Hydrocarbon Generation and Expulsion of the Upper Triassic T 3 x 5 Source Rocks in the Western Sichuan Depression: Assessment for Unconventional Natural Gas

Tight‐sand gas in the Jurassic and shale gas within the fifth member of Xujiahe Formation (T 3 x 5 ) in the Western Sichuan Basin (WSD) are currently regarded as the most prolific emerging unconventional gas plays in China. This study conducted a conventional evaluation of T 3 x 5 source rocks in the WSD, and investigated their hydrocarbon generation and expulsion characteristics, including intensity, efficiency and amount. The results show that, the T 3 x 5 source rocks are thick (generally >200 m), and have a high total organic content (TOC), ranging from 2.5 to 4.5 wt%. It is thus indivative of a great hydrocarbon generation potential when they underwent high thermal evolution (R o >1.2%) in the area. In addition, an improved method of hydrocarbon generation potential is applied, indicating that the source rocks reached a hydrocarbon expulsion threshold with vitrinite reflectance (R o ) reaching 1.06%, and that the comprehensive hydrocarbon expulsion efficiency is about 60%. The amount of hydrocarbon generation and expulsion from T 3 x 5 source rocks is 3.14×10 10 t and 1.86×10 10 t, respectively, with a residual amount of 1.28×10 10 t within them. Continuous‐type tight‐sand gas is predicted to have developed in the Jurassic in the Chengdu Sag of the WSD because of the good source‐reservoir configuration; the Jurassic sandstone reservoirs are tight, and the gas expelled from the T 3 x 5 source rocks migrates for very short distances vertically and horizontally. The amount of gas accumulation in the Jurassic reservoirs derived from T 3 x 5 source rocks is up to 9.3×10 8 t. Geological resources of shale gas are up to 1.05×10 10 t. Small differences between the amounts calculated by the volumetric method and those obtained by hydrocarbon generation potential method may be due to other gas accumulations present within interbedded sands associated with gas shales.