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Lithofacies and Depositional Environment of the Eocene Kongdian Shale, Cangdong Depression, Bohai Bay Basin, China


The Eocene Kongdian Formation is a key target for unconventional hydrocarbon exploration in China and comprises organic-rich shale unit deposited in a lacustrine deep-water setting. Mineral components are dominated by dolomite, quartz and clay minerals with minor amounts of plagioclase, calcite, potassium feldspar, and analcite. The brittle mineral (quartz, feldspar and carbonate) content ranges from 39% to 77% by weight (average: 64 wt. %). Five primary lithofacies were identified based on the detailed observation of cores using petrographic and scanning electron microscope examination of thin sections and other data analyses: (1) massive dolomitic mudstone, (2) laminated dolomitic mudstone, (3) laminated silty mudstone, (4) laminated mixed mudstone, (5) massive mixed mudstone. The lithofacies vary in petrological and organic content. Different characteristics between these five lithofacies reflect the difference of depositional environment and depositional mechanism. Major depositional processes inferred from Kongdian facies and sedimentary structures include suspension settling, turbidity currents, and underflows. In the prodelta to semi-deep lacustrine, the turbidity currents and underflows are widely developed, which formed the superimposed deposition combination of laminated silty mudstone, massive mixed mudstone and laminated mixed mudstone. In the semi-deep to deep lacustrine, the water was relatively quiet with a small amount of terrigenous clast input, and the carbonate and clays were more developed, forming laminated dolomitic mudstone and massive dolomite mudstone from suspension deposition.