrams, and major ion ratios to analyze the hydrochemical characteristics, origins, and carbon sink effects of rivers in a typical subtropical border polje. The results show that: (1) The main types of river water in the border polje are allogenic water, surface rivers recharged by allogenic water, karst underground rivers recharged by allogenic water, and karst groundwater. The total ion concentration shows an increasing trend. The average concentration of Ca2+ in karst groundwater is 91.06 mg/L, which is 2.3, 4.3, and 12.4 times that of underground rivers recharged by allogenic water, surface water recharged by allogenic water, and allogenic water, respectively. (2) After the allogenic water enters the karst area, the concentrations of Ca2+ and $\mathrm{HCO}_{3}^{-}$ gradually increase, and the calcite saturation index shifts positively. In the context of an accelerated water cycle due to heavy rainfall in the subtropical monsoon region, the reaction time of allogenic water with carbonate rocks is insufficient after entering the karst area to strengthen erosion. Whether it is direct infiltration of allogenic water, or karst underground rivers or surface water recharged by allogenic water, it has significant potential for increasing the carbon sink. Figures and Tables | References | Related Articles | Metrics

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Effects of algae-derived organic matter source on sediment mineralization in the karst reservoir HUANG Siyu, PU Junbing, PAN Moucheng, LI Jianhong, ZHANG Tao 2024, 31(5): 387-396.  DOI: 10.13745/j.esf.sf.2024.2.7 Abstract ( 641 )   HTML ( 1 )   PDF (1932KB) ( 33 )   In the karst reservoir, there are a lot of $\mathrm{HCO}_{3}^{-}$ in the water column, which can promote algae to function of biological carbon pump and lead the algae organic matter to be deposited under the water column. Meanwhile, algae organic matter is mineralized on the sediment suffer in order to affect the organic matter burial, carbon cycle of karst aquatic environment. To study effect and of algae-derived organic matter source on sediment mineralization, we selected the Dalongdong Reservoir as the object and analyzed carbon cycle stability of karst reservoir aquatic environment. The results revealed that: organic carbon in the surface sediment come from the algae source (20.9%-65%) and soil source (11.8%-53.4%), which organic carbon of algae source was mainly deposited on the downstream and soil source' was on the upstream. The potential mineralization on the upstream was higher than that on the downstream, because the mineralization process of organic carbon was influenced by organic source difference, especially algae-source; with biological carbon pumping effect and inorganic carbon protection, potential burial volume in the surface sediment of karst reservoir was higher than that in the karst soil, which showed that sediment structure in the karst reservoir was stable. Figures and Tables | References | Related Articles | Metrics

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Geochemical characteristics of trace elements and their implications in the small karst basin, Southwest China WU Qing, HUANG Fen, GUO Yongli, XIAO Qiong, SUN Ping’an, YANG Hui, BAI Bing 2024, 31(5): 397-408.  DOI: 10.13745/j.esf.sf.2024.2.10 Abstract ( 724 )   HTML ( 0 )   PDF (3997KB) ( 52 )   Trace elements play a significant role in resources and environmental research in karst areas. In this paper, we examine the geochemical characteristics of trace elements in a typical karst underground river located in a peak cluster depression and discuss their implications for karst spatial structure and hydrology. In the Maocun underground river basin, trace elements such as Sr, Cr, Ni, Co, and Mn primarily originate from the dissolution of carbonates. Intense precipitation can drive deep karst water flow, whereas weak precipitation has limited driving force. Spatial and temporal variations in the ratios of ρ(Ni)/ρ(Co), ρ(Sr)/ρ(Ca2+) and ρ(Sr)/ρ(Mg2+) indicate that the LLS and BY subsystems exhibit limited karst development, while greater karstification is observed from SGY to MC. The ratios of ρ(Sr)/ρ(Ca2+)and the slopes of ρ(Sr) relative to ρ(SiO2) increase with the degree of karst development, making them potential indicators of karstification. The spatial and temporal variations of ρ(Sr) are closely related to the lithologic characteristics of the strata, allowing the division of the basin into non-karst areas, transition zones from non-karst to karst areas, and karst areas based on geological background. Using the changes in ρ(Sr) from non-karst to karst areas and applying the principle of mass conservation, we calculated the average proportions of karst water contributions to the SGY, SWQ, LLS, and BY springs during the dry season to be 51.50%, 50.46%, 65.48%, and 22.16%, respectively. Trace elements can indicate the degree of karstification and quantitatively estimate the proportions of karst water sources, providing scientific guidance for addressing complex resource and environmental issues in karst areas. Figures and Tables | References | Related Articles | Metrics

