GAS FORMING MECHANISMS AND ACCUMULATION MODELS OF THE XUJIAHE FORMATION IN HEBAOCHANG REGION, THE SOUTH OF SICHUAN BASIN

The Upper Triassic Xujiahe Formation is a reservoir forming system with self-generation and self-storage.In Hebaochang region of the Sichuan Basin,the thickness and range of sand rocks is relatetively larger in the Second Member,Fourth Member and Sixth Member of the Xujiahe Formation.However,the reservoir rock mainly has low porosity and low permeability.According to the capillary pressure curve,the capi-llary resistance of the secondary migration of gas is about 2.5 MPa to 3.5 MPa.Heights of gas column are between 150 m and 300 m.Because of the gentle regional structure,gas accumations are mainly controlled by the sand body distribution and reservoir properties.Although the excellent reservoir rock has less capillary resistance,the distribution area is relatively small,and usually belongs to lenticular or strips sand body.The secondary migration of gas depends mainly on the gas expansion and the gas accumulates in nearby regions.