Organic matter enrichment and palaeoenvironmental comparison of Alinian fine-grained sedimentary rocks in lake facies

The Yan'an Formation in the Ordos Basin and the Xintiangou Formation in the Sichuan Basin are typical lake facies sedimentary formations developed in the Alinian period. They contain a large amount of organic-rich fine-grained sedimentary rocks and are the main sites for shale oil and gas enrichment. In this study, geochemical tests, including major, trace, and rare earth elements and TOC tests, were carried out on the fine-grained sedimentary rocks of the shallow lake Yan'an Formation and deep lake Xintiangou Formation to reconstruct their depositional palaeoenvironments, and to explore their organic matter enrichment mechanisms in different depositional environments under lithological differences. The conclusions of this study are as follows: 1) The total organic carbon (TOC) of fine-grained sedimentary rocks increases with increasing mud content and decreasing grain size; 2) The palaeoenvironments of the fine-grained sedimentary rocks in the shallow and deep lake facies are generally consistent, but the redox conditions and deposition rate of the water column in the deep lake facies are stronger than those in the shallow lake facies, while the input of terrigenous detrital, palaeosalinity, and stagnant environment are relatively weaker; 3) The organic matter enrichment mode of lake facies fine-grained sedimentary rocks is a typical palaeoclimatic and palaeoproductivity-dominated “Production mode”; 4) The difference in depositional environments under lithological differences is mainly related to productivity factors, such as palaeoclimate (CIA) and palaeoproductivity (P/Ti). The results of this study help to understand the formation mechanism of lake facies fine-grained sedimentary rocks, distinguish the hydrocarbon generation potential of different lithologies, and provide a basic geological theoretical basis for the exploration of shale oil and gas in the study area.