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Sedimentary Model of Fine-Grained Sediments: A Case Study From the Holocene Qinghai Lake


Fine-grained sediments and sedimentary rocks make up as much as 75% of the present and past sedimentary records. However, they have been relatively understudied because of their grain size and prone to weathering character. The recent burst of interest in unconventional petroleum resources has led to a better understanding of fine-grained sediments, in terms of their sedimentary processes, depositional environments, and lithofacies. The Qinghai Lake, situated on the Northeast Qinghai-Tibet Plateau, is the largest saline lake in China with a closed catchment. Its Holocene sediments is dominated by fine-grained sediments. A sedimentary model is proposed in this study for the Holocene fine-grained sediments of Qinghai Lake, by integrating short cores, petrographic and geochemical data, satellite images, and physical modeling results. Based on energy level and relative water depth, lacustrine environments are subdivided into littoral zone, sublittoral zone, and profundal zone. The littoral environment is dominated by reworked sandstone, siltstone, and oolite sand, with a typically low total organic carbon (TOC). Satellite images show hypopycnal, meopycnal and homopycnal sediment plumes, which are driven by wind and inertial force. They can capture algae by flocculation processes, and usually spread along shoreline to distal basin. However, the algae-containing flocculated particles would be reworked and transported to sublittoral and profundle zones, with few organic matter deposited in situ. In contrast, the profundal zone is dominated by organic-rich mudstone associated with fallout biological debris both of autochthonous and allochthonous origins. Mudstone rich in allochthonous organic matters is commonly deposited by hyperpycnal flow, and typically accumulates in proximity to river mouth. They tend to interbed with thick hyperpycnal sandstone that forms favorable petroleum reservoir. The accumulation of such deposits are mainly controlled by sediment supply, river discharge, climate, lake level and salinity. Mudstone rich in autochthonous organic matters is commonly deposited by organic material (e.g., marine snow) and flocculation of algae and clay particle induced by hypopycnal, meopycnal and homopycnal flows and typically accumulate far away from river mouth. The distribution of such deposits are mainly controlled by salinity, concentration of sediment plume, characteristic of clay mineral, and biological productivity. The sublittoral zone, a transitional environment between the littoral and profundal zones, is dominated by organic-poor muddy siltstone and silty mudstone. The proposed sedimentary model is helpful to provide a better understanding on evaluation of hydrocarbon potential of source rocks and identification of target zones for unconventional resources.