Relationship between element geochemical characteristics and organic matter enrichment of Wenchang Formation mudstones in Enping Sub-sag 21, Pearl River Mouth Basin

The elements of mudstones can well preserve the original sedimentary records. To define the element geochemical characteristics of the Wenchang Formation mudstones in Enping Subsag 21 and its relationship with enrichment of organic matter, 10 debris samples of the mudstones from well YJ-1 were collected, on which geochemical tests of organic carbon, major elements and trace elements were conducted. The geochemical characteristics of major and trace elements were examined, and paleo-environmental indicators such as paleo-climate, paleo-water depth, paleo-oxygenation facies, and paleo-productivity of the Wenchang Formation in the subsag were systematically analyzed. Compared with the content of major and trace elements in the Upper Continental Crust (UCC), major elements of the Wenchang Formation mudstones in Enping Subsag 21 are relatively enriched in Al₂O₃, K₂O, and MnO, and poor in CaO, MgO, and Na₂O. Trace elements are rich in Li, Be, Zn, Rb, CS, Ba, Pb, Th, U, and poor in Sr, V, and Cr. Paleoenvironmental indicators show that climate of Enping Subsag 21 in the Wenchang period was warm-moist to warm-humid. The paleo-lake was relatively deep and stable. Redox environment of the lake was mainly sub-reductive. Paleo-productivity of the lake was relatively high, and the vertical difference was significant. Correlation between organic carbon content of mudstones and paleo-productivity parameters is poor, but there is a good positive correlation with reducibility of the lake. Enrichment of organic matter was controlled by oxidation-reduction degree of the lake. Reduction condition of the lake was conducive to the preservation of organic matter. Enrichment of organic matter belongs to the "preservation mode". Further analysis shows that paleo-climate is the key factor for enrichment of organic matter in the Wenchang Formation. Warm and humid climate has a certain positive impact on productivity of the lake. Meanwhile, the precipitation was large, and the lake was deep, which was conducive to a reducing environment at the lake bottom, and thus conducive to the preservation of organic matter.