Gas-bearing characteristics and preservation conditions of Upper Ordovician Wufeng-Lower Silurian Longmaxi shale in western Hubei

Abstract

The Upper Ordovician Wufeng to Lower Silurian Longmaxi formations are important exploration targets for shale gas in the western Hubei province. The lithology, sedimentary facies, gas-bearing characteristics and preservation conditions of the Wufeng-Longmaxi formations were analyzed with the samples of more than 20 survey wells. The Wufeng Formation is dominated by black carbonaceous siliceous shale of deep-water siliceous argillaceous shelf facies, while the Longmaxi Formation is dominated by gray-black carbonaceous shale of deep-water argillaceous shelf facies. The Wufeng-Longmaxi formations and the lower part of Lower Silurian Xintan Formation consist of two three-level sequences, and the gas content conforms to the characteristics of TST1 > TST2 > HST2 > HST1. The gas content of the Wufeng Formation is slightly higher than that of the Longmaxi Formation. A good gas potential has been found to the north of Tianyangping Fault and the east of Xianfeng Fault. Since most wells were in the deep shelf facies area, sedimentary facies have little effect on the plane distribution of shale gas content. The studies of preservation conditions showed that both the Xintan Formation as the roof and the Ordovician as the floor have good sealing properties. The burial history of Wufeng-Longmaxi formations can be divided into four stages, and shows the characteristics of "early uplift time" and "rapid uplift rate" when compared with the Sichuan Basin. The burial depth and the distance to normal fault have certain influence on the gas content. However, the matching pattern between fault and fold is the most important factor affecting gas content. Two structural patterns are beneficial to shale gas accumulation, one is being far away from normal fault and locating at the syncline wing or core, the other is being far away from normal fault and locating at the footwall of fault and the target stratum is not connected to the fault in the up-dip direction.

Keywords

• preservation condition
• shale gas
• gas content
• wufeng-longmaxi formations
• western hubei