Thermal maturity of crude oils in the Baiyun sag, Pearl River Mouth basin, South China Sea: Insights from biomarkers，aromatic hydrocarbon and adamantane compounds

Abstract

The Baiyun Sag in the Pearl River Mouth Basin is a hydrocarbon-rich depression in which oil and natural gas fields coexist. The complex characteristics of oil and condensate in the sag have been long-term issues, and one of the questions is the formation stage of black oil, volatile oil and gas condensate in the sag. In this study, to accurately estimate the thermal maturity of oil in the Baiyun Sag, maturity parameters derived from normal alkanes, terpanes, steranes and aromatic hydrocarbons were systematically investigated. In addition, adamantane parameters were used to verify both the oil and condensate maturity. The results show that the saturated hydrocarbon biomarker parameters are not suitable for providing a complete understanding of the oil maturity in the Baiyun Sag. A qualitative evaluation method for various maturity stages of oil was proposed based on the methylphenanthrene distribution fractions, phenyldibenzothiophene maturity parameters, and diamantane concentration. Subsequently, the maturity was quantitatively evaluated using empirical equations based on methyl diamantine. The equations were selected by comparing the calculated equivalent reflectance (Rc) with the qualitatively determined maturity stage of the crude oil. Finally, the calculated vitrinite reflectance values indicated an overestimation of the maturity of light oils and condensate oils, which were thought to be produced in the late oil window (Ro: 1.0%∼1.3%) to wet gas stage (Ro: >1.3%). The new results indicate that the black oils with Rc values ranging between 0.72% and 1.05% was generated during the peak oil window. The Rc values of gas condensates in the eastern and northeastern structural belts range from 1.20% to 1.24%, suggesting that they are products of the late oil window. The maturity levels of gas condensates in the Panyu Low Uplift are the highest (Rc: 1.14%–1.46%) in the sag. They were produced in the late oil to wet gas stages. The oil maturity characteristics confirm the oil exploration potentials of the eastern and northeastern structural belts of the Baiyun Sag.