The formation and distribution of sedimentary facies of the Wufeng Formation reflect the evolution of Guangxi Movement and significantly impact shale reservoir quality in southern Sichuan Basin, China. This study characterizes the sedimentary facies and their evolution of Ordovician-Silurian transition shale based on detailed core descriptions, full-scale imaging of large slabs, and field emission scanning electron microscopy of argon-ion polished sections. There only exist fine-grained turbidite deposits, hemipelagic deposits, and shallow shoal deposits for the Wufeng shale. Fine-grained turbidite deposits consist primarily of clastic quartz and clay minerals and can be divided into nine subdivisions. Hemipelagic deposits are mainly composed of quartz, detrital carbonate, and clay minerals. Shallow shoal deposits are dominated by clay minerals, dolomite, and calcite, with carbonates primarily of autochthonous origin. The fine-grained turbidite deposits predominantly occur within the Dicellograptus complanatus and D. complexus graptolite biozones, while hemipelagic deposits are confined to the Paraorthograptus pacificus biozone, and shallow shoal deposits are restricted to the Metabolograptus extraordinarius biozone. Formation and distribution of the three sedimentary facies are closely related to the Guangxi Movement. During the strong tectonic compression stage, sufficient sediment supply and intensive volcanic eruption favored the formation of the fine-grained turbidite deposits. Along with waning tectonic activity and reduced terrestrial input, hemipelagic deposits formed and then shallow shoal deposits. Sedimentary facies exert first-order controls on shale reservoir quality, with hemipelagic deposits exhibiting optimal reservoir characteristics. Laboratory analyses reveal that hemipelagic facies possess the highest porosity (3.34–4.15%) and TOC content (2.91–4.10%) due to biogenic quartz enrichment and minimal allochthonous dilution, whereas fine-grained turbidites show degraded properties (porosity: 1.58–3.81%; TOC: 0.15–2.6%) from high-energy siliciclastic influx. Shallow shoal deposits display intermediate values (porosity: 3.92%; TOC: 3.25%), constrained by carbonate cementation.

Organic matter exhibits significant heterogeneity and complexity, with varying pore structures across different types influenced by multiple interacting factors. This paper introduces a “two categories, six subcategories” classification scheme based on morphological observations using a combination of argon ion polishing and scanning electron microscopy (SEM). Organic matter is classified into two main categories: depositional organic matter and migrated organic matter, based on whether migration has occurred. Depositional organic matter is further subdivided into three types based on microscopic characteristics: bioclasts, compacted kerogen, and in situ remnants from post-hydrocarbon generation. Migrated organic matter is categorized into three types: organic matter in intragranular pores, organic matter in intergranular pores, and bitumen in microfractures. Bioclasts can be further classified into alginite, zooclasts, acritarchs, and encapsulated organic matter based on maceral type. Zooclasts, acritarchs, encapsulated organic matter, and compacted kerogen—types of depositional organic matter—have few or no pores. This is primarily related to the nature of the hydrocarbon-generating precursor materials, with compacted kerogen being influenced by low thermal maturity and diagenetic compaction. In contrast, pores are more developed in alginite, in situ remnants from post-hydrocarbon generation, and all forms of migrated organic matter, largely due to the expulsion of gaseous hydrocarbons during thermal evolution. The pores in alginite reflect both the original structural properties of the hydrocarbon-generating precursor materials and the thermal evolution process. Depositional organic matter exhibits a stronger oil-generating potential and a higher gas-generating potential, while migrated organic matter primarily possesses a stronger gas-generating capability. Specifically, organic matter enriched in alginite, in situ remnants from post-hydrocarbon generation, as well as migrated organic matter in intragranular pore and intergranular pore, exhibit a higher hydrocarbon-generation potential.