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Fractionation of stable isotopes and the carbon-water cycle in Yangtze River YUAN Yaqiong, SUN Ping’an, YU Shi, HE Shiyi 2024, 31(5): 409-420.  DOI: 10.13745/j.esf.sf.2024.2.8 Abstract ( 786 )   HTML ( 2 )   PDF (4329KB) ( 46 )   The karst carbon cycle exhibits rapid dynamic responses and is sensitive to environmental changes. The water cycle can influence the two driving factors of the karst carbon cycle (water and CO2), making it an important influencing factor. This study analyzes samples collected in June and October 2016 from the main stream, major tributaries, reservoirs, and lakes of the Yangtze River. The study examines δD, δ18O, inorganic carbon content, and isotope fractionation characteristics and controlling factors, revealing the impact of the water cycle on the karst carbon cycle in the Yangtze River Basin. The results show that the spatial variation in δD and δ18O compositions in the Yangtze River Basin’s water bodies reflects continental, latitudinal, and altitudinal effects, and varies with the seasonal changes in rainfall. Inorganic carbon mainly originates from the weathering of carbonate rocks, and δ13CDIC values are primarily controlled by the relative contributions of carbonic acid weathering and sulfuric/nitric acid weathering of carbonate rocks to $\mathrm{HCO}_{3}^{-}$. Hydrological processes in the Yangtze River Basin significantly impact the karst carbon cycle. In the upstream permafrost region, soil freezing results in mantle-derived and atmospheric CO2 participating in the weathering of carbonate rocks, significantly increasing δ13CDIC values. During the summer and autumn monsoon rains, the rapid decline in precipitation δD and δ18O, coupled with soil CO2 accumulation and the decrease in carbon isotope values, leads to a decrease in δD, δ18O, and δ13CDIC values in the water. Hydrological processes also affect the “biological carbon pump” effect, which is stronger during normal flow periods, and the stratification effect in reservoirs becomes more pronounced. Figures and Tables | References | Related Articles | Metrics

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Influence of lead and zinc on geological carbon sink under oxygen-rich conditions LI Liang, JIANG Zhiwei, WU Bingjin, WEI Dongwen, WANG Wenhai 2024, 31(5): 421-429.  DOI: 10.13745/j.esf.sf.2024.2.12 Abstract ( 456 )   HTML ( 0 )   PDF (3646KB) ( 42 )   Since the Industrial Revolution, the increasing demand for resources has led to the overexploitation of mineral resources, resulting in a series of geological and environmental problems. Research in mining has primarily focused on geological and environmental hazards, mine restoration technologies, and the health hazards posed by heavy metals exposure. However, few scholars have studied the relationship between heavy metals and geological carbon sinks. With the “dual-carbon” goal becoming a national strategy, research on the mechanisms of heavy metals and geological carbon sinks has significant scientific importance. This study investigates the influence of lead and zinc on geological carbon sinks under open system conditions by examining the dissolution characteristics of different rock samples in various concentrations of lead and zinc solutions. The findings are as follows: (1) With increasing experimental time, the pH value of all groups showed a decreasing trend, while the conductivity showed an increasing trend. (2) A 0.5 mg/L lead solution may inhibit rock dissolution, as the $\mathrm{HCO}_{3}^{-}$ concentration initially increased significantly on the 10th day before gradually decreasing. When lead-zinc ore powder from the Lingchuan County lead-zinc mine was added, the $\mathrm{HCO}_{3}^{-}$ concentration in the solution doubled. Data showed that the $\mathrm{HCO}_{3}^{-}$ concentration in the 0.1 mg/L lead solution with added lead-zinc ore powder was significantly lower than in the control group without the ore powder. This suggests that Pb2+ may act as an inhibitor, affecting the dissolution of rock samples in the solution. (3) In the 1 mg/L and 5 mg/L zinc solutions with added lead-zinc ore powder, the $\mathrm{HCO}_{3}^{-}$ concentrations in the limestone, hornfels, and limestone+hornfels groups were significantly higher than in the control groups without the ore powder, indicating that zinc may act as a catalyst. (4) Scanning non-in situ SEM images of the dried rock samples revealed that both low and high concentrations of lead solutions increased the number of dissolution pits compared to the lead-free solution. The dissolution steps were also more irregular in high-concentration lead solutions, accompanied by replacement reactions. From low to high concentrations of lead solutions, the development of porosity and the formation of cracks were observed. Figures and Tables | References | Related Articles | Metrics

