Gas-bearing characteristics and controls of the Cambrian Shuijingtuo Formation in Yichang area, Middle Yangtze region

The Yichang area in the Middle Yangtze region is a new exploration target for shale gas outside the Sichuan Basin. The shale of the Cambrian Shuijingtuo Formation has an enormous gas resource potential in view of gas shows in most wells. Gas contents vary in different wells. We investigated the gas-bearing characteristics and controls of shale in the Cambrian Shuijingtuo Formation in the Yichang area integrating geological and geochemical data. The black rock series of the Shuijingtuo Formation in this area is mainly carbonaceous shale and gray shale, about 50-140 m thick. They have a high organic carbon content and are mainly of typeⅠ, generally thermally over-mature, showing a good hydrocarbon generation potential. The on-site measured gas content is 0.32-5.48 m³/t, and the cumulative thickness of shale with gas content >2 m³/t is 44.05 m, which reflects a good overall gas content of shale in this area. The gas-bearing capacity of the Shuijingtuo Formation shale is affected by various factors such as sedimentary facies, organic carbon content, mineral composition, porosity, extent of fracture and formation pressure. The black carbonaceous shale deposited in the continental shelf facies has higher gas content than the gray calcareous shale of the slope facies, while the limited platform facies has the lowest gas content. The gas content has a positive correlation with the organic carbon and quartz mineral contents, a weak negative correlation with the carbonate mineral content, and a poor correlation with the clay mineral content. Organic carbon content has a more significant control on gas content. Shale porosity and pore distribution variations are important factors leading to the differences in gas content. In addition, shale gas content is also closely related to reservoir fracture development and formation pressure.