The fractal dimension quantitatively describes the complexity of the shale pore structure and serves as a powerful tool for characterizing the evolution of shale reservoirs. Thermal simulation experiments were conducted on the low-maturity transitional shale from the Shanxi Formation in the Ordos Basin. The initial samples consisted mainly of quartz (39.9%) and clay minerals (49.9%) with moderate-to-good hydrocarbon generation potential. Samples from different thermal maturation stages were analyzed through geochemical, mineralogical, and pore structure experiments to reveal the evolution of mineral compositions and pore structure parameters. The fractal dimensions of the pore structure were calculated using both the FHH and capillary bundle models. Correlation coefficients and principal component analysis (PCA) were employed to explore the factors influencing the fractal dimension and its evolutionary patterns during reservoir development. The results indicate that (1) with increasing thermal maturity, the quartz content gradually increases while the contents of clay minerals, carbonate minerals, pyrite, and feldspar decrease. (2) The evolution of porosity follows five stages: a slow decrease (0.78 < Ro < 1.0%), a rapid increase (1.0% < Ro < 2.0%), a relatively stable phase (2.0% < Ro < 2.7%), a rapid rise (2.7% < Ro < 3.2%), and a slow decline (Ro > 3.2%). The evolution of the pore volume (PV) and specific surface area (SSA) indicates that the proportion of pores in the 5–20 nm and 20–60 nm ranges gradually increases while the proportion of pores smaller than 5 nm decreases. (3) The fractal dimension of shale pores (D₁, average value 2.61) derived from the FHH model is higher than D₂ (average value 2.56). This suggests that the roughness of pore surfaces is greater than the complexity of the internal pore structure at various maturities. The D_M distribution range calculated from the capillary bundle model was broad (between 2.47 and 2.94), with an average value of 2.84, higher than D₁ and D₂. This likely indicates that larger pores have more complex structures. (4) D₁ shows a strong correlation with porosity, PV, and SSA and can be used to reflect pore development. D₂ correlates well with geochemical parameters (TOC, HI, etc.) and minerals prone to diagenetic alteration (carbonates, feldspar, and pyrite), making it useful for characterizing the changes in components consumed during pore structure evolution. (5) Based on the thermal maturation process of organic matter, mineral composition, diagenesis, and pore structure evolution, an evolutionary model of the fractal dimension for transitional shale was established.

Pore structure plays a critical role in evaluating shale “sweet spots”. Compared to marine shale, lacustrine shale has more diverse lithofacies types and greater heterogeneity in pore structure due to frequently changing environmental conditions. Using methods such as mercury intrusion porosimetry (MIP), field emission scanning electron microscopy (FE-SEM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD), this study investigates the micropore structures and heterogeneity of different lithofacies in the Jurassic Dongyuemiao Member lacustrine shale. Image processing and multifractal theory were employed to identify the controlling factors of pore structure heterogeneity. The key findings are as follows. (1) Based on mineral content and laminae types, the lithofacies types of Dongyuemiao lacustrine shale are classified into four types: shell–laminae mixed shale (SLMS), silty–laminae clay shale (SLCS), clast–laminae clay shale (CLCS), and clay shale (CS). (2) Based on genesis, shale reservoirs’ pore and permeability space are categorized into inorganic pores, organic pores, and micro-fractures. Inorganic pores consist of inter-particle pores and intra-particle pores. Pore size distribution curves for all four lithofacies exhibit two main peaks, with pore sizes concentrated in the ranges of 2–10 nm and 50–80 nm. Mesopores and macropores dominate, accounting for over 80% of the total pore volume. Mesopores are most developed in CLCS, representing 56.3%. (3) Quartz content is positively correlated with the multifractal dimension, while clay content shows a negative correlation. Higher quartz content, coupled with lower clay content, weakens pore structure heterogeneity. A negative correlation exists between total organic carbon (TOC) and the multifractal dimension, indicating that higher organic matter content enhances organic pore development and increases microscopic heterogeneity. (4) Porosity heterogeneity in SLMS is effectively characterized by D₀-D_max, while in the other three lithofacies, it is characterized by D_min-D₀. Permeability across all lithofacies correlates with D₀-D_max. In CS, SLMS, and SLCS, permeability is positively correlated with D₀-D_max, with higher values indicating greater permeability heterogeneity. In CLCS, permeability is negatively correlated with D₀-D_max, such that lower values reflect stronger heterogeneity.

