Productivity of Eocene ancient lake and enrichment mechanism of organic matter in Huadian Basin

Based on the geochemical research of mudstone and oil shale of Huadian formation, the productivity of Eocene Paleo-Huadian lake was qualitatively and quantitatively recovered by using the organic carbon method, and the water environments, paleoproductivity change during the lake evolution and enrichment mechanism of organic matter were discussed. The results show that the evolution order of paleoproductivity in Huadian Basin is middle oil shale member, upper coal member and lower pyrite member. The highest paleoproductivity achieves at 1 033.09 g/(m·a) during the sedimentary period of middle member of Huadian formation. The ancient lake is in eutrophic state. The bacteria and algae are dominate in organic matters and the lake water is in steady hypoxic and fresh-brackish reduction environment during this stage, which reflects that the high paleoproductivity is mainly contributed by algae and directly influences the origin and abundance of organic matters. The alternative salinity change of lake water makes the bottom water be in a steady hypoxic environment, which is a favorable environment for organic matter conservation. The lakes during the lower member and upper member of Huadian formation are in obvious mesotrophic state and fresh hypoxic environment, and the average paleoproductivity is 143.82 g/(m·a) and 153.26 g/(m·a) respectively. High paleoproductivity is the first condition for organic matter enrichment of oil shale, and steady hypoxic environment in bottom water is the most favorable environment for organic matter enrichment.