During deposition of the 2nd member of Upper Triassic Xujiahe Formation in Chuanxi foreland basin

the variation of accommodations in different locations within the basin are not unitary

due to the tectonic movement.According to the differential subsidence of basement and relative changes of lake level

it is believed that the water depth within the foreland basin can be divided into an increasing zone

a decreasing zone

as well as an invariant zone.The uneven changes of accommodations in the same period in a foreland basin are characterized as accommodation transformation system. Accommodation transformation system can be subdivided into several internal structural units

such as transformation body and transformation plane. During the deposition of the middle and lower 2

nd

member of Xujiahe Formation

the accommodations in Chuanxi basin appear to be elongated in NE direction along Longmenshan structural belt

and can be divided successively into 7 accommodation transformation zones basinward

including primary accommodation transformation zone (Ⅰ)

primary accommodation increasing zone(Ⅱ)

primary accommodation transformation zone(Ⅲ)

primary accommodation decreasing zone(Ⅳ)

secondary accommodation transformation zone(Ⅴ)

secondary accommodation increasing zone(Ⅵ)

and secondary accommodation decreasing zone(Ⅶ).These accommodation transformation systems have mainly been caused by the tectonic stress fields that lead to the alternating occurrence of compressional subsidence and nappe uplifting. Presently

the primary accommodation transformation zones are the most prolific gas areas where the largest number of gas reservoirs have been discovered

and they are also the practical exploration target areas at current technical conditions.