Geology and geochemistry of source rocks of the Eastern Pearl River Mouth Basin, South China Sea

Abstract

The Pearl River Mouth Basin (PRMB), with mostly Cenozoic sediments, is one of the largest offshore oil-bearing basins in the South China Sea. It covers an area of 175,000 km 2 with the maximum sedimentary thickness of 12 km. Large structural belts trending NE-SW were developed in the basin. From north to south they are the Northern Fault Terrace-Uplift, Northern Depression, Central Uplift and Southern Depression Belts, respectively. The evolution of the PRMB had been in two stages, i.e. first faulting (from Paleocene to Early Oligocene) and then subsiding (starting from Late Oligocene), and the so-called Double Structural Layers were thus developed. Therefore, the sedimentary rocks of the PRMB are characterized by terrestrial followed by marine facies. Three terrestrial (accendingly known as Shenhu, Wenchang and Enping Formations, respectively) and six marine formations (accending named Zhuhai, Zhujiang, Yuehai, Wanshan and Quaternary Formations, respectively) were deposited. Geochemical statistical study of possible source rocks in the PRMB showed that both the terrestrial Enping and Wenchang Formations were richer in organic matter while both the marine Zhujiang and Zhuhai Formations are relatively poor. The type of organic matter is dominantly composed of Types II to I for the Wenchang Formation and mainly Type III for the other three overlying formations. Furthermore, in thermal maturity, the Zhujiang Formation is either immature or marginally mature, Zhuhai, marginally mature-mature, Enping, mature-highly mature and Wenchang, highly-overmature. Hence, the principal source rocks in the PRMB are the Wenchang and Enping Formations, and the Zhuhai and Zhujiang Formations could also be oil sources. Geochemical investigations indicated that the source rocks in the PRMB are characterized by lower saturates to aromatics ratio, commonly found 18a(H)-oleanane and resin-derived “W” and “T” compounds in some source rocks, enriched high molecular weight n-paraffin hydrocarbons and C29 steranes except for those of Wenchang and partly of the Enping Formations and predominant C30 hopane for Wenchang Formation in contrast to the others, which contained either predominant C31 homohopane or C29 norhopane or C27 trisnorhopane. However, the more abundant C30 4-methyl steranes discovered in crude oils of the PRMB were only identified from a thin setting of Wenchang Formation occurring in Huilu Lowhigh. Therefore, it was deduced that there must have been a certain setting of the Wenchang Formation which contains enriched C30 4-methylsteranes which are widely distributed in the depositional centers of the separated Depressions of the PRMB. They are possible oil sources of the reservoirs found in the PRMB.