The pore structure plays a critical role in evaluating shale “sweet spots”. Compared to marine shale, lacustrine shale has more diverse lithofacies types and greater heterogeneity in pore struc-ture due to frequently changing environmental conditions. Using methods such as mercury intru-sion porosimetry (MIP), field emission scanning electron microscopy (FE-SEM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD), this study investigates the micropore structures and heterogeneity of different lithofacies in the Jurassic Dongyuemiao Member lacustrine shale. Image processing and multifractal theory were employed to identify the controlling factors of pore structure heterogeneity. The key findings are as follows: (1) Based on mineral content and laminae types, the lithofacies types of Dongyuemiao lacustrine shale are classified into four types: shell-laminae mixed shale (SLMS), silty-laminae clay shale (SLCS), clast-laminae clay shale (CLCS), and clay shale (CS). (2) Based on genesis, shale reservoirs pore and permeability space are cate-gorized into inorganic pores, organic pores, and micro-fractures. Inorganic pores consist of in-ter-particle pores and intra-particle pores. Pore size distribution curves for all four lithofacies ex-hibit two main peaks, with pore sizes concentrated in the ranges of 2-10nm and 50-80nm. Meso-pores and macropores dominate, accounting for over 80% of the total pore volume. Mesopores are most developed in CLCS, representing 56.3%. (3) Quartz content is positively correlated with the multifractal dimension, while clay content shows a negative correlation. Higher quartz content, coupled with lower clay content, weakens pore structure heterogeneity. A negative correlation exists between total organic carbon (TOC) and the multifractal dimension, indicating that higher organic matter content enhances organic pore development and increases microscopic heteroge-neity. (4) Porosity heterogeneity in SLMS is effectively characterized by D0-Dmax, while in the other three lithofacies, it is characterized by Dmin-D0. Permeability across all lithofacies correlates withD0-Dmax. In CS, SLMS, and SLCS, permeability is positively correlated with D0-Dmax, with higher values indicating greater permeability heterogeneity. In CLCS, permeability is negatively correlated with D0-Dmax, such that lower values reflect stronger heterogeneity.

In recent years, the successful application of gravity-flow deposit theory in major petroliferous basins in China had attracted extensive attention in the field of sedimentology and had become a key research frontier. This study utilized core, drilling, logging, and microphotograph data, along with low-temperature nitrogen adsorption and high-pressure mercury injection experiments. It discussed the characteristics of gravity-flow deposits, sedimentary microfacies, sedimentary models, and the significance of gravity-flow deposits to pore heterogeneity in shale reservoirs, focusing on the first submember of the Dongyuemiao Member (referred to as the Dong 1 Member) in the Fuling area of the Sichuan Basin. The results indicated the development of four types of mudrock in the Dong 1 Member: massive to planar laminated shell mudrock (F1), planar laminated bioclastic mudrock (F2), planar laminated silty mudrock (F3), and massive mudrock (F4). These corresponded to debris flow deposits (F1, F2), turbidite deposits (F3), and suspension deposits (F4). According to the characteristics of lithofacies combinations and sedimentary features, four sedimentary microfacies were identified: gravity-flow channel, tongue-shaped, lobate, and semi-deep lake mud. The Shell Banks were disturbed by earthquakes, tides, storms, and other activities. Silt, clay, fossil fragments, plant debris, and other materials were deposited under the influence of gravity, mixing with surrounding water to form an unbalanced and unstable fluid. When pore pressure exceeded viscous resistance, the mixed fluid became unbalanced, and gravity flow began to migrate from the slope to the center of the lake basin. A sedimentary unit of gravity-flow channel-tongue-shaped-lobate was developed in the Fuling area. The Fuling area’s gravity-flow depositional system resulted in distinct microfacies within the Dongyuemiao Member, each exhibiting characteristic lithofacies associations. Notably, lobate deposits preferentially developed lithofacies F3, which is distinguished by significantly higher clay mineral content (60.8–69.1 wt%) and elevated TOC levels (1.53–2.45 wt%). These reservoir properties demonstrate statistically significant positive correlations, with clay mineral content strongly influencing total pore volume and TOC content specifically enhancing mesopore development (2–50 nm pores). Consequently, the F3 lithofacies within lobe deposits emerges as the most prospective shale gas reservoir unit in the study area, combining optimal geochemical characteristics with favorable pore-structure attributes.