The characteristics and formation of maximum flooding (MF) black shales are important aspects in defining the geology of fine-grained reservoirs. The MF black shales are located at the bottom of the Longmaxi Formation on the Upper Yangtze Platform, corresponding to graptolite zone LM1. Seismic interpretation, X-ray diffraction entire rock analysis, total organic carbon (TOC) tests, and field emission scanning electron microscopy analysis indicate that the MF black shales have an average content of 49.3% quartz (85% clay size), 10.5% calcite, 8.4% dolomite, and 23.4% clay minerals. The quartz content increases basinward, whereas the clay mineral content decreases. The shale has developed during rapid sea level rise, with a thickness of 0.5–2.8 m that gradually thickens basinward. The TOC content, averaging 5.4%, gradually decreases basinward, with four distinct stacking patterns. The mineral composition and thickness of the Longmaxi shale are related closely to rapid transgression, biology, and volcanism during the period of sedimentation. Rapid transgression has led to a decrease in terrestrial input and shale thickness. In addition, biological activity and volcanism have caused the prevalence of microcrystalline quartz. Shales with high TOC content are related to anoxic conditions, along with low sedimentation rates and high primary productivity. The combination of an anoxic water column, weak dilution, and enhanced organic matter (OM) supply have enhanced the preservation of the OM. The four TOC stacking patterns are related to the water depth. The supply of clay minerals decreases with increasing water depth, whereas the degradation and recycling of OM decrease the TOC content. The sediment accommodation increases with increasing water depth, resulting in four TOC stacking patterns.

CBM well pattern optimization design and deployment is a key part of CBM development plan formulation. Reasonable well pattern deployment can not only optimize CBM production, but also reduce CBM development costs and improve economic benefits. In response to this problem, taking Daning block in the Ordos Basin as the research object, the coal seam conditions and coal reservoir characteristics in the study area were systematically analyzed, and the suitable well type and well pattern orientation for CBM development were determined. The results show that the No.5 coal seam of Shanxi Formation in the Daning block has the characteristics of large thickness, simple structure, high gas content and low permeability. The well type of CBM development should be mainly directional wells, supplemented by horizontal wells, with the corresponding well pattern for large-scale development, and the well pattern orientation is NE66°; using the CBM productivity numerical simulation software COMET3, the current use of the Daning block is analyzed. The productivity was predicted based on the well pattern and well spacing, and the productivity under the open scheme of different well pattern and well spacing was simulated and evaluated. Numerical simulation results show that the CBM recovery rate is only 19.03% after 3086 d of well pattern and well spacing used in the current CBM development in the Daning block, indicating a low degree of CBM resource production. When the well spacing is 300 m×80 m and 400 m×100 m (that is, the well spacing ratio is 4∶1), the cumulative gas production of a single well in the five-spot well pattern is the largest. That is, when the well row spacing ratio is 3∶1), the cumulative gas production of a single well in the rectangular well pattern is the largest. Therefore, it is considered that the five-point well pattern should be selected when setting the well row spacing ratio of 4∶1 in the Daning block, and the 3∶1 Rectangular well pattern should be selected for the well spacing ratio of 1. After the well pattern and well spacing are optimized, the CBM recovery rate reaches 44.18% and 36.85% respectively, and the CBM resource production degree is greatly improved.

Shale has horizontal bedding of diverse origins in differential permeability.An integrated analysis of core data,full-diameter images of enlarged thin section and scanning electron microscope(SEM)images of argon-ion-milled samples,shows that the gas-bearing Wufeng-Longmaxi shale in southern Sichuan Basin develops four types of horizontal bedding,that is,grading type composed of claystone,grading type composed of siltstone and claystone,alternating siltstone and claystone type,and page type.The grading type of claystone constitutes the multi-layer superimposed siltstones in parallel,with siltstone and claystone depositing in graded bedding.The grading type composed of siltstone and claystone is composed of parallelly alternating silty and clayey beds,where the silty beds mainly in clast-supported texture,has abrupt boundary at the bottom and gradual contact with the clayey bed on top,featuring normal grading as a whole.The alternating siltstone and claystone type features abrupt contact between both beds and lamina with no grading.The page type constitutes parallel bedding of very thin clayey lamina with weakly normal grading.The four types of horizontal bedding are of different genesis.The grading type of claystone and grading type of siltstone and claystone are of relatively low-energy turbidite origin,with the former derived from even weaker hydrodynamic conditions;the alternating siltstone and claystone is of contourite origin such as shelf facies;and the page type is of pelagic origin from suspended sediment deposition.The horizontal bedding serves to directly affect shale permeability.The page type is characterized by the abundance of organic matter and organic pores,ranking top in permeability;the alternating siltstone and claystone type takes the second place in permeability with better sorting;while the two grading types come at last in permeability with poor sorting and low organic matter content.

