Main controlling factors for gas preservation conditions of marine shales in southeastern margins of the Sichuan Basin

South China has experienced complex tectonic movements during geologic history and the marine shales there are high in thermal maturity, thus the preservation condition is one of the major factors controlling shale gas enrichment and production. Based on the exploration practices of the Jurassic shale gas on southeastern margins of the Sichuan Basin in recent years, a comparative study was performed on the preservation conditions of conventional gas reservoirs and shale gas reservoirs and revealed that top and bottom seals of high quality are the indispensable conditions for shale gas preservation. The self-sealing ability and the huge gap up to 2-8 times between vertical and lateral permeability of shale gas reservoirs determine that the lateral diffusion is the basic pattern of shale gas migration. The major controlling factors for shale gas preservation and their criteria of discrimination were further studied. The following conclusions were obtained. The strength of tectonic modification is the major controlling factor for shale gas preservation. Broad and gentle structures with continuous seals and an anticlinal setting are more favorable for the enrichment of shale gas, and a closed evolutionary environment with late uplifting is more favorable for the preservation of shale gas. Shale gas preservation is more effective in areas with undeveloped faults, areas with a good fault sealing ability, downthrown blocks with fault barriers, and areas far away from outcrops or zones with hiatus. The pressure coefficient is a comprehensive discrimination criterion for sealing condition, so a high or super-high pressure of a reservoir is a signal of good preservation condition for a shale gas play. The pressure coefficient is in positive correlation with shale gas production in the study area. In the areas within the southeastern Sichuan Basin except for those with strong erosion or hiatus, the Wufeng and Longmaxi Fms are high in pressure coefficient as a whole and have good preservation conditions for shale gas. In contrast, the pressure coefficient lowers progressively toward southern and eastern margins of the basin and the overall shale gas preservation conditions are quite unfavorable.