Sequence stratigraphy of the Feixianguan Formation in the Yuanba gas field, northeastern Sichuan

The Feixianguan Formation in the Yuanba gas field, northeastern Sichuan may be divided, on the basis of cores, thin sections, well logs and seismic data, into two third-order sequences:SQ1 representing the 1st and 2nd members, and SQ2 representing the 3rd and 4th members of the Feixianguan Formation. Grounded upon the carbonate platform, both SQ1 and SQ2 are made up of the transgressive and highstand systems tracts and lack of the lowstand systems tract. During the deposition of the transgressive systems tract of SQ1, the bulk of the Yuanba area is built up of the open platform deposits on the shallow-water carbonate platform, and continental shelf deposits near the east. During the deposition of the highstand systems tract of SQ2, there are gradations of the open platformplatform margin-slope deposits from west to east. Till the deposition of SQ2, the whole area graded into the shallowwater carbonate platform dominated by the transgressive and highstand systems tracts representing the open platform deposits in the transgressive phase and evaporate platform deposits in the highstand phase.