Abstract: For the first time, the discovery of XX16–1 oilfield in southern slope of Laizhouwan sag revealed that the mixed sedimentary reservoirs of terrestrial detritus and carbonate were developed in lower 3~(rd) member of Shahejie formation in southern slope of Lanzhouwan sag. Based on the coring data, slice and scanning electron microscope, the evolution process of sedimentation and tectonics were systematically analyzed. The results indicated that the braided river delta was developed in lower 3~(rd) member of Shahejie formation, the mixed carbonate-terrestrial detritus sedimentary reservoirs and the interlayer mixed reservoirs were mainly developed. The former had significant dissolution, which effectively improved the storage space. The latter was mainly of micritic dolomite, and the dense layers were formed in the diagenetic process of argillaceous laminated layers, which could effectively preserve the primary reservoir space. However, the micro cracks formed by tectonism could effectively improve the seepage channels, and therefore, the effective reservoirs were formed. Based on the understandings of reservoir size, physical properties and production capacity, it is pointed out that carbonate–terrestrial clastic rock reservoir is the dominant reservoir in this area, which is the key point of future exploration and development.