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Relative Contribution of Different Source Rocks Determined by Enhanced End-member Oil Mixing Experiment in Zhuyi Depression, Pearl River Mouth Basin, South China Sea

Abstract

The mudstone and coal in the Eocene deep-water lacustrine Wenchang (Ew) formation and the Oligocene shallow-water lacustrine to swamp Enping (Ee) formation are the main source rocks in the Zhuyi Depression, Pearl River Mouth Basin, South China Sea. The relative contribution of the Ee and Ew formations in different oil fields is still unknown. An enhanced end-member oil mixing experiment was established to investigate quantitatively the relative contribution of the Ee and Ew formations. Oil-source correlation and hydrocarbon accumulation process analysis indicate that the samples HZ33 and HZ9 can represent the oil derived from the Ew formation and the Ee formation respectively. The oil sample HZ33 is a black oil with high concentration of Steranes and Terpanes, rich in C30-4-Methyl-24-Ethyl-Cholestanes, C16-Sesquiterpanes, Drimanes and poor in Bicadinanes and Adamantanes. The oil sample HZ9 is a condensate with low concentration of Steranes and Terpanes, poor in C30-4-Methyl-24-Ethyl-Cholestanes, C16-Sesquiterpanes, Drimanes and rich in Bicadinanes and Adamantanes. Two end-member oils were mixed at different volume ratios (8:0, 7:1, 6:2, 5:3, 4:4, 3:5, 2:6, 1:7 and 0:8) and an enhanced template to determine the mixing ratio was built. The template was composed by the biomarker concentration indexes, such as C30-4-Methyl-24-Ethyl-Cholestanes, Adamantanes, et al and the geochemical parameters changing linear, such as Drimanes/Pentamethyldecalin, C16-Sesqui-terpanes/Homodrimane, Bicadinanes/C30-Hopane, et al. As the differences in the physical properties of the HZ 33 and HZ9, most of the geochemical parameters change nonlinearly. This makes it difficult to determine the mixing ratio accurately. The enhanced template solved this problem. The relative contribution of the Ee and Ew formations in different oil fields was calculated using the above template. The results showed that the relative contribution of the Ee formation was underestimated. In the Huizhou and Enping sags, the relative contribution of the Ee formation can be higher than 50%. This result has also been verified by the numerical simulation of end-member oil mixing and the basin modeling of the Zhuyi Depression. This study provides a tentative method for the quantitative study of relative contribution from different source rocks in the same basin.