Occurrence Patterns and Controlling Mechanisms of In Situ Gas Content in Varied Marine Siliceous Shale Subfacies: A Case Study of the Wufeng–Longmaxi Formation in the Anchang Syncline, Northern Guizhou, China

Abstract Significant discrepancies in in situ gas content and occurrence characteristics exist among different siliceous shale subfacies of the Wufeng–Longmaxi Formation, yet the underlying controlling mechanisms remain poorly understood, particularly in normal-pressure shale gas reservoirs. This study systematically investigated the gas-bearing heterogeneity and its dominant controls in the Anchang syncline, a representative normal-pressure block in northern Guizhou. A suite of laboratory analyses, including TOC determination, XRD mineralogy, FE-SEM, low-temperature N2 adsorption, and high-pressure methane adsorption, were employed. The results indicate that siliceous shales are divided into mixed-siliceous shale facies (S-2) and clay-bearing siliceous shale facies (S-4). S-2 is dominated by organic matter pores, exhibiting a higher total pore volume and specific surface area. In contrast, S-4 features abundant clay-related inorganic pores with underdeveloped organic pores. S-2 demonstrates superior methane adsorption capacity, with an average maximum absolute adsorption capacity of 3.676 cm3/g compared to 3.456 cm3/g for S-4. The TOC content and hydrophilic clay mineral proportion are the primary controls on adsorption capacity. SDR model results reveal that S-2 has a higher average in situ total gas content of 4.293 cm3/g with an adsorbed gas ratio of 79.27%, while S-4 averages 3.622 cm3/g with an adsorbed gas ratio of 87.47% and limited free gas. Gas-bearing properties are synergistically controlled by TOC, pore structure, mineral composition, and water saturation. Crucially, the higher water saturation of S-4, driven by its clay-rich composition, allows water to easily invade pores under normal-pressure conditions, weakening capillary sealing and causing a far more severe free gas loss than in S-2. Integrated analysis reveals that S-2 displays superior gas-bearing properties and more favorable occurrence conditions, positioning it as the prime exploration target in the Anchang syncline.