Sedimentary geochemistry and paleoenvironment of oil shales in the Maoming Basin in the northwestern continental margin of the South China Sea

The global shale oil resources are far more than conventional ones, and the development and utilization of shale (oil-bearing shale) resources will change the global energy landscape. The oil shale resource reserves in the Maoming Basin, Guangdong, South China, account for 16.72% of the national total, possessing tremendous resource potential. Focusing on the analysis and research of the geochemical characteristics of oil shales in the Eocene Youganwo Formation in Maoming Basin. systematic sample collection and analysis on the contents of major and trace elements were conducted, and the sedimentary geochemical characteristics and sedimentary environment were deeply analyzed. Results are: (1) The oil shales of the Youganwo Formation showed enrichment in Al₂O₃ and Fe₂O₃ relative to the average value of the upper continental crust (UCC) but deficient in MgO, Na₂O, P₂O₅, MnO₂; (2) During the sedimentation, the parent rock of the oil shales was subjected to strong chemical weathering, which caused exchange of potassium in clay minerals. However, some high field strength elements and rare earth elements were less affected by chemical weathering. The effect of sediment recycling on the geochemical properties of oil shale sedimentation was relatively small; (3) The provenance was consisted of intermediate acidic igneous rocks between granite and granodiorite in lithology similar to the Precambrian tonalite assemblage (TTG) in chemical composition. The Maoming oil shales were deposited in a warm and humid climate in lake water with low salinity and strong stratification. The bottom water was in a reducing environment, which was conducive to the enrichment and preservation of organic matter; (4) The water temperature difference between surface and bottom layers caused lake water stratification, while lake transgression deepened the water, which weakened the influence of wind and rivers on the bottom layer of water and resulted in oxygen deficiency in the bottom layer, providing favorable conditions for the enrichment and preservation of organic matter and the formation of the oil shales.