Abstract:

The formation of the gas pools in Xujiahe Formation was controlled by its sequence stratigraphic framework.Ⅲ-1 sequence is constituted by lowstand system tract formed of clastic rocks,transgressive system tract formed of sandstone,arenaceous rock and highstand system tract formed of arenaceous rock and coal beds;Ⅲ-2 sequence contains respectively lowstand system tract formed of lacustrine facies and transgressive delta sediments,transgressive system tract formed of argillaceous rocks;Ⅲ-3 sequence is composed of lowstand system tract with huge thickness of fan deposits in the internal part of the basin,the transgressive system tract with lacustrine and regressive delta sediments,the highstand system tract formed of terrigenous debris.Both Ⅲ-2 and Ⅲ-3 sequences show the framework of progradation from periphery to the interior part.It is suggested that the transgressive system tract contains thick mudstone and abundant organic matter,and it should be the main source rock and regional caprock;the lowstand and highstand tracts contain sandstones with favourable porosity and permeability,and should be considered as reservoir rocks.