Controls on Differential Development of Intra-Platform Shoal Reservos in the Middle Permian Qixia Formation, Sichuan Basin: A Comparison of Central and Southern Sichuan

Intra-platform shoal reservoirs of the Middle Permian Qixia Formation are important targets for deep natural gas exploration in the Sichuan Basin. Core and thin-section observations, well-log interpretation results, and isotope data are integrated to compare reservoir-rock assemblages, pore types, petrophysical properties, and diagenetic features in central and southern Sichuan. Central Sichuan samples are dominated by fabric-retentive and crystalline dolostones containing intercrystalline pores and intercrystalline dissolution pores, with localized vugs and fractures. Southern Sichuan subsurface samples comprise a mixed limestone–dolostone assemblage with pores along grain or crystal boundaries and fractures; moldic pores are observed in regional outcrop samples. Among the interpreted intervals included in the statistics, intervals with porosity ≥5% account for 16.7% in central Sichuan (n = 430) and 7.6% in southern Sichuan (n = 422). Some medium- to coarse-crystalline dolostone groups have higher Sr isotope ratios than the limestone and finely crystalline dolostone groups, consistent with isotopic alteration. Mean δ18O values are more negative in southern Sichuan regional outcrop samples than in central Sichuan well samples; these differences require interpretation alongside mineral composition and sample setting. Reservoir differences between the two areas are expressed by contrasting precursor lithologies, dolomite replacement textures, and degrees of pore modification, with dissolution and localized filling further modifying pore geometry and preservation.