Sedimentary facies and evolution of Chang 9 oil formation in Yanchang Formation , Zhijing-Ansai area, Ordos Basin

Zhijing-Ansai area is located in the middle of Shanbei slope of the Ordos Basin. Based on detailed observations of well core, sandstone thickness, sandstone strata thickness ratio, authigenic chlorite development characteristics, and paleontological characteristics, the authors studied the distribution regularity of sedimentary facies and the sedimentary evolution. The results show that a suite of meandering fluvial deltas-lacustrine sediments were developed in the study area. The delta front deposits were mainly developed in the northern part of the study area, whereas shallow lake sediments were developed in the southern part of the study area. The size of the lake basin in Chang 92 oil formation was smaller than that in Chang 91 oil formation. During the depositional period of Chang 91 oil formation, the lacustrine basin began to expand, and lake shoreline moved to the land, with the expansion of the lake area. Form Chang 92 oil formation to Chang 91 oil formation, the water depth changed from shallow to deep, and then a set of high resistance mudstones with hydrocarbon-generating potential were deposited, and the deltaic front subaqueous distributary channel sand bodies became high quality reservoirs.