Abstract: Oil and gas shows are active in the red-brown sandy mudstone interval under the unconformity surface between Jurassic and Cretaceous strata on the southern slope of Mosuowan swell. High-yield commercial oil flow has been gained from several wells drilled by Sinopec in the sandy mudstone interval and then Yongjin oilfield has been discovered. However, the classification of horizon of the interval has always been controversial. On the basis of the latest 2D seismic data and combining with well logging-seismic calibration, the paper performs 2D & 3D merging detailed seismic geological interpretation in the study area. Combined with the comparative analysis of core and microscopic features under microscope, it is considered that the red-brown sandy mudstone interval which is widely distributed at the top of Jurassic in Mosuowan swell should be classified into the Middle Jurassic Toutunhe formation. The re-classification of the horizon identifies that the major target zone in the study area is the Toutunhe formation which has favorable conditions for large-scale hydrocarbon accumulation, and indicates further exploration direction on the southern slope of Mosuowan swell.