细粒沉积物类型及演化是周缘板块构造演化的响应,并深刻影响页岩的储层品质.通过川南地区五峰组页岩岩心精细描述、大薄片全尺度成像和氩离子抛光片场发射扫描电镜分析,揭示晚奥陶世—早志留世之交海相页岩沉积类型及演化.结果表明:①川南地区五峰组发育细粒浊流沉积、半远洋沉积和陆棚浅滩沉积3大沉积类型:细粒浊流沉积物质主要来自于陆源,可清晰见到9个典型的沉积序列;半远洋沉积物以陆源和内源混合沉积为主,递变型水平层理发育;陆棚浅滩沉积以陆源成因和原地成因物质混合为主,沉积物主要以悬浮沉降的方式沉降.②细粒浊流沉积主要发育于笔石带WF1—WF2,半远洋沉积主要发育于笔石带WF3,而陆棚浅滩沉积主要发育于笔石带WF4.③细粒沉积类型及演化反映了周缘板块构造演化:笔石带WF1—WF2沉积期,周缘板块构造挤压强烈,陆源供给充分,故细粒浊流沉积发育;笔石带WF3沉积期,周缘板块构造活动减弱,海平面快速上升,故半远洋沉积发育;笔石带WF4沉积期,周缘板块构造活动进一步减弱,全球海平面快速下降,故陆棚浅滩沉积发育.细粒沉积类型直接影响页岩的储层品质,细粒浊流沉积孔隙度和总有机碳含量相对较低,而半远洋沉积孔隙度和总有机碳含量相对较高.

黑色页岩甜点类型、识别标准及分布规律直接影响勘探层位优选及钻井靶点设计.为此,以四川盆地南部(以下简称川南地区)上奥陶统五峰组—下志留统龙马溪组海相黑色页岩为例,结合岩石学、层序地层学等理论,综合分析了相对海平面变化、裂缝类型与页岩储层品质之间的关系.研究结果表明:①川南地区五峰组—龙马溪组黑色页岩可划分出沉积型和裂缝型 2 类甜点,其中,沉积型甜点细分为早期海进型、快速海进型、晚期海进型和近滨海进型 4 类,裂缝型甜点分为网状微裂缝型和网状宏观裂缝型2 类.②早期海进型和近滨海进型以半远洋沉积为主,粉砂和黏土混杂堆积,TOC较低;快速海进型以远洋沉积为主,微晶石英发育,TOC高,有机孔发育;晚期海进型以等深流沉积为主,TOC偏低,无机孔相对发育.③裂缝型甜点储层宏观裂缝和微裂缝发育,但基质孔隙度和渗透率不一定高.④早期海进型甜点分布于笔石带WF2—WF3,快速海进型甜点分布于笔石带LM1,晚期海进型甜点分布于笔石带LM2—LM4 及LM5,近滨海进型甜点分布于笔石带LM1—LM4,网状微裂缝型甜点主要分布于笔石带LM1,网状宏观裂缝型甜点的分布受断层规模和构造转换带变形程度控制.⑤笔石带LM6—LM8 黏土矿物含量高、成岩收缩缝发育的位置可形成网状微裂缝型甜点;构造转换带与调节带具有弱变形与弱改造特征,网状宏观裂缝发育,可形成网状宏观裂缝型甜点.结论认为,川南地区快速海进型和近滨海进型甜点勘探已取得重大突破,早期海进型和晚期海进型是下一步勘探的重要目标;以沉积学及裂缝特征为判识依据的甜点类型划分为页岩气深化勘探和效益开发提供了理论支撑.

