The effect of sedimentary facies on Longmaxi shale gas in southern Sichuan Basin

To identify the impact of deposition facies on Longmaxi shale gas enrichment in southern Sichuan Basin, the authors studied sedimentary characteristics, stratigraphic distribution, lithology, sedimentary structures, organic carbon content, mineral composition, reservoir space type and other features as well as the impact of sedimentation on favorable shale distribution with the purpose of providing a basic evidence for the evaluation of shale gas enrichment conditions. By the observation and description of drilling cores from six shale gas wells in the study area and according to log response, experimental analysis, Longmaxi Formation was divided into three subfacies, which are shallow water muddy shelf, half-deep water shelf and deep water shelf. It is shown that shale gas accumulation is affected by organic carbon content, the development and distribution of shale, mineral composition, and the development of pores, which are all controlled by sedimentary facies. The deeper the water, the closer to the deposition center, TOC content is higher; Longmaxi Formation is thick but the effective section is much thinner, and it becomes thicker where it is closer to the deposition center. Organisms deposited in deepwater make the effective shale have higher organic matter content and more brittleness; Main pore types in Longmaxi shale are clay mineral layer pores and organic pores, and organic pores are more developed in the section with higher TOC content. Deep-water shelf is the most favorable facies for shale gas enrichment with abundant organic matter content, high brittleness and abundant organic pores.