Compound structures

such as coaxial superimposed

cross-over and mitre types

are common in western Sichuan depression as a result of multistage tectonic events. They have been formed early

thus would have been favorable for timely migration and accumulation of gas prior to the tightening of Xujiahe Formation sandstones. Compounding of structures would be favorable for the formation of crush structures

which would have improved the percolating conditions of the reservoirs and productivity of gas wells

and would have established communication between deep source rocks and shallow reservoirs

leading to the formation of gas pools far from their sources. The large fracture in western Sichuan depression has been formed as a result of multistage tectonic events and has led to the destruction of gas pools. Medium and small gas pools might have been preserved in areas far from the large fracture

due to the inhomogeneity of the reservoirs.