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.