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

The Pearl River Mouth Basin in the South China Sea has accumulated >2 km of Eocene sediments in its deep basin, and has become the exploration focus due to the recent discoveries of the HZ25-7 oil field in the Eocene Wenchang (E2w) Formation. In this study, the geochemical characteristics of potential source rocks and petroleum in the HZ25-7 oil field are investigated and the possible origins and accumulation models developed. The analytical results reveal two sets of potential source rocks, E2w and Enping (E2e) formations developed in the study area. The semi-deep-to-deep lacustrine E2w source rocks are characterized by relatively low C29 steranes, low C19/C23 tricyclic terpane (<0.6), low C24 tetracyclic terpane/C30 hopane (<0.1), low trans-trans-trans-bicadinane (T)/C30 hopane (most <2.0), and high C30 4-methyl sterane/ΣC29 sterane (>0.2) ratios. In contrast, the shallow lacustrine and deltaic swamp-plain E2e source rocks are characterized by relatively high C29 steranes, high C19/C23 tricyclic terpane (>0.6), high C24 tetracyclic terpane/C30 hopane (>0.1), variable yet overall high T/C30 hopane, and low C30 4-methyl sterane/ΣC29 sterane (<0.2) ratios. The relatively low C19/C23 tricyclic terpane ratios (mean value: 0.39), low C24 tetracyclic terpane/C30 hopane ratios (mean value: 0.07), high C30 4-methyl sterane/ΣC29 sterane ratios (mean value: 1.14), and relatively high C27 regular sterane content of petroleum in the HZ25-7 oil field indicate that the petroleum most likely originated from the E2w Formation mudstone 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 peak filling occurs after 12 Ma. Thin sandstone beds with relatively good connectivity and physical properties (porosity and permeability) 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 a migration pathway. Thus exploration should focus in these areas in the future.