Sequence Filling Patterns and Oil-Gas Distribution Rules of the Xujiahe Formation in the Sichuan Foreland-like Basin

This study analyzed the surface sections of the Xujiahe Formation of the upper Triassic in the Sichuan foreland-like basin,and the sedimentary facies and sequence of drilling cores and logging data.In combination with the regional geology and oil and gas geological characters,this study also focused on the sedimentary order of the Xujiahe Formation,filling patterns,and gas-oil distribution rule.Two super long-term sequence cycles (SLSC1,SLSC2)and 5long-term sequence cycles(LSC1—LSC5)were indentified,which correspond to the lower and upper Xujiahe Formation of the coupling process between basin and mountain,and 5regional lake transgression-lake regression sedimentary cycles,respectively. The stratigraphic isopach and lithofacies paleogeography maps of whole basin have been compiled based on the isochronous stratigraphic unit defined by the half cycle of long-term base level rising and falling.The results indicate that the subsidence and depocenter of members 2and 3of the Xujiahe Formation was located in the western Sichuan depression;member 4migrated to northeastern Sichuan;members 5and 6 to the northeastern Sichuan depression.Different sedimentary evolution stage and frontal depression are characterized by various filling patterns and oil and gas distribution rules.Accompanied by the depocenter of two basin-mountain coupling subsystem(low Xujiahe basin and upper Xujiahe basin)and the migration of the subsidence-depocenter of excellent source-reservoir-cap rock assemblage to the northeast from the western Sichuan depression,the oil and gas distribution are also characterized by the migration from south to north and to northeast and the development around foreland rise area.