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Geochemistry, Origin, and Accumulation of Petroleum in the Eocene Wenchang Formation Reservoirs in Pearl River Mouth Basin, South China Sea: A Case Study of HZ25-7 Oil Field

Abstract

Hydrocarbon exploration in Pearl River Mouth Basin (PRMB), South China Sea, began relatively late (in 1980) because of restrictions caused by the deep-water setting. In recent years, an increasing number of exploratory wells have been drilled into Eocene rocks in the Huizhou Depression, northern PRMB, and new hydrocarbon resources (HZ25-7 oil field) have been detected in the Eocene Wenchang Formation (E2w), a unit that has been regarded as only a source rock for more than 30 years. A detailed investigation into the geochemistry of potential source rocks and petroleum in the HZ25-7 oil field will aid in a better understanding of their origins and to predict the petroleum distribution in the E2w rocks in the PRMB. Therefore, this study investigated the geochemical characteristics of potential source rocks and petroleum in the HZ25-7 oil field based on more than 260 Eocene mudstone samples, 100 E2w sandstone reservoir samples, and 10 oil samples from these newly drilled wells to provide an interpretation of the possible origins and accumulation models of this field. Laboratory methods used in this study including the Rock-Eval, gas chromatography–mass spectrometry, thermal-burial evolution modeling, petrophysical analysis, and fluid inclusion analysis. The results reveal that two sets of potential Eocene source rocks, E2w and Enping (E2e) formations, developed in the study area. The E2w source rocks are characterized by relatively low bicadinane/C30 hopane (< 3.0) and high 4-methyl sterane/ΣC29 sterane (> 0.4) ratios. However, the E2e source rocks are characterized by high bicadinane/C30 hopane and low 4-methyl sterane/ΣC29 sterane (<0.4) ratios. The geochemistry characteristics of petroleum in the HZ25-7 oil field indicate that the petroleum most likely originated from the E2w Formation in the Huizhou Depression. One stage of continuous charging is identified in the HZ25-7 oil field; oil injection is from 16 Ma to present and the peak filling occurs after 12 Ma. Thin sandstone beds with relatively good physical properties (porosity >5% and permeability >0.1 mD) and continuities in the E2w Formation are favorable conduits for the lateral migration of petroleum. This petroleum accumulation pattern implies that the E2w Formation on the western and southern margins of the Huizhou Depression are favorable for petroleum accumulation because they are located in the pathway of petroleum migration, and therefore, more attention should be paid to them in the future.