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Study on the effect of submerged plants on the stability of karst carbon sink SUN Caiyun, ZHENG Bingqing, LI Jun, FU Hongming, SUN Rongqing, LIU Honghao, LIAO Zuying, JIANG Hongsheng, WU Zhenbin, XIA Shibin, WANG Pei 2024, 31(5): 430-439.  DOI: 10.13745/j.esf.sf.2024.2.9 Abstract ( 756 )   HTML ( 3 )   PDF (2120KB) ( 33 )   Karst carbon sinks are an important means of achieving carbon neutrality, and their stability is a key scientific issue that needs to be addressed. Approximately 45% of annual photosynthesis on Earth occurs in aquatic environments, yet how submerged plants in karst areas affect the stability of karst carbon sinks remains unknown. This study focused on submerged plants in three karst rivers. We employed quadrat sampling, pH-drift technology, and elemental stoichiometry to qualitatively and quantitatively examine the effects of submerged plants on the stability of karst carbon sinks. Our results showed that there were 8, 5, and 7 species of submerged plants in the ZDR, CTR, and HXR, respectively. The Shannon-Wiener diversity index and Simpson dominance index ranked as ZDR>HXR>CTR. In the three karst rivers, Vallisneria natans, Ottelia acuminata, Potamogeton wrightii, and Hydrilla verticillata were the dominant species, all of which had the ability to utilize $\mathrm{HCO}_{3}^{-}$. The annual carbon sequestration rates of submerged plants in the ZDR, HXR, and CTR were 8.56×103 g·m-2·a-1, 4.83×103 g·m-2·a-1, and 3.88×103 g·m-2·a-1, respectively, with an average of 5.76×103 g·m-2·a-1, which are 37.65 and 40.56 times higher than those of grasslands and man-made forests, respectively. The higher the diversity of submerged plants in rivers, the higher the carbon sequestration. Overall, submerged plants play a crucial carbon pump role in karst aquatic ecosystems, thereby enhancing the stability of karst carbon sink. Figures and Tables | References | Related Articles | Metrics

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Biomass and distribution characteristics of dominant shrubs under varying degrees of rocky desertification in the karst region of southern Yunnan YANG Huaju, LI Canfeng, YANG Kehao, ZHANG Xilu, WANG Chuanyu, WANG Xingrong, HE Xu, PENG Xuefeng, ZHANG Liankai 2024, 31(5): 440-448.  DOI: 10.13745/j.esf.sf.2024.2.11 Abstract ( 749 )   HTML ( 2 )   PDF (3372KB) ( 35 )   Shrubs play a crucial role in vegetation restoration in karst rocky desertification areas, with their biomass directly influencing the restoration of rocky desertification and the carbon budget. To explore the biomass characteristics and distribution patterns of dominant shrub species in response to rocky desertification succession, this study focused on three dominant shrub species in the karst areas of southern Yunnan: Osteomeles schwerinae, Osyris lanceolata and Dodonaea viscosa. We analyzed their crown width, tree height, basal diameter, and the biomass of various organs (roots, stems, leaves, and flowers/fruits). The results showed that: (1) Under different rocky desertification gradients, the average biomass of Osteomeles schwerinae and Dodonaea viscosa exhibited an “inverted V” trend, while Osyris lanceolata showed a different pattern. (2) The order of biomass allocation among the organs of the three dominant shrubs under various rocky desertification gradients was: stems > roots > leaves > flowers/fruits. (3) The biomass estimation for the three shrubs was best described by linear and power functions, with the best predictive variables being basal diameter (D) and the product of basal diameter squared and tree height (D2H). This study provides fundamental data for vegetation restoration and ecosystem carbon budget research in karst rocky desertification areas Figures and Tables | References | Related Articles | Metrics

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Weathering process and carbon sink effect of carbonates in typical karst small basin CHEN Fajia, XIAO Qiong, HU Xiangyun, GUO Yongli, SUN Ping’an, ZHANG Ning 2024, 31(5): 449-459.  DOI: 10.13745/j.esf.sf.2024.2.13 Abstract ( 693 )   HTML ( 4 )   PDF (2603KB) ( 60 )   Carbonate weathering is a process of absorbing CO2 from the atmosphere, exhibiting a carbon sink effect. Utilizing the hydrogeochemical characteristics of water in karst areas to reveal the weathering process of carbonate rocks and calculate karst carbon sink fluxes in basins is an important aspect of global change research. A case study of the Guancun Groundwater (GGW) in Liuzhou, Guangxi, with monitoring results from four seasons in 2021, showed that: (1) The main anion in the basin is $\mathrm{HCO}_{3}^{-}$, and the main cation is Ca2+. The equivalent concentrations and TDS of anions and cations reflect the intense weathering of carbonate rocks. (2) Based on the spatiotemporal distribution characteristics of its hydrochemistry, the GGW belongs to the HCO3-Ca type. Besides the weathering of carbonate rocks by H2CO3, the weathering by H2SO4 and HNO3 also occurs in water-rock interactions. (3) Considering the combined water-rock interaction process involving H2CO3, H2SO4 and HNO3, the average CO2 consumption by H2CO3 weathering of carbonate rocks is calculated using δ13CDIC as 2.38 mmol/L. The net CO2 consumption is expressed as [$\mathrm{HCO}_{3}^{-}$]mol-[Ca2++Mg2+]mol, with an average net CO2 consumption of 1.98 mmol/L. The inorganic carbon sink intensity (Cm) of the GGW is 86.37 tCO2·km-2·a-1. (4) However, the inorganic carbon sink intensity (Cm) of this groundwater, calculated using the hydrochemical-runoff method, is 94.49 tCO2·km-2·a-1. The net CO2 consumption is 91.41% of the CO2 sink calculated by the hydrochemical-runoff method. (5) The carbon sink intensity in 2021 was 1.91 times that of 2008, providing a reference for assessing the potential to increase the karst carbon sink through artificial intervention. Figures and Tables | References | Related Articles | Metrics