Controlling factors of organic matter enrichment in continental shale: A case study of the Jurassic Da’anzhai member in the Sichuan Basin

Abstract

The continental lake basin is a favorable accumulation area of shale oil and gas; however, the organic matter enrichment regularity in continental lake facies is still controversial, which hinders the exploration of continental shale oil and gas in the Sichuan Basin. In this study, the relationship between a sedimentary environment and organic matter enrichment of continental shale was analyzed by petrology and organic and inorganic geochemistry tests of 153 samples. The results show that different lithologic associations in the Da’anzhai member have different contents of organic matter. Among them, the TOC content of unit II (shale-limestone interbed) is the highest, mainly, type II kerogen, followed by unit III (shale clip shell limestone belts), mainly, type III kerogen. Geochemical indicators show that different paleoenvironmental factors play different leading roles in organic matter enrichment in different periods of the Da’anzhai member. Unit Ⅱ is deposited in an arid environment with less fresh water supply; therefore, the water cycle is hampered. Due to the effect of salinity stratification, anoxic water was formed, which was beneficial to the preservation of algae and plankton in the lake basin, resulting in the formation of type Ⅱ2 kerogen. Unit III is deposited in a semi-humid and semi-arid climate, and its water is in an oxidizing environment. Precipitation and river runoff increase the input of terrigenous debris and higher plants to form type III kerogen. This study has guiding significance for the cause of organic matter enrichment in the Da’anzhai member and the prediction of favorable zones.