The Ediacaran Dengying Formation 2nd Member (hereafter 2nd Member) in the Sichuan Basin is influenced by major tectonic events including the Caledonian, Indosinian, and Himalayan orogenies and this strata has experienced a complex hydrocarbon accumulation history, resulting in inconsistent gas–water contacts. To elucidate this complex history, this study investigates the diagenetic mineral filling sequence within the Dengying 2nd Member in the Penglai area. We integrated data from analytical techniques such as cathodoluminescence (CL), in situ trace element analysis, U–Pb geochronology, and fluid-inclusion microthermometry. Based on these analyses, this study established the paragenetic sequence, incorporating both diagenesis and hydrocarbon accumulation, for the Dengying 2nd Member. This sequence comprises eight distinct phases of mineral precipitation and hydrocarbon emplacement: fibrous dolomite, granular dolomite, fine crystalline dolomite, first-phase bitumen, medium crystalline dolomite, saddle dolomite, second-phase bitumen, and quartz. From this sequence, we propose a four-stage hydrocarbon accumulation model for the Dengying Formation: (1) primary migration and accumulation during the Indosinian period; (2) oil cracking to gas during the Yanshanian period; and (3) and (4) two distinct stages of gas pool adjustment during the Himalayan period. Corresponding to these stages, this study developed distinct accumulation models and simulated migration and accumulation processes during key stages. The results indicate that the distribution of paleo-oil pools exerts significant control over the location of present-day gas accumulations. Initial oil charge was controlled by the distribution of carrier beds and hydrocarbon charging pathways, with water zones observed more frequently in the lower intervals of the Dengying 2nd Member. Subsequently, gas generated from oil-cracking filled these carrier beds, with areas of gas enrichment correlating with zones of high paleo-oil saturation. Finally, during the later adjustment stages, fault activity induced gas remigration and leakage, significantly impacting the final trapping configuration and preservation of gas accumulations.

The transitional shale of the Upper Permian Longtan Formation in the Sichuan Basin is characterized by high organic matter abundance, wide distribution, and significant resource potential. This study systematically analyzes the sedimentary environment and geochemical characteristics of the Longtan Formation, clarifying the evolutionary patterns of its sedimentary environment, detrital influx, paleoclimate, paleosalinity, and paleoredox conditions. The results indicate that the Longtan Formation can be divided into five Members. The Long 1 to Long 4 Members were predominantly deposited in littoral-marsh facies, whereas the Long 5 Member represents tidal flat–lagoon facies. Geochemical analyses indicate that the Long 4 Member exhibits the highest TOC content (3.37%–11.50%, avg. 6.39%), characterized by black shale interbedded with thin coal seams and pyrite bands. This interval corresponds to relatively low detrital input (Zr: 103–1124 ppm, avg. 697 ppm; Ti: 2589–3909 ppm, avg. 3408 ppm), a warm and humid paleoclimate (CIA: 88.4–96.8, avg. 93.6; Sr/Cu: 1.6–6.8, avg. 3.5), moderately elevated paleosalinity (Sr/Ba: 0.36–2.46, avg. 1.59), and oxic to intermittently reducing water conditions (U/Th: 0.21–0.68, avg. 0.37), making it the main interval for organic matter enrichment. From the Long 1 to Long 5 Members, the depositional environment evolved with progressively shallower water, decreasing terrigenous input, and increasingly oxic conditions. These changes, combined with high productivity and intermittently reducing conditions, collectively governed the accumulation and preservation of organic matter across the formation.