Genetic types of mudstone in a closed-lacustrine to open-marine transition and their organic matter accumulation patterns: A case study of the paleocene source rocks in the east China sea basin

Organic mudstone can be a high-quality source rock in petroleum basins. The accumulation patterns of various types of mudstone can also serve as a useful record of paleogeography. Due to the impact of tectonic movements, the East China Sea Basin (ECSB) saw its deposition setting transform from closed-lacustrine to open marine settings. This provides an opportunity to explore organic matter accumulation patterns within such a transformation setting. This study classified the constituent mudstones according to their differences in composition and sedimentary structure as follows: coaly mudstone or carbonaceous mudstone, calcareous mudstone, clayey mudstone, and siliceous mudstone. Considering the different sedimentary structures, siliceous mudstone can be subdivided into laminated, wavy, and slumped mudstones. Using integrated multiple indices of the paleo-environment, which includes clay mineral content, trace mineral content, and micro-paleontology assemblages, we reconstructed the paleography setting during the Paleocene and inferred that during this time, the ECSB was characterized by sedimentary differentiation. We discerned two different organic mudstone accumulation patterns by combining the results from rock-eval pyrolysis and paleo-environmental indices: 1) A closed-lacustrine mudstone accumulation pattern characterized by abundant sediment supply and dominated by allogenous mudstone formation by the flocculation process. The fresh to slightly salty water environment provided adequate nutrition to microorganisms and an appropriate temperature led to the increased accumulation of type II1 and II2 kerogen. 2) An open-marine mudstone accumulation pattern which revealed a sediment-starved setting. The flocculation process and fine-graded turbidities were the main processes leading to mudstone accumulation. More plants were concentrated in shallow water with declining salinity, leading to peat and mass carbon mudstone accumulation.