Geochemical characteristics and origin of crude oil and natural gas in different areas of Wenchang-A Sag, Pearl River Mouth Basin, South China Sea

Complex hydrocarbon distributions characterize the Wenchang-A Sag. Systematic study of the geochemical characteristics of crude oil, natural gas and source rocks and their genetic relationship is still needs to be completed. The Rock-Eval, kerogen maceral, vitrinite reflectance, saturated hydrocarbon gas chromatographymass spectrometry, natural gas components, carbon isotopes, and light hydrocarbon were performed. 1) Crude oil is classified based on four factors: wax content, the presence of C27 diasteranes, the regular steranes alpha alpha 20RC27-alpha alpha 20RC28-alpha alpha 20RC29, and the bicadinanes characteristics. Class I crude oil has high wax and C27 diasteranes. For Class II crude oil, the regular steranes are in 'L'- shaped distribution, and the content of bicadinanes is shallow. Class III crude oil has soft wax and C27 diasteranes, and regular steranes in the reverse 'L'-shaped distribution, with a high peak degree of bicadinanes. For Class IV crude oil, regular steranes are in 'V'-shaped distribution, with high peak bicadinane. 2) Class I crude oil comes from source rocks in area C. Class II crude oil comes from source rocks in areas D and E. Class III crude oil comes from areas A, and B. Class IV crude oil comes from source rocks in area A. 3) The source of natural gas in Group I is hydro propylene, and natural gas in Group II is humic. Natural gas in Group III is mixed. Groups I and II are kerogen cracking gas, and group III is a mixture of crude oil secondary cracking gas and kerogen cracking gas. Natural gas in Groups I and II mainly come from local source rocks, and Group III has mixed source characteristics. In the future, oil exploration can continue in Areas C and D, and more favorable areas for gas exploration are Areas C, D, and E.