Structure, burial, and gas accumulation mechanisms of lower Silurian Longmaxi Formation shale gas reservoirs in the Sichuan Basin (China) and its periphery

ABSTRACT The Longmaxi Formation shale has a unique gas accumulation mechanism resulting from its multiphase tectonic movements and burial characteristics. This formation’s structure, burial, and gas accumulation mechanisms were studied based on structural interpretations, geochemical characterization, core observations and descriptions, burial history study, and comparisons of production rate and gas content between shale gas wells located in different structural settings. This study indicates that as shales are more strongly deformed tectonically, their sealing capabilities weaken. From the center to the margins of the basin, the uplift occurred earlier, the burial depths became shallower, and the residual strata area became smaller. These key factors have led to the formation of disrupted shale gas reservoirs at the basin margins with the quality of shale gas reservoirs degrading compared to those of the interior of the basin. Based on the migrated gas source from the basin interior and reservoir sealing capacity, three gas accumulation models are proposed: available migrated gas source + sealed reservoir, available migrated gas source + open reservoir, and unavailable migrated gas source + open reservoir. The most favorable model is the available migrated gas + sealed reservoir model because it has the presence of additional migrated gas source from the central basin and offers good sealing capabilities. Therefore, good shale gas reservoirs are primarily found in the areas of moderate burial depth for gas generation and continuous strata distribution with less tectonic disruptions and later uplift, and they have supplemental migrated gas from the interior area of the Sichuan Basin.