Abstract: The J、K、L and M groups(Pre-I group,Oligocene-lower Miocene)have good conditions for hydrocarbon accumulation in the Malay Basin. ①Tectonic movements favor the accumulation of hydrocarbon. In early Miocene,the stage of extensional fracturing,numerous lacustrine source rocks were deposited. In early-middle Miocene,the stage of tectonic subsidence,accompanied by basin inversion,reservoir and seal systems of the Pre-I group's petroleum system were formed. ②The lacustrine deposits are excellent source rocks, which are rich in algae and have higher maturity and organic content. ③Reservoirs are available, such as the braided river sandstone in the K group and the estuarian channel sandstone and intertidal sandstone in the J group. ④Cap rocks are well developed. The main cap rock of Pre-I group is the delta-shallow marine mudstone deposited in the transgressive period of the Basin. The secondary cap rock is the lacustrine mudstone in K、L and M groups. ⑤Migration pathways are still open. Both upward and lateral migration occurs ⑥The source-reservoir-seal assemblages are favorable. Both the self generation and self preservation assemblage and lower generation and upper preservation assemblage occur, which are favorable to oil and gas accumulation. The petroleum system of Pre-I group is located in the southeastern part of the Malay Basin. The oil and gas distribution in the Malay Basin is controlled by the distribution、quality and maturity of source rocks、forming time of structures and the migration of hydrocarbon.