Triassic upper group of the Yanchang Formation in Xiasiwan oilfield, as the next section of the main reservoir, 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, the authors find layer system replacement resources for the steady development of Xiasiwan oilfield.