Abstract: Since conducting thorought oil exploration in the Pearl River Mouth Basin( PRMB) advanced achievement on the study of local geology and geochemistry has been gained. It has been proven that the oil formation and accumulation in the PRMB is characterized by "terrestrial-originated while marine-reser-voired". That is, both terrestrially lacustrine Wenchang mudstone and coaly Enping Formation are the main sources of the oil whilst the overlying marine sandstone and mudstone of Zhuhai and Zhujiang formations contribute reservoir beds and caprocks. Such a good combination of source rock, reservoir bed and caprock enables the oil exploration in the PRMB successfully. However, with upgrading the exploration degree more and more data suggests that the marine mudstone occurred in both Zhuhai and Zhujiang Formations can not only be as the caprock but also be as subordinary source of the oil when buried at a certain depth because it possesses a certain organic abundance. But there has not yet been any proof for one to confirm that the marine mudstone has ever provided oil for the PRMB. To find the evidence, geochemical features of two lowmature crude oils and one respective Zhuhai and Zhujiang mudstone were studuied in detail. Results showed that both Zhuhai and Zhujiang mudstones were organic-riched, and while buried at the depth below 2500 meters the Zhuhai source rock became marginally mature Moreover, geochemical analyses for saturated and aromatic fractions indicated that benzohopanes, the biomarker indicator of lower thermal maturity, were ubiquatous in both the crudes and the extracts, whereas C₃₀ 4-methyl steranes were common compounds in the oils but not in the extracts, thus suggesting that the low maturity of the oils should be attributed to the mix up of the oil with lower maturation level, of which the would-be source rock was Zhuhai mudstone owing to the absence of benzohopanes in the terrestrial source rocks. The finding was of great importance because Zhuhai Formation is widespread, especially in Baiyun Depression, in which the formation was buried at the depth around 3000 meters according to seismic section and therefore just entered the OGM. hence was probably an important oil source.