Sedimentary facies of fourth and fifth members of upper Triassic Xujiahe formation, Sichuan basin

Stratigraphic division and correlation of the upper Triassic Xujiahe formation in Sichuan basin has long been a controversial issue,and the research on sequence stratigraphy recently has established an isochronous stratigraphic framework of Xujiahe formation.Based on the stratigraphic framework,the sedimentary facies of the fourth and fifth members of Xujiahe formation in the whole Sichuan basin is thoroughly investigated.The fourth member of Xujiahe Formation consists dominantly of fine and medium grained sandstone,and delta plain and front deposit are well developed,while the fifth member mainly contains mud shale,and lacustrine deposit is developed.The fourth member of Xujiahe Formation contains thick sandstone with favorable porosity and permeability,and it should be considered as reservoir rock.The fifth member should be the main source rock as well as regional cap rock.