The influence of sedimentation rate and sedimentary facies on organic matter enrichment in source rocks in the Lufeng Sag, Pearl River Mouth Basin, South China Sea

Introduction Organic matter enrichment in the source rocks of rift lacustrine basins is crucial for the formation of oil and gas, and sedimentation rates and sedimentary environments are two important factors influencing organic matter enrichment. Methods Integrating organic geochemistry (TOC and rock pyrolysis), inorganic geochemistry (major and trace elements), and sedimentation rate reconstruction, this study investigates the organic matter enrichment of Eocene Wenchang and Enping source rocks in the Lufeng Sag, Pearl River Mouth Basin. A model of the organic matter enrichment of source rocks in the Lufeng Sag was established based on paleoclimate, sedimentary facies, and sedimentation rates. Results The organic matter enrichment in the Lufeng Sag was primarily governed by the coupling of paleoclimate, sedimentary facies, and sedimentation rates. Discussion During the deposition of the fourth member of the Wenchang Formation (E 2 wc 4 ), strong fault activity and rapid tectonic subsidence maximized the accommodation and water depth, establishing a stable, reducing, semi-deep to deep lacustrine environment. This period, characterized by a warm and humid climate, facilitated high paleoproductivity. Specifically, the synergistic effect of high primary productivity and high sedimentation rates (>500 m/Ma, averaging 634.86 m/Ma) in the deep lacustrine facies led to the development of excellent source rocks with a mean TOC of 2.82 wt. %. In contrast, the shore-shallow lacustrine and braided river delta front, developed under moderate sedimentation rates (100-500 m/Ma) and weakly reducing conditions, yielded good to average source rocks. During the subsequent decline phase (Enping Formation), weakened tectonism and an arid climate resulted in shallow, oxidized water bodies and lower productivity, significantly diminishing the organic matter preservation potential. Overall, the combination of a warm-humid climate, high sedimentation rates, and a strongly reducing environment during the intensive rifting stage provided the most favorable conditions for high-quality source rock formation.