Development patterns of reservoir-seepage units and differential enrichment mechanisms of natural gas in tight sandstones of the Triassic Xujiahe Formation, western Sichuan Depression, Sichuan Basin

This study characterized the types, geological features, and differences in gas-bearing properties of reservoir-seepage units, investigated the controlling effects of geological structures, reservoir fractures, and petrophysical properties on gas-bearing potential, and elucidated the differential enrichment mechanisms of natural gas. Using seismic data, imaging logs, core observations, high-pressure mercury injection (HPMI), and production data, we aim to address the challenges of diverse reservoir-seepage unit types and unclear primary controlling factors of natural gas enrichment in the tight sandstones of the Triassic Xujiahe Formation in the western Sichuan Depression, Sichuan Basin. As indicated by the research results, four types of reservoir-seepage units are identified in the study area: fault-fracture, fold-fracture, fault-fold-fracture, and matrix types. The matrix-type units, characterized by underdeveloped tectonic fractures, exhibit inferior petrophysical properties and low gas-bearing potential. In contrast, the fault-fracture, fold-fracture, and fault-fold-fracture types, which are governed by tectonic deformation, feature well-developed fracture networks, enhanced physical properties, and higher gas-bearing potential. Meanwhile, fracture-rich units, medium- and high-angle fractures significantly enhance vertical permeability and expand the effective range of gas charging. Fracture density and the proportion of medium- and high-angle fractures typically decrease with increasing distance from faults, with the 200 ~ 600 m range identified as the optimal interval for gas enrichment. Furthermore, the fold width-to-height ratio exerts a clear influence on fracture intensity; areas proximate to fold hinges exhibit higher fracture densities and a greater proportion of medium- and high-angle fractures, providing superior conduits for gas accumulation. Finally, effective pore preservation is critical for gas enrichment in matrix-type units, and those with chlorite rims and superior sorting represent the most favorable sites for accumulation.