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Hydrocarbon Generation Potential of Late Eocene Enping Formation Mudstones in the Huilu Area, Northern Pearl River Mouth Basin, South China Sea

Abstract

Late Eocene Enping Formation mudstones in the Pearl River Mouth Basin, South China Sea, which formed during a lake-flooding event, are one of the most important sets of cap rocks in the basin. However, the generation potential of Enping Formation mudstones remains poorly understood and controversial. Despite a lack of documentation, these mudstones are presumed to have hydrocarbon generation potential. In this study, we provide new organic geochemical data (Rock-Eval and gas chromatography–mass spectrometry data) for these mudstones based on several new wells drilled into Eocene strata in the Huilu area, northern Pearl River Mouth Basin, and associated thermal-burial evolution modeling results and geological background of the Late Eocene. We then discuss their hydrocarbon generation potential. Geochemical analyses indicate that these rocks have total organic carbon (TOC) contents of 0.55 to 3.3% (mean: 1.31%) and are dominated by type III kerogen derived from terrestrial plant material. The episodic rifting movement, which was associated with the introduction of a large quantity of terrestrial higher plants by rivers, led to organic matter (OM) accumulation and preservation that formed type III kerogen in Enping Formation mudstones. The rapid transverse facies change led to the geochemical heterogeneity of the source rocks. The OM in these rocks reached an early-to–mid-mature stage in uplifted areas (Ro = 0.5%–1.0%) and a late-mature stage in sags (Ro = 1.0%–1.3%), stages wherein hydrocarbons can be generated. Therefore, the Enping Formation mudstones may have held a potential for hydrocarbon, especially gas, generation during the lake-flooding period and hence deserve attention in future exploration. More research needs to be conducted on the generation potential of Enping Formation mudstones as few mudstone samples from deeper sags in the Huilu area are currently available for investigation.