The Upper Triassic in western Sichuan depression can be divided into 2 source-reservoir-cap rock combinations. The peak of hydrocarbon generation and expulsion in the lower combination occurred during middle-late stages of Late Triassic; while that in the upper combination occurred in Late Jurassic. Both mechanical compaction and infilling of authigenic minerals would have contributed to the tightening of reservoirs in Xujiahe Fm

which occurred at the end of the Middle Jurassic Upper Shaximiao through to Late Cretaceous. Analysis of fluid inclusions indicates that gas reservoirs in the 2

nd

member of Xujiahe Fm were charged mainly at the end of the deposition of the 4

th

member of Xujiahe Fm through to the end of Middle Jurassic. Multiple phases of tectonic movements resulted in polyphase traps in western Sichuan depression

of which the large Yanshanian palaeohigh had the best timing with hydrocarbon migration and accumulation. Based on the stages of hydrocarbon generation and expulsion

tightening of reservoirs

fluid charging and trap formation

it can be determined that the main reservoiring stage in the Upper Triassic in western Sichuan depression would be from middle-late Indosinian to middle-late Yanshanian.