Geochemical characteristics, palaeoenvironment and formation model of Eocene organic-rich shales in the Beibuwan Basin, South China Sea

Lacustrine organic-rich shales in the Eocene are the most important source rock in the Beibuwan Basin. Palynological, petrographical and geochemical data were applied to define their palaeoenvironment, formation and petroleum potential. TOC values range from 2.84 to 14.77% and the kerogens of these dark gray or brown shales with numerous lamalginites can be classified as oil-prone type-I and type-II1. Biomarkers are characterized by a high abundance of C30 4-methylsteranes, a low content of oleanane and diterpanes, relatively abundant pentacyclic terpanes, which indicate that the organic matter in the shales is typically algal and microbial origin. Abundance of amorphous organic matter, the presence of low level gammacerane, small amounts of pyrite, Pr/Ph ratios of 1.38–2.42, and V/V + Ni ratios of 0.74–0.84 suggest that the organic-rich shales were formed in a less oxic to anoxic, stratified, fresh-brackish lake environment with a relatively low sedimentation rate. For this starved lake basin, the bloom and death of algae in nutrient-rich surface water supplied a large amount of organic matter to the lacustrine sediments. The oxygen-depleted bottom waters of the palaeo-lakes favored the accumulation and preservation of sedimentary organic matter, resulting in the formation of the organic-rich shale, a high-quality source rock. Future exploration or assessment of the petroleum potential of the basin could be assisted by considering the proposed formation model of the organic-rich shales, and their distribution relative to potential traps.