Paleoenvironmental Characteristics of Lacustrine Shale and Its Impact on Organic Matter Enrichment in Funing Formation of Subei Basin

Abstract

Organic matter in depositional environment is the essential material for oil and gas generation. Total organic carbon (TOC) is one of the important parameters for estimating the hydrocarbon generation potential of shale oil and predicting sweet spots. The TOC of the second member of the Funing Formation (Ef2) ranges from 0.25% to 2.30%. TOC is higher in the upper shale and lower in the lower shale of the Funing Formation, showing a significant enrichment difference. However, there have been few reports on the study of the main controlling factors for the differential enrichment of organic matter in Ef2. This study aims to reconstruct the paleoenvironment of lacustrine shale in Ef2. Additionally, this study aims to clarify the influence of the paleoenvironment on the differential enrichment of organic matter in Ef2. For this purpose, systematic mineralogical and geochemical analyses were conducted on 72 samples from a representative well. The results indicate that, based on parameters such as paleoclimate (chemical index of alteration, CIA), paleosalinity (Sr/Ba), paleoredox conditions (Cu/Zn), paleoproductivity (P/Ti), water depth (Rb/K), and terrigenous clastic input (Al, Ti), the paleoenvironment during the deposition of the Ef2 shale clearly exhibited significant changes. During the early stage, the climate was hot and dry, with shallow water, weak chemical weathering, low productivity, and salinity ranging from saline to brackish. In the later stage, the climate became warm and humid, with deeper water, moderate chemical weathering, high productivity, and salinity ranging from brackish to freshwater. There are significant errors in directly using the Sr/Ba index to evaluate the paleosalinity of Ef2 shale. Carbonate minerals and calcium-rich bioclasts may increase the Sr/Ba ratio, and the corrected Sr element content is only 44.29% of the original sample. The enrichment of organic matter is clearly controlled by productivity levels and climatic conditions. The higher the paleoproductivity and the warmer and more humid the climate, the more enriched the organic matter becomes. Fundamental differences in paleoproductivity govern the enrichment of organic matter during the deposition process of the Ef2 shale. The organic matter enrichment pattern in the Ef2 shale represents a typical productivity model.