Abstract: Triassic upper group of the Yanchang Formation in Xiasiwan oilfield,as the next section of the main reservoir,after decades of exploration and development,its reserve resources become increasingly inadequate.By analysis on oil-bearing section in the Xiasiwan area in recent years,we found that the lower group of Triassic Yanchang Formation Chang 7-Chang10 segments showed good oil and gas resources.By discussing the sedimentary facies,sand body shape,reservoir-cap,structural features,lithological assemblage of the lower group of Yanchang Formation under section Chang 7-8,we can find layer system replacement resources for the steady development of Xiasiwan oilfield.