Characterization of lacustrine mixed fine-grained sedimentary rocks using coupled chemostratigraphic-petrographic analysis: A case study from a tight oil reservoir in the Jimusar Sag, Junggar Basin

Mixed deposits are sediments consisting of external clastic (epiclastic or terrigenous), intrabasinal components and pyroclastic components. The mixture, comprising variable amounts of the three components, is defined as mixed sedimentary rocks. The Permian Lucaogou Formation (P(2)1) in the Jimusar Sag of the Junggar Basin is a promising tight oil target in western China, the fine-grained mixed sedimentary rocks of which are rich in organic matter (OM) and two sweet spot intervals with relatively high porosity. However, the sediment composition, provenance and deposition environmental settings have not been studied in detail. In this study coupled chemostratigraphic-petrographic analysis were used to reconstruct their depositional environments. The results show that the fine-grained sedimentary rocks have three major sediment sources, external clastic input (terrigenous clastics), intrabasinal autochthonous to parautochthonous components (carbonates, siliceous skeletal debris and OM) and pyroclastic input. Main lithofacies include siltstone/fine sandstone, mudstone, dolomite and tuffite. The silt/sandstones were mainly sourced from rocks with calc-alkaline composition, while the tuffaceous sedimentary rocks were sourced from high-K calc-alkaline rocks. Elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions, whereas the fine-grained clastic sediments were deposited under relatively humid conditions. The muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones. Variations of lithofacies and OM accumulation of different intervals reflect changing deposition environmental settings, and the frequently altered high TOC content rocks and good reservoirs are benefit for tight oil formation. The work may provide some useful insights and serve as a reference for studying other mixed fine-grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere.