PETROLEUM SYSTEM, OIL AND GAS MIGRATION AND ACCUMULATION IN FUSHAN DEPRESSION, BEIBUWAN BASIN OF SOUTH CHINA SEA

There are two petroleum subsystems in the Fushan Depression in vertical succession. The conduit for oil and gas migration of the lower subsystem is the reverse deep fault system in the Liusan Section of the Eocene, and the conduit of the upper system is the broom-like shallow fault system in the Liuyi Section of the Eocene and the Weizhou Group of the Oligocene. The reverse deep fault system is an effective seal for oil and gas accumulation on the north-dipping large monoclinic zone in the Fushan Depression, as the broom-like shallow faults provide conduits for hydrocarbon escape due to lack of effective traps. Therefore, the lower oil and gas subsystem is more prosperous than the upper one for oil accumulation. Spatially, controlled by the Lingao and Changliu faults, there formed two sub-depressions in which deposited the source rocks in the Liushagang Group of Eocene. Thus the area can be divided into Eastern Bailian oil and gas subsysterm and Western Huangtong petroleum subsystem in a plane view. The Bailian subsystem is obviously better than the Huangtong subsystem because of its thick source rocks, high maturity and high quality reservoirs. Therefore, the exploration potential of the Bailian subsystem is higher than that of the Huangtong subsystem. In conclusion, there are a total of four oil and gas subsysterms in the Fushan Depression with the Eastern Bailian subsystem as the most favorable, which may account for more than 50% of hydrocarbon resources in the depression. It is believed that the accumulation of oil and gas is source controlled in the Fushan Depression. Oil and gas usually accumulated in the traps around the hydrocarbon generation centers. Both the Bailian and Huangtong sub-depressions are the centers of hydrocarbon generation. Around the hydrocarbon generation centers there are two favorable hydrocarbon accumulation zones, i.e. the Bailian subsystem and Huangtong subsystem. The region from Huachang to Qiaotou is located in the overlapping area of the two subsystems and is expected as the most prosperous target for oil and gas exploration.