Sedimentary Facies Analysis of the Lower Bitumen-bearing Sandstone Member of Kepingtage Formation,Silurian in Tazhong Area,Tarim Basin

The lower bitumen-bearing sandstone Member of Kepingtage Formation in Tazhong Area is one of the major oil and gas-rich layers in Palaeozoic clastic rock series in Tarim basin.Based on description of drilling cores,combined with logging,seismic and test data,it is showed that transgressive shore-tidal-braided-river delta depositional system was developed in target stratum.Littoral facies is made up of up-nearshore and down-nearshore microfacies,whose lithology includes fine sandstone,siltstone,pebbled anisomerous sandstone.Tidal flat facies is made up of tidal sand bar,tidal channel,sand flat microfacies.The sediments of tidal channel mainly include sandstone,gritstone and pebbled sandstone.Its sedimentary structures include wedge shaped cross bedding,tabular cross-stratification,and herringbone cross-bedding.There are some erosion surfaces and lots of pyrite in the bottom of tidal channel.Braided-river delta is made up of underwater distributary channel,interdistributary bay,and sand sheet microfacies.During marine transgressive period,there are four depositional periods sandbodies from the basin central to the south overlapping onto Tazhong low uplift one by one.According to sedimentary overlap evolution law,the microfacies of these four periods of sandbodies are littoral,tidal flat and braided-river delta facies in the plane.In terms of controlling factors,distribution range and transgression direction,the four periods microfacies have similarity and regularity.