Tide-Dominated Estuarine and Deltaic Deposition of the Upper-Triassic Xujiahe Formation in the Yuanba Area,Sichuan Basin

Through outcropped profiles at the margin of Sichuan basin and study of core-sections in the Yuanba area,it has been revealed that the Xujiahe Formation is not the continental depositions as commonly inferred to since 1950s,but the product of coast system products.The 2nd Member of Xujiahe Formation belongs to tide-dominated estuarine in origin,including the facies of alluvial Channel,tidal-fluvial channel,UFR sand flat,tidal bar,mud flat and salt swamps.The 4th Member of the Formation is regarded as the product of tide-dominated delta,including the facies of delta-plain channel,distributary-mouth tidal bar,delta front,prodelta,mud flat and salt swamps.Both environments of tide-dominated estuary and delta are temporary phenomenon in geological history.When sediment supply is sufficient,this area becomes tidal flat-strand plain environment(deposition),which corresponds to the coal-bearing fine-grained successions of the 3rd and 5th Member of Xujiahe Formation.Five types of reservoir facies are divided based on the close relationship between reservoir quality and sedimentary facies.Exploration suggestion is provided in this study based on experiences in the Xujiahe Formation in Anyue Area.