Uncertain provenance and tectonic setting of shale has constrained the exploration and exploitation of natural gas in the Wufeng–Longmaxi Formation in the southern Sichuan Basin, China. Therefore, this study analyzed the mineral petrology and geochemistry to assess the effects of sorting, recycling, weathering, and paleoclimate on the deeply buried shale. The findings revealed that the mineral composition is dominated by biogenic quartz, terrigenous clay minerals, and carbonate minerals. Combinations of clay lamina and silty lamina occur in black shale. The geochemical characteristics of these sedimentary rocks remain unaffected by sedimentary sorting and recycling processes. The clastic sediments primarily originate from the felsic igneous rock source that underwent moderate weathering during its initial deposition in the collision environment. Based on the comprehensive analysis of the paleoclimate and paleoprovenance characteristics, Wufeng–Longmaxi Formation characteristics are considered to have undergone six stages, among which the stage of relative enrichment of organic matter corresponds to LM1, LM2–3, and LM4. Following the Hirnantian glacial period, the era has witnessed global warming, marked by glacier melting, a rise in global sea level, and a decrease in terrestrial debris supply, collectively facilitating organic matter enrichment. During the LM5 and LM6–8 periods, the increase in source supply caused by the Leshan–Longnvsi and Qianzhong–Xuefeng uplift inhibited organic matter enrichment.

Most scholars believe that transgression events can contribute positively to organic matter enrichment and shale gas sweet spot development, while whether or not regression events are conducive to shale gas sweet spot development remains to be further discussed. Variations in organic carbon content (TOC), and major and trace elements at the Rhuddanian–Aeronian stage in the Luzhou area, the southern margin of Sichuan Basin, were analyzed in this paper. We discuss differences in paleoenvironment organic matter enrichment and shale sweet spot development during transgression and regression. A transgressive system tract (TST) occurred during the early Rhuddanian stage, while early highstand system tracts (Ehst-1 and Ehst-2) occurred during the late Rhuddanian stage and Aeronian stage, and a late highstand system tract (LHST) developed during the late Aeronian stage. A rise in sea level during the TST in the upper Yangtze resulted in an anoxic environment, where a continuous upwelling current brought about a large number of nutrients in the seawater, significantly increasing paleoproductivity. Strong tectonic subsidence, weak chemical weathering, and a rising sea level together led to a low terrigenous debris supply in the catchment area. Therefore, paleoproductivity and redox conditions were the primary controlling factors of organic matter enrichment at the TST stage, with a clastic supply of secondary importance. With the advance of the Guangxi orogeny, the organic matter enrichment at the EHST-1, EHST-2, and LHST stages was mainly controlled by redox conditions and debris supply. A comparison of the key physical parameters and geochemical indicators of shale reservoirs from these four system tracts suggests that two shale sweet spot types (type I and II) were developed during the Rhuddanian–Aeronian stage, occurring in the TST and EHST-2 stages. High TOC and high microcrystalline quartz content are key to developing type I sweet spots, while enhanced anoxic conditions in the bottom water caused by ephemeral, small-scale sea level rises are the main determinant of class II sweet spots in the later EHST stage.

High graphitization is responsible for low-resistivity shale development with poor reservoir quality. This paper provides an explanation of organic matter graphitization and determines the impact of high graphite content on low-resistivity shale reservoir quality at the Wufeng-Longmaxi Formation in the Southern Sichuan Basin. Fine veins are frequently developed at shale samples with Ro > 3.5%, graphitized organic matter > 25%, and resistivity < 5 Ω•m, which are dominated by three mineral assemblages: brunsvigite, barite-hyalophane-barium feldspar-potassium feldspar-anhydrite, and calcite-ankerite. These filling minerals are characterized by an Eu positive anomaly and high Ba, Fe, and Mn contents, suggesting that low-resistivity shale was modified by magmatic-related low-temperature hydrothermal fluid. Temperature measurements of brine inclusions and a semi-empirical geothermometer of chlorite show that low-temperature hydrothermal fluid experienced the chlorite stage (150–180 °C), the low-sulfidation stage (120–150 °C), and the low-temperature calcitization stage. Paleozoic fault systems and late Permian hydrothermal activities associated with the Emeishan mantle plume control the graphitization of low-resistivity shale. The water formation and seawater infiltrated into the deep crust along the Paleozoic basement faults under gravity, developing alkaline hot brine through mantle plume heating and then causing a water-rock reaction with basement rocks. They migrated upward along deep and large Paleozoic faults through convective thermal circulation in the Tiangongtang area, the Shuanglong-Luochang area, and the Xuyong area. Cation exchange and redox reactions occurred during the interaction between high-temperature hydrothermal fluid and cool wall rocks. The migration of alkaline hot brine via the Wufeng-Longmaxi shale introduced a subsequent water-rock reaction, resulting in the development of hydrothermal mineral assemblages that intricately filled fractures. It increased formation temperature and enhanced thermal maturity and graphitization of organic matter at the Wufeng-Longmaxi low-resistivity shale, resulting in a wide distribution of low-resistivity shale at the Changning Block.

