Deep shale gas from the Longmaxi Formation has been discovered in recent years in both Luzhou and Yuxi in southern Sichuan, and deep shale gas will become an important exploration direction for shale gas in China. The Hechuan Tongnan region is located in the gentle slope area of the ancient uplift in central Sichuan, adjacent to the western Chongqing and Luzhou regions. Drilling reveals active oil and gas display in the Silurian Longmaxi Formation, characterized by well leakage, overflow, gas invasion, and abnormal gas logging. The maximum total hydrocarbon content is 12.96–100

Fine-grained sedimentary rocks (FGSRs) in lacustrine basins are traditionally interpreted as low-energy suspension deposits. However, extensive shale oil exploration has revealed pronounced lithofacies heterogeneity within mudstone successions, necessitating a re-evaluation of their dynamic depositional processes. This study integrates data from eleven cored boreholes, along with petrographic and geochemical analyses, to characterize lithofacies variability, depositional processes, and environmental evolution of the Upper Cretaceous Qingshankou Formation FGSRs in the Songliao Basin. Sedimentological and geochemical proxies indicate that FGSR deposition occurred in brackish-saline waters with moderate stratification and persistent anoxia under warmhumid climatic conditions. A revised depositional framework identifies eight lithofacies and delineates four lacustrine depositional environments that were shaped by intermittent marine incursions: (i) sublittoral proximal deposits, consisting of interlaminated mudstones and siltstones associated with storm-induced tempestites and muddy hyperpycnal flows; (ii) sublittoral distal deposits, dominated by wave-enhanced sediment gravity flows with tripartite microstratigraphy recording turbulent-to-laminar flow transitions; (iii) semi-profundal deposits, comprising hybrid event beds and bottom-current deposits with rhythmically discontinuous silt laminae and mud ripples, the latter formed through bedload transport of clay aggregates under sustained current velocities; (iv) profundal deposits, including laminated varves, organic-rich suspension mudstones, and tuff layers. Their high organic carbon content and minimal bioturbation establish them as important targets for unconventional shale oil exploration. These findings challenge the conventional paradigm of deep-lacustrine quiescence, highlighting the pervasive influence of energetic depositional processes in shaping shale heterogeneity and organic matter preservation. The results have significant implications for the exploration and characterization of lacustrine shale reservoirs on a global scale.

Based on the observation and analysis of cores and thin sections, and combined with cathodoluminescence, laser Raman, fluid inclusions, and in-situ LA-ICP-MS U-Pb dating, the genetic mechanism and petroleum geological significance of calcite veins in shales of the Cretaceous Qingshankou Formation in the Songliao Basin were investigated. Macroscopically, the calcite veins are bedding parallel, and show lenticular, S-shaped, cone-in-cone and pinnate structures. Microscopically, they can be divided into syntaxial blocky or columnar calcite veins and antitaxial fibrous calcite veins. The aqueous fluid inclusions in blocky calcite veins have a homogenization temperature of 132.5–145.1 °C, the in-situ U-Pb dating age of blocky calcite veins is (69.9±5.2) Ma, suggesting that the middle maturity period of source rocks and the conventional oil formation period in the Qingshankou Formation are the sedimentary period of Mingshui Formation in Late Cretaceous. The aqueous fluid inclusions in fibrous calcite veins with the homogenization temperature of 141.2–157.4 °C, yields the U-Pb age of (44.7±6.9) Ma, indicating that the middle-high maturity period of source rocks and the Gulong shale oil formation period in the Qingshankou Formation are the sedimentary period of Paleocene Yi'an Formaiton. The syntaxial blocky or columnar calcite veins were formed sensitively to the diagenetic evolution and hydrocarbon generation, mainly in three stages (fracture opening, vein-forming fluid filling, and vein growth). Tectonic extrusion activities and fluid overpressure are induction factors for the formation of fractures, and vein-forming fluid flows mainly as diffusion in a short distance. These veins generally follow a competitive growth mode. The antitaxial fibrous calcite veins were formed under the driving of the force of crystallization in a non-competitive growth environment. It is considered that the calcite veins in organic-rich shale of the Qingshankou Formation in the study area has important implications for local tectonic activities, fluid overpressure, hydrocarbon generation and expulsion, and diagenesis-hydrocarbon accumulation dating of the Songliao Basin.

