Factors controlling the distribution of oil shale layers in the Eocene Fushun Basin, NE China

Abstract The Fushun Basin is a typical continental basin filled with organic-rich fine-grained sedimentary rocks, such as coal, oil shale and greyish green mudstone. Currently, research on this basin predominantly focuses on the enrichment mechanisms of organic matter in coal and oil shale and the controlling factors of coal seam transformation to oil shale. However, during the transformation stage, the deposition of oil shale ceased and was converted to interbeds of grey green mudstone and thin oil shale layers. The control mechanism of this type of sedimentary succession is still not clear. In this study, the thick oil shale in the upper part of the Jijuntun Formation (Unit I) and the high-frequency interbedded mudstone and thin oil shale in the Xiloutian Formation (Units II and III) were analysed as key strata. Organic and inorganic geochemical data were analysed to reconstruct the transformation of the various types and sources of organic matter and controlling factors including the palaeoclimate, palaeoweathering, bioproductivity, terrigenous clastic input, palaeosalinity, and redox conditions. Among these, Unit I was affected by humid climate conditions predominantly. The water depth and volume were large, and the input of terrigenous debris was low. Freshwater algae blooms and anoxic conditions led to a supply of organic matter and that was well preserved. This further resulted in a high abundance of organic matter in the sedimentary rocks. Unit II was affected by semihumid-semiarid and arid climate conditions. The lake became shallower while terrigenous clastic input and lake palaeosalinity increased; these changes represented the transition stage from freshwater to brackish water. The lake still exhibited suboxic/anoxic conditions, but the biological living space decreased, resulting in a limited organic matter supply and a low abundance of organic matter in the sediment. Unit III was affected by semihumid-semiarid climate conditions, and the palaeosalinity continued to increase. The lake had brackish water, with a high bioproductivity, a decreased water volume and a limited supply of organic matter. However, brackish water stratification occurred in the lake, which created a strongly anoxic environment and good preservation conditions, resulting in a secondary high abundance of organic matter. A comprehensive analysis indicates that the environmental change caused by the palaeoclimate was the main factor driving the oil shale deposition to cease and the onset to deposit grey green mudstone and thin oil shale in the Fushun Basin, Northeast Asia.