Transgressive Lacustrine Shale Sedimentary Environment, Enrichment Model and Mechanisms for Organic Matter in the Paleocene Funing Formation, China

Abstract

The relationship between marine transgression and the distribution of lacustrine organic matter has restricted shale oil and gas exploration for decades. In this study, the research objective is to analyze the sedimentary environment and evaluate its influence on organic matter in transgressive lacustrine shale. The study uses various analyses including total organic carbon (TOC), Rock-Eval pyrolysis, gas chromatography-mass spectrometry (GC-MS), trace element and isotope analysis. Finally, the study proposes an enrichment model for organic matter. The lacustrine shale of the second member of the Funing Formation (E₁f²) is divided into three sequences. The results indicate that the depositional environment of the organic matter during this period was an arid and humid, reduced, closed, rift lake basin. In the first sequence, high salinity resulted from increased evaporation, leading to low primary biological productivity. At this time, the lake basin belonged to a salinized closed lake basin. Intermittent transgressions began in the second sequence, with the deep lake area still being dominated by a reducing environment. The third sequence saw the environment evolve into a closed lake basin characterized by a warm and humid freshwater environment with high primary productivity. Marine transgressions introduce a substantial amount of marine plankton, nutrient elements, as well as more CO₂ and CO₃^2– into the lake, leading to increased primary productivity. The sedimentary model for transgressive lacustrine source rocks proposed here serves as an example for similar transgressive lake basins.