Shale oil has become a global hotspot of unconventional exploration and development. In this study, the latest drill core and experiment analyses of the Qingshankou Formation in the northern Songliao Basin were used to evaluate its lithofacies classification, sedimentary environment, pore types, pore-throat structure characterization, and shale oil potential. Lithofacies classification was determined according to the total organic carbon (TOC) content, sedimentary structure, and rock mineral content. Laminae genesis and micro-sedimentary structures indicate the deposition of fine-grained sedimentary rocks (FGSRs) in a semi-deep to deep lacustrine environment; however, evidence also suggests partial reworking by storm events and bottom current flows. FGSRs mostly comprise type I kerogen, with small amounts of type II1. The average vitrinite reflectance of the FGSRs was 1.37%, indicating middle to high stages of thermal maturation within the oil generation window. The N2 adsorption experiment indicated that silty mudstone (SM), silty fine mixed sedimentary rock (SFMR), and argillaceous fine mixed sedimentary rock (AFMR) had ink-bottle-shaped and slit-shaped pores, and the lithofacies were dominated by mesopores, accounting for 77.4%, 71.9%, and 80.8% of the total pore volume, respectively. Mercury injection capillary pressure analysis indicated that SM and SFMR had an average pore-throat radius of 0.01–0.04 μm, whereas AFMR and CM were dominated by nanopores, mainly distributed in the range of 0.004–0.0063 μm. Based on the comprehensive studies of TOC content, pore development, and brittleness, we concluded that organic-rich laminated SM and SFMR should be the focus of shale oil exploration of the Qingshankou Formation in the northern Songliao Basin, followed by organic-rich or organic-moderate laminated and layered AFMR, as well as calcareous fine mixed sedimentary rocks.

松辽盆地古龙凹陷有丰富的页岩油气资源.但是目前对古龙凹陷页岩油储层的岩石学特征和页岩油的状态,尤其是储集空间的类型及其组合关联性认识不清.通过对古龙凹陷白垩系青山口组岩芯的精细描述及其薄片分析和电子显微镜观察及三维CT深入研究,发现古龙青山口组页岩油的储集空间具有多样性和多尺度性.除了纳米孔缝外,还有页理缝.根据页理缝的规模和与油气的关系,可分为5类:①纳米缝,缝宽在10～50 nm,缝长50～100 nm,或更长;两端尖中间宽,微弯曲呈蠕虫状;本身也是重要的储集空间,是纳米级油元的重要组成部分,与纳米级孔油元和微米级油基关系密切;②微米缝,宽0.1μm到数微米,长数十微米到数百微米,与纳米级油元和微米级油基关系密切;③中微缝,宽数微米到数十余微米,缝长数百微米,与微米级油基关系密切;④大微缝,宽数十微米到100μm,长数百微米到数毫米,与微米级油基和微微缝及中微缝关系密切和⑤大页理缝(宽数百微米,肉眼明显可见),与各级微裂缝关系密切.此外,可见高角度倾斜或直立的裂缝,由于这些页理缝顺层发育,所以往往当做页理对待.通过研究,认为页理缝主要是嫩江组沉积末期(嫩末)和明水组沉积末期(明末)的构造反转褶皱过程中形成的.另外,还发育了大量的顺层方解石脉,根据方解石脉的宽度分为3类:①小型介于0.1～1.0 mm;②中型介于1.0～5.0 mm;③大者介于0.5～1.0 cm,最大宽度2.5 cm.较大的顺层方解石脉由垂直页理的纤柱状方解石组成.大方解石普遍发育共轭裂缝和挤出构造,是古龙凹陷嫩末和明末古应力反转的结果,也是古应力恢复的重要依据.经应力恢复认为嫩末和明末,可能一直延续到依安组的最大应力来自于水平方向(东西方向),在1500 m和2500 m深度水平最大挤压应力分别可达139.16 MPa和204.27 MPa,而垂向最小应力则分别为35.44 MPa和59.07 MPa.所以,在这种应力状态下导致顺层发育了大量页理缝和顺层纤柱状方解石脉.此外,在页理面上还发育了一系列摩擦镜面、擦痕、阶步、光面、剪裂面、鳞片构造、碎片构造等,揭示了沿页理发育了强烈的顺层剪切.四级纳微缝与大页理缝密集发育,在顺层面方向构成了裂缝空间联通网络,使页岩在顺层面方向渗透率较好或很好;裂缝空间联通网络与纳米和微米孔一起构成了一个三维的特殊缝孔体,与碳酸盐岩的缝洞体相当.纳米孔缝和微米孔缝及页理缝对于松辽盆地青一段页岩油的勘探开发具有重要意义.

缝洞体系是碳酸盐岩油气藏勘探的一个重要对象.以往的研究发现断溶体是一种重要的构造缝洞体系.最近我们在研究碳酸盐岩构造缝洞体系时发现了一个新的构造缝洞体系:褶溶体.褶溶体是一种褶皱作用控制下的溶洞裂缝体系.根据褶皱部位可以分为3种褶溶体:1)背斜褶溶体;2)向斜褶溶体;3)翼部褶溶体.背向斜褶溶体与褶皱过程中的虚脱和轴面节理有关;翼部褶溶体与层间滑动和翼部褶断有关.以背斜褶溶体最为重要,因为最易于富集油气,形成缝洞油气藏.褶溶体的形成与岩性、岩层厚度和褶皱的形成过程中的应力状态和岩层破裂过程及古气候、古地理等因素密切相关.