吐哈—三塘湖盆地煤层气资源丰富,但低煤阶煤层气成藏机制认识不足制约了该区煤层气的勘探开发.为此,在系统分析煤系地层地质特征的基础上,从沉积环境、构造特征、封盖条件和水文地质条件等方面分析和总结了煤层气富集主控因素和成藏模式,并明确了该区煤系气的有利勘探区带.研究结果表明:①古地貌和古环境控制煤岩的发育程度,稳定的正向构造是煤层气富集指向区,顶底板稳定的泥岩封盖条件是煤层气富集的必要条件,地表水补给是晚期生物气生成和富集的关键;②该区煤层气成藏可划分为缓坡区多气源补给煤层气富集、山前带复杂断块浅层煤层生物气、深层砂煤共储煤系气富集以及南部残余凹陷深部洼槽区煤层气富集4种模式;③条湖凹陷—马朗凹陷北部斜坡带、台北凹陷北部山前带西段浅层是煤层气最有利目标区,淖毛湖凹陷东部、沙尔湖凹陷东洼槽是煤层气较有利目标区,台北凹陷核桃沟—柯柯亚、鄯勒、照壁山—红旗坎、鄯善—温吉桑、疙瘩台构造带以及条湖凹陷中部构造带是煤系气富集有利区.结论认为,低煤阶厚煤层煤层气可以获得高产工业气流,吐哈—三塘湖盆地深部煤层气资源具有良好的勘探开发前景.

我国低煤阶含煤盆地地质背景复杂,煤层气富集成藏具有自身的特殊性,煤层气勘探一直处于小型试验阶段.为了促进我国低煤阶煤层气的勘探开发,在系统分析、总结我国含煤盆地类型、煤层气成因、赋存状态、富集模式的基础上,梳理了目前勘探开发工作面临的挑战与科研攻关目标,并提出低煤阶煤层气的有利勘探方向.结果表明:①我国低煤阶煤层气具有盆地类型多元、煤层气成因类型多元、赋存状态多元、富集类型多元的成藏特征;②低煤阶煤层气勘探面临目标区优选、深层煤系气共探共采系统评价、钻完井及增产改造工艺等3个方面的挑战;③应加强对煤层气成藏过程及动力学机制、气源成因及其资源贡献、吸附态和游离态煤层气空间分布规律及地质控制因素等的研究攻关,开展煤层气资源有效性和可采性评价,形成一套深部煤系气勘探开发评价方法.结论认为:①我国浅层煤层气资源有利勘探区包括二连盆地吉尔嘎朗图、霍林河、白彦花等凹陷,海拉尔盆地伊敏、呼和湖、陈旗等凹陷,鄂尔多斯盆地彬县、焦坪、黄陵、乌审旗东部等地区,以及准噶尔盆地南缘地区;②深层煤层气资源则主要分布在准噶尔、吐哈、三塘湖、海拉尔和三江等盆地或盆地群.

广义的煤系气是指赋存于煤系地层中的全部天然气,而狭义的煤系气则主要指赋存于煤层中的煤层气及其附近致密砂岩储层中的天然气.为了提高煤层气的开发效益,从烃源岩分布、煤与砂岩的组合类型、沉积相、封闭体系等方面分析了我国狭义煤系气的成藏特征及其控制因素.结果表明:①我国含煤盆地煤系地层煤与砂岩组合配置关系多样;②广覆式烃源岩分布为煤系气的富集提供了充足的气源基础;③沉积相控制了煤系气生储盖组合的配置关系,构成煤系气成藏的先天物质基础,河流、三角洲沉积体系煤层和砂岩均发育,是煤系气共生成藏最有利的沉积相带;④封闭体系对煤层(系)气的控制作用贯穿于煤层(系)气生成、聚散、富集及成藏等全过程,可划分出3种煤系气气藏类型——自生自储型煤层气藏、煤层气—砂岩气共生气藏、煤成砂岩气藏,其中前者是目前煤层气勘探开发的主要对象.进而指出了下一步的煤系气勘探方向:①以沁水、鄂尔多斯、准噶尔、海拉尔、鸡西等煤系气资源量较大的盆地为重点,开展煤系气勘探和评价:②针对不同煤系气气藏类型开展有针对性的勘探工作,在浅部自生自储型煤层气富集区进行煤层气勘探,对深部煤层气—砂岩气共生气藏进行煤层气+砂岩气综合勘探,对煤成砂岩气藏重点开展砂岩气勘探.

