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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 km2 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.

A thorough petroleum exploration of the Pearl River Mouth Basin (PRMB), South China Sea, began in 1983. At present, several oilfields have been found in the PRMB, mainly distributed in Dongsha Massif, Huizhou and Xijiang Depressions as well as Huilu Lowhigh, and one of them has been developed recently. The crude oils found in the basin can be classified into two types. One is normal waxy type, and the other is cyclic type, which may be caused by minor biodegration and is restricted to the Liuhua District of Dongsha Massif. However, on the basis of geochemical characteristics, all the crude oils are thermally mature, indicating that they are derived from source rocks which have entered the main oil generation period but their maturity is not high enough to reach the overmature stage. Moreover, in the biomarker distribution, the oils also share many similarities. Almost all the oils contain abundant C 30 4-methylsteranes with 24-ethyl side chain, ubiquitous oleanane and lower concentration of gammacerane, and possess high ratios of Ni/V, pristane to phytane and C 30 hopane over total C 29 steranes as well as high paraffin wax and low sulphur content, indicating that they originated from terrestrial organic matter deposited in lacustrine and marsh coal-forming environments. However, some characteristics resemble Brazilian offshore oils of salinewater lacustrine environment. The oils found in the PRMB can also be classified into three main genetic types based on the relative values of pristane over phytane ratio, C 29 sterane preference and the composition of the carbon isotope. Type I oils occurred in the Huizhou and the Xijiang Depressions and their adjacent Dongsha Massif. It has higher ratios of pristane to phytane (1.80–5.54 and 3.21 on the average scale) and heavier carbon isotopic composition, indicating that their source rocks contain much more abundant terrestrial higher plant input. Type II, encountered in Huilu Lowhigh and its bounding area of Dongsha Massif, has moderate ratios of pristane to phytane (1.78–2.94 and 2.39) and C 29 sterane similar to Type I. Type III is distributed in the Liuhua District of Dongsha Massif and has lower pristane to phytane ratios (ranging from 1.49 to 1.97), smaller C 29 sterane preference, higher C 30 4-methylsterane content and lighter carbon isotopic composition (less than −29%) even though they have undergone minor biodegration, implying that the source rocks of the oils are enriched with lacustrine organism input.