COMPARATIVE STUDY ON THE LOWER SILURIAN LONGMAXI SHALE RESERVIORS BETWEEN THE NORTH AND SOUTH JIAOSHIBA AREA, SE SICHUAN BASIN, CHINA

Owing to successful shale gas exploration and development, wells were drilled into the Longmaxi shale formation in the Jiaoshiba area, Southeast Sichuan Basin, China. However, shale gas production in Longmaxi varies greatly between the North and South Jiaoshiba areas. To determine the factors controlling the shale gas accumulation mechanism, the total organic carbon (TOC) content, mineralogy, porosity, permeability, total gas content, gas preservation conditions, and tectonic activities of Longmaxi Shale in both the North and South Jiaoshiba areas were studied in detail. The results show that the concentration of organic matter and minerals in the Longmaxi shale differed slightly between the South and North Jiaoshiba areas, and the lower Longmaxi Shale (Long 1 Member), which is used in commercial shale gas production, is characterized by a high quartz content and low clay content. In addition, the Longmaxi shale in the North Jiaoshiba area is characterized by high porosity and low permeability, while that in the South Jiaoshiba area has low porosity and high permeability. Two different reservoir types were identified in the Jiaoshiba area. The major storage spaces are pores in the Longmaxi shale reservoirs located in the North Jiaoshiba area, while the storage capacity of shale reservoirs in the South Jiaoshiba area is predominantly provided by fractures. Shale gas mainly exists as free gas in the Northern Jiaoshiba area due to the good gas preservation conditions. However, owing to the development of abundant faults in the South Jiaoshiba area, a large amount of free gas was lost, causing absorbed gas to dominate the shale reservoir in this area.