Abstract: As the largest oil and gas basins in the Southern South China Sea, the Zengmu and Baram-Sabah basins have roughly similar recoverable reserves but great differences in oil and gas ratio, and distribution patterns. Based on the data of oil source correlation, combined with the data of paleoclimate, paleovegetation, depositional evolution and faices differentiation, two distribution patterns for marine-continental transitional source rocks has been concluded by this paper. It suggests that the oil from shore side of the Zengmu Basin and the Baram-Sabah Basin are mainly generated from coaly source rocks, while the oil from offshore Zengmu Basin from terrestrially sourced marine deposits. Up to the paleoclimate and paleovegetation, the coaly source rock has rather strong ability of oil generation. Based on the depositional facies of source rocks, two distribution patterns for the marine-continental transitional source rocks have been recognized by the authors for the first time, which are the “deltaic coaly facies -terrestrially sourced marine facies source rocks” and the “deltaic- turbidite -coaly source rocks”. Under the control of the type of source rocks, the oil in the Zengmu Basin is enriched in the delta plain and inshore region, while the oil in the Baram-Sabah Basin has a wide coverage.