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.

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.