Sedimentary characteristics of the middle and lower 3rd member of Shahejie Formation in Gunan Sag,Bohai Bay Basin

Based on the regional geological data and the theory of petrology,sedimentology and lithofacies paleogeography,this paper analyzed sedimentary facies markers of cores from 10 wells including color,composition,granularity,sedimentary structure and etc.The facies makers were integrated with logs of 45 wells to study the division of sedimentary microfacies and sedimentary environment evolution of the middle and lower 3rd member of Shahejie Formation in Gunan Sag,and analyzed their relation to reservoirs..Three main sedimentary facies are identified in the middle and lower 3rd member of Shahejie Formation in Gunan Sag,namely fan delta,delta and lacustrine,which are further divided into nine sub-facies and sixteen microfacies.Tempestite sediments is found for the first time in this sag,which is considered as a positive factor for improving the quality of reservoir.During the depositing of the lower 3rd member of the Shahejie Formation,small-scale delta and fan delta were developed in the north of the sag,whereas delta were developed in the south.These deltas gradually withered along with the expansion of the lake,being favorable for the development of source rock.And during the deposition of the middle 3rd member of the Shahejie Formation,the deposits supply increased and the net-to-gross ratio of the sandbody in the sag enlarged,which were favorable for the quality of reservoir.