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The Depositional Environment of Dolomite-Rich Oil Shale from Northwest China: A Model for a Potential Shale Oil System?

Abstract

Developing unconventional resources (i.e., shale oil) has high priority in Junggar Basin, Northwest China. Permian sections around the Junggar Basin are typified by black shale, commonly interpreted as deep lake. A detailed sedimentological investigation of middle Permian-aged mudstones, Lucaogou Formation, exposed in Jingjingzigou of Bogeda Mountain has revealed that deposition was dominated by suspension. This comprehensive investigation of mud-dominated successions combines petrographic and geochemical techniques to determine: (a) the physical sediment transport processes responsible for sediment dispersal, (b) the nature of primary production within the water column, and (c) the origin of abundant dolomite component. 368 samples were collected from 1000m-high Jingjingzigou section. A total of 32 thin sections were analyzed. TOC (including total sulfur), Rock-Eval, carbon isotope of kerogen, major and trace elements were analyzed for the 32 samples, X-Ray Diffraction, carbonate C/O and Sr isotopes were assigned for each sample. Results show a succession of oil shale, dolomitic mudstone and dolomite layers. Lucaogou formation can be divided into 7 parts. Thin bands of tuff and limestone are interbedded at the bottom part 1. The part 2 consists of shale and mudstone with interbedded siltstone. The part 3 and 4 consists of shale and dolomitic mudstone. The part 5 is dominated by dolomite-rich siltstone. At the top of part 6 and 7, there are intercalations of dolomite in thick oil shale. Except for a few ripples seen in part 3, horizontal bedding is the predominant structure, indicating a low energy depositional environment and suspension processes. The sediment petrographic study reveals the presence of 3 major microfacies that ranges from delta front to deep lake. The distribution of these microfacies indicates an overall deep lacastrine environment that fluctuates between drought and moist conditions. The long term changes in lithofacies reflect two and a half 3rd order sequence cycles, where the 2 maximum flooding horizon are respectively located on the boundary of part 1&2, and part 4&5. Geochemistry study indicates high primary production that average TOC is 5.4%. The production is much higher in part 5–7, about 3 times of part 1–4, which suggests the upper Lucaogou Formation may be better for shale oil exploration.