Geological characteristics and gas bearing capacity of shale from transitional marine-continental facies of Longtan Formation: A case study from the Well LD1 in Gulin-Xuyong, southern Sichuan

The transitional marine-continental coal-measure shale of Permian Longtan Formation is widely developed in Yangtze Block. Taking the shale gas investigation Well LD1 in Gulin-Xuyong area of southern Sichuan as an example, the geological conditions of shale gas of Longtan Formation are evaluated by using data of logging well and core sample testing.Compared with marine shales of Wufeng-Longmaxi Formations, which are of high-yield shale gas wells such as JY1 and N203 of in Sichuan Basin, the Longtan Formation has a smaller shale thickness of single layer,generally less than 5m, but the total cumulative shale thickness is more than 40 meters. Shale of Longtan Formation has a higher organic carbon content, its kerogen is mainly type Ⅱ₂, and its maturity of organic matter has already reached the over-mature stage (the average value of R_O is 3.21%).In Longtan Formation, the contents of clay minerals are relatively high, the contents of brittle minerals are relatively low, and the porosity is generally greater than 3%.The abnormal value of shale gas in Longtan Formation is high, reaching 5.9m/13 layers.The value of highest anomaly is more than 8 times the background value, and the average total gas content in the upper part of Longtan Formation can reach 4.08 m³/t.