The control effect of burial evolution time limit on the organic matter pore structure of shale: A case study of the lower Silurian Longmaxi Formation shale from the periphery of the Sichuan Basin in southern China

The most critical reservoir spaces in shale formations are organic matter (OM) pores, predominantly governed by thermal maturation processes. Notably, the Lower Silurian Longmaxi shale exhibits comparable thermal maturity levels across different areas of the Sichuan Basin periphery while demonstrating significant variations in OM pore structure. In conjunction with regional tectonic influences, areas with lower gas content correspond to earlier stratum uplift times, indicating that burial duration significantly affects OM pore structure. In this study, we examine the Longmaxi shale from the Changning area in southern Sichuan Basin (LCS) and the Youyang area in southeastern Chongqing (LYC), which have similar thermal evolution degrees but different burial durations. Through carbon and sulfur analysis, asphalt reflectance testing, and focused ion beam electron microscopy, we analyzed the differences in OM pores structure and determined the impact of burial duration on these structures. Our results show distinct differences in OM pores structure between LCS and LYC. The OM pore diameters in LCS primarily range from 200 to 1000 nm, whereas those in LYC mainly range from 30 to 400 nm. The number of OM pores in SEM with per unit view in LCS is significantly higher than in LYC. The prolonged burial duration of LCS, exceeding 200 million years (Ma), has allowed for sufficient thermal evolution, leading to substantial hydrocarbon gas generation and superior OM pores structure. In contrast, LYC has a burial duration of approximately 150 Ma, resulting in smaller OM pore diameters and fewer pores compared to LCS. The extended burial duration ensures that the shale undergoes adequate thermal evolution, generating large amounts of hydrocarbon gases and forming OM pores with excellent storage performance. The Yanshan Movement, occurring during the Cretaceous-Jurassic period, is responsible for the varying strata uplifting times in the Yangtze Platform of South China. Formed by the subduction of the Pacific plate under the Eurasian plate, this movement created compressive stress at their junction. The time required for extrusion stress to propagate from the lower Yangtze to the upper Yangtze differs, resulting in longer shale evolution times in the upper Yangtze compared to the middle and lower Yangtze. These findings underscore the necessity of incorporating burial duration considerations in shale gas resource assessments. • The most effective reservoir space in shale is the organic matter (OM) pores. • The effect of burial time has an influence on OM pores structure. • The long burial time effect ensures the shale undergoes sufficient thermal evolution to generate OM pores with excellent storage. • The effect of burial time must be considered in evaluating shale gas exploration and development potential.