Sedimentary facies evolution of Upper Palaeozoic formation in Ordos Basin

This study of sedimentary facies evolution of the Upper Palaeozoic formation is based on drilling and geological information.It analyses palaeogeographic environment, rock type,sedimentary facies type and distribution in the Benxi Stage (incursion), Taiyuan Stage (incursion spread), Shanxi Stage (regression),Shihezi Stage (paralic) and Shiqianfeng Stage (continental lacustrine).As a result,these five stages formed a transgression and regrssive cycle including from coastal facies to delta facies,fluvial facies and lacustrine facies of continental sedimentary in marine-continental transition,and controlled the formation of the favorable reservoir facies which comprised delta facies distributary chanel psepholite,submerged distributary chanel psepholite and tidal flat facies bar.The hydrocarbon source rocks is coal-bearing strata in the tidal flat facies.The caprock is related to shaly rocks in the flood plain.The complicated overlapping and juxtaposition between the source and reservoir rocks caused the formation of lithologic gasses, which was largely controlled by sandbody distribution.