Coal measure gas is broadly defined as all natural gas occurring in coal measure strata while narrowly defined as the coalbed methane (CBM) in coal beds and the natural gas in the adjacent tight sandstone reservoirs. In this paper, the accumulation characteristics and control factors of narrowly defined coal measure gas in China were analyzed from the aspects of source rock distribution, coal–sandstone combination type, sedimentary facies and closed system to improve CBM development benefit. And the following research results were obtained. First, there are various coal–sandstone combination relationships in coal measure strata of coal bearing basins in China. Second, the widely-distributed source rocks provide sufficient gas sources for the enrichment of coal measure gas. Third, sedimentary facies dominate the combination relationships of source–reservoir–caprock assemblages of coal measure gas, so the inborn material base of coal measure gas accumulation is formed. The sedimentary systems of fluvial facies and delta facies are the most favorable sedimentary facies for the paragenesis and accumulation of coal measure gas for their coal beds and sandstones are developed. Fourth, the closed system controls the whole process of generation, enrichment and accumulation of CBM (coal measure gas). Three types of coal measure gas reservoirs are identified: self-source and self-reservoir CBM reservoirs, paragenetic CBM–sandstone gas reservoirs, coal-derived sandstone gas reservoirs, among which the first type is currently the main target of CBM exploration and development. Finally, the next exploration direction of coal measure gas was pointed out as follows. First, the exploration and evaluation of coal measure gas shall be focused on the giant basins with abundant coal measure gas, such as Qinshui, Ordos, Junggar, Hailar and Jixi. Second, coal measure gas reservoirs shall be explored specifically based on different types. It is necessary to carry out CBM exploration in the self-source and self-reservoir CBM enriched zones in the shallow layers, integrated exploration of CBM and sandstone gas in the paragenetic CBM–sandstone gas reservoirs in the deep layers, and sandstone gas exploration in coal-derived sandstone gas reservoirs.

二连盆地为我国典型低煤阶褐煤分布区,煤层气资源丰富,但煤层气富集成藏机制认识不足制约了该区低煤阶煤层气的勘探开发.为此以二连盆地吉尔嘎朗图凹陷低煤阶煤层气为研究对象,从煤层分布、含气性、煤层气成因、生物成因气模拟实验、保存条件等方面研究了该区煤层气富集的主控因素,并指出了下一步的勘探方向.研究结果表明:①浅水湖盆聚煤环境下,凹陷中部—缓坡带厚煤层发育,厚煤层弥补了含气量的不足;②含煤段堆积过程中,浅水湖泊周期性出现使得煤层上覆泥岩周期性发育,盖层条件有利;③凹陷中部—缓坡带位于地下水承压区,水动力侧向封堵有利于煤层气富集;④研究区煤层气为生物成因,原位条件下煤样产气0.25 mL/g,现今仍有生物气生成.结论认为:①厚煤层发育区、具备生物气生成以及良好的封盖条件并处于水动力承压区为吉尔嘎朗图凹陷煤层气富集成藏的关键;②吉尔嘎朗图凹陷煤层气富集模式为生物气+承压水封堵煤层气富集,中部—缓坡带L12-S88井区为下一步煤层气建产的有利区.

Objective The Tarim Basin is China’s largest ore-bearing interior basin,and contains mainly marine oils.The Kuqa depression,a secondary structural unit within the northern Tarim Basin,is composed of Mesozoic and Cenozoic clastic sedimentary rocks dominated by continental oil.Previous research suggests that the crude oils especially condensates in the Kuqa depression are rich in abnormally