南海北部湾盆地福山凹陷含油气系统与油气运聚成藏规律

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

福山凹陷纵向上可分为下部和上部含油气子系统,前者的油气运聚输导为始新统流三段深层反向断裂系统,而后者则为始新统流一段、渐新统涠洲组帚状浅层同生断裂系统。深层反向断裂系统具有良好的封闭性,在福山凹陷区域北倾大单斜背景下是封堵油气聚集成藏的关键因素;浅层帚状同生断裂对油气成藏的控制作用以输导、逸散为主,致使上部含油气子系统缺乏有效圈闭;因此,下部含油气子系统优于上部。横向上由于受临高断裂和长流断裂构成的断槽控制,形成以始新统流沙港组烃源岩为主的2个生烃沉积次洼,据此可以在平面上将该区划分为东区白莲和西区皇桐含油气子系统。白莲子系统烃源岩有效厚度、成熟度、储层质量等比皇桐子系统具有明显优势,勘探前景优于皇桐子系统。总之,福山凹陷纵、横向上可划分为4个相互交叉叠合的含油气子系统,且以东区白莲子系统中的下系统油气成藏条件最佳,其集中了凹陷50%以上的资源。福山凹陷油气运聚成藏遵循"源控论"规律,油气运聚多围绕生烃洼陷富集成藏,以白莲次洼和皇桐次洼为生烃中心,大致可划分出白莲子系统和皇桐子系统2个有利油气聚集的环带,在两环带叠合处的花场—桥头地区为最有利油气富集区。

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 monoclinie 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.