Geochemical characteristics and depositional environment of Paleogene lacustrine source rocks in the Lufeng Sag, Pearl River Mouth basin, South China Sea

Abstract

A systematic geochemical analysis of samples from newly drilled wells in the Lufeng Sag, Pearl River Mouth Basin has been carried out to characterize source rock quality, determine organic matter input and reconstruct the depositional environment of the Enping and Wenchang formations. Total organic carbon (TOC) content ranges from 0.46 to 1.57 wt% in the Enping formation, and from 0.53 to 6.10 wt% in the Wenchang formation. Kerogen types I-II were identified in the Wenchang formation samples, while kerogen types II-III were identified in the Enping formation samples. Maturity parameters suggest marginally mature to mature levels for most samples with higher maturity for the Wenchang formation samples. Biomarker assemblage and trace element ratios of the Wenchang and Enping formations are distinctively different, indicating predominantly aquatic origin organic matter input under suboixc-anoxic water conditions for the Wenchang formation and more allochthonous terrestrial organic matter input under oxic water conditions for the Enping formation. Moderate to high primary productivity and temperature drove water stratification, together with weak detrital material dilution led to the enrichment of organic matter in the Wenchang formation. While the low primary productivity, poor preservation conditions and high dilution led to the relatively poor source rock quality in the Enping formation.