Oil shale paleo-productivity disturbed by sea water in a coal and oil shale bearing succession: A case study from the Paleogene Huangxian basin of Eastern China

Paleo-productivity has been found to have extensive non-uniformity in the vertical sections of the oil shale layers in the coal and oil shale-bearing successions near the sea which have been disturbed by sea water. It is very important for researchers to analyze the paleo-productivity differences in order to benefit the utilization and exploitation of oil shale. There were a series of coal and oil shale-bearing successions influenced by sea water located in the Huangxian Basin of Eastern China. These include the Oil Shale 1, Coal Bed 1, and Oil Shale 2 deposits. By using sampling analysis, this study determined that Oil Shale 2 (OS2) had been influenced twice by sea water during its deposition process. Six stages were included in the OS2-CB1 depositional process, which were used to build a depositional model. In the first stage, the depositional environment of the oil shale was a fresh water lake, and fresh organisms were the materials of the oil shale. The lake was determined to be a eutrophic lake. In the second stage, the paleo-productivity was down due to a transgression in the tectonic movements, and also due to the fact that the area of the eutrophic lake was shrinking. In the third stage, fresh water was the main supply once again due to the termination of the transgression, and the paleo-productivity was found to be locally increased. A mesotrophic lake lay in the areas near the sea, while a eutrophic lake was located in the middle section of the area. Also, oligotrophic lake deposits were found near the provenances. In the fourth stage, a transgression occurred in the Huangxian Basin due to the basin's subsidence, and the paleo-productivities decreased. A mesotrophic lake type was the main depositional environment in the majority of the areas. In the fifth stage, a fresh water supply was dominant. The paleo-productivities improved, and a eutrophic lake was present. In the sixth stage, a peat swamp developed within the entire basin to the stable tectonic activity. The peat swamp was preserved and coalificated to form a coal bed (CB1). This study's results allowed for the interpretation of the vertical differences in the deposits of oil shale, and the determination the mechanism of the coal and oil shale symbiosis was then possible.