Paleoenvironmental controls on organic-rich lithofacies of Eocene saline lacustrine in the Chentuokou Depression, Jianghan Basin

Abstract

Source rocks characterized by shale-dolomite interbeds were widely developed in oil group Ⅱ, the lower Member of Xingouzui Formation in Jianghan Basin. A breakthrough has been made recently in shale oil exploration of shale-dolomite interbeds, while the presence of gypsum and salt deposites in saline lake on shale porosity restrains the process of shale oil exploration. Objective It is of great significance for shale oil exploration to clarify the controlling factors of the organic-rich shale lithofacies with low content of gypsum and salt. Methods Well C1 penetrated stratigraphic unit of the Xingouzui Formationin the Chentuokou Depression, Jianghan Basin was studied. Based on the core and thin section observations and the analyses of whole rock mineral compositions, organic carbon contents (TOC), and geochemical elements, a systematic study on lithofacies, sedimentary environment, and main controlling factors of organic matter accumulation were conducted. Results The classification of fine-grained sediments and the saline lacustrine sedimentary model in Jianghan Lake Basin were established. Lithofacies of oil group Ⅱ, the lower Member of Xingouzui Formation Xingouzui Formation in Well C1 can subdivided into seven types: organic-rich laminated dolomite facies, organic-rich layered dolomite facies, organic-contain massive dolomite facies, organic-rich laminated mixed shale facies, organic-rich layered mixed shale facies, organic-contain layered siltstone, and organic-poor massive gypsum-salt rock facies. The co-evolution of paleoclimate, paleosalinity, and paleoredox resulted in the diversity of lithofacies development, and the main controlling factor of organic matter accumulation was the anoxic palaeo-water body. The favorable lithofacies for shale oil exploration are organic-rich/organic-contain laminated/layered dolomite/mixed shale, which deposited in semi-deep lake with brackish water, anoxic condition. Conclusion This study reveals the controlling factors for the development of organic-rich lithofacies and its depositional environment of oil group Ⅱ, the lower Member of Xingouzui Formation in the Chentuokou Depression, which can provide valuable insights for shale oil exploration of Xingouzui Formation in related Depression in Jianghan Saline Basin.

Keywords

• jianghan basin
• eocene
• saline lacustrine deposit
• organic-rich lithofacies
• paleoenvironment
• organic matter accumulation