Gas Bearing Characteristics And Main Controlling Factors Of Continental Shale Sequence In Western Sichuan Depression

Continental shale of Sichuan Basin has wide prospect of exploration,but it's different to marine sequence.Because of the complex internal structure,studies of this shale are weak,mainly in these areas,such as gas-bearing characteristics,occurrence of shale gas and their proportion,and controlling factors of gas-bearing characteristics.Therefore we observe field outcrops,ues the data from cores and well logging,and combined with geochemistry experimental dates to study the continental shale geological features in Sichuan Basin.At the same time,we conduct desorption experiment to analyzed gas-bearing characteristics and the proportion of absorbed gas and free gas,and then discussed the main controlling factors in the aspects of absorbed gas and free gas.The results showed that,the main lithology of continental shale sequence in T₃x⁵was interbed of gray black,dark gray shale and siltstone,which was various in thickness and showed strong heterogeneity.The type of kerogen of source rock was type III,which characterized by high abundance of organic matter and mature to high mature thermal evolution degree.The reservoir space includes several types,organic pores developed in shale and microporous developed in sandstone reservoirs.Shale sequence of T₃x⁵ showed high gas content in drilling and logging progresses.Test by desorption method showed the gas content with the average of 1.1m³/t.While in general,shale had larger gas content than siltstone reservoirs.The content of gas was a mixture of absorbed gas and free gas,which was mainly absorbed gas.The content of absorbed gas was more than 70%,while free gas less than 30%.The gas saturation of 15 wells in T₃x⁵was from 20%to 77%,with the average of 45.3%.Gas bearing layers had large thickness and in general had well gas-bearing characteristic.Shale reservoirs in T₃x⁵also had comparably high testing gas production,large disparity of gas production which was from 0.1 to 10×10⁴m³/d,among which half wells had production more than1.5×10⁴m³/d,it's showed that continental shale sequence had comparably high production per day.The gas-bearing characteristics of shale in T₃x⁵ was controlled by several factors.Different lithologies had different influences on absorbed gas:the amount of absorbed gas in shale reservoirs was larger than that in siltstone and fine sandstone reservoirs.Amount of absorbed gas in T₃x⁵ increased by the increasing of amount of clay minerals and organic matters.Increasing of porosity could not only beneficial to accumulation of absorbed gas but also free gas.