Biomarker compositions and geochemical significance of crude oils of Baiyun Sag, Pearl River Mouth Basin

Crude oils from the Baiyun Sag of Pearl River Mouth Basin have high abundance of diagnostic biomarkers including novel C15 sesquiterpene, special configurated tricyclic sesquiterpenes and tetracyclic sesquiterpenes (X, Y, Z and X1, Y1, Z1), and bicadinanes, which indicate the contributions from terrestrial higher plants to the organic matters. These oils are also rich in oleanane, which is a diagnostic biomarker for angiosperm organic matter. Based on the analysis of 13 crude oils from the Baiyun Sag, the compositions and geochemical significance of typical biomarkers were revealed. Results indicate that C24-des-oleanane (Y1) and C27 tetracyclic (Z1) may share a similar biological origin. Novel C15 sesquiterpene, C24-des-oleanane and C27 tetracyclic, and bicadinanes may be derived from different higher plants according to the molecular structure and abundance correlation analysis of the related compounds. In addition, the biomarker compositions of crude oils from different regions are significantly varied, which will benefit the oil group classification in the Baiyun Sag. The distribution of novel C15 sesquiterpene and bicadinanes was influenced by sedimentary environment, and may be more enriched in oxidized environment. The distribution patterns of C24-des-oleanane, C27 tetracyclic and oleanane are principally affected by biogenic input, but are less influenced by sedimentary environment fluctuation. The relatively high abundance of C24-des-oleanane, C27 tetracyclic and oleanane in the crude oil in the northeastern and eastern Baiyun area reflects that the Dongsha Uplift and the Yunli Low Uplift may be an important source of terrigenous organic matter in this area.