Depositional environment, sequence stratigraphy and sedimentary mineralization mechanism in the coal bed- and oil shale-bearing succession: A case from the Paleogene Huangxian Basin of China

It has been a long dispute in the forming mechanism of coal and oil shale in coal-and oil shale-bearing succession because the depositional environments between coal and oil shale are distinctly different. In this study, the Huangxian Basin was chosen as a typical example to study the mechanism of sedimentation of the coal and oil shale. Eleven facies and four facies associations have been recognized from petrologic, sedimentological and paleogeographic characteristics. The four facies were formed in alluvial fan, fan delta, braided stream, braided stream delta, shore-shallow lake-lacustrine bog, shallow lacustrine and deep lacustrine environments. Two types of sequence boundaries (regional unconformities and regional exposed non-sedimentary surfaces) and double systems tract boundaries (maximum flooding and first flooding surfaces) are identified. Sequence stratigraphic frameworks with two sequences (each including three systems tracts) are established to analyze the depositional evolution. We find that the Huangxian Basin is a paralic continental faulted lacustrine basin with four different types of coal bed and oil shale combinations: oil shale/coal bed/oil shale (OS-CB-OS), coal bed/oil shale (CB-OS), coal bed/mudstone/oil shale (CB-M-OS) and oil shale/coal bed (OS-CB). The formation mechanisms of the coal bed and oil shale associations are interpreted to be: a) the basin first experienced transgression, giving rise to the CB-OS or CB-M-OS combinations where oil shale was overlain by coal beds, and b) continental, non-lacustrine deposition developed in the basin and was terminated by subsequent transgression, forming lacustrine deposits. Later, the basin was silted into a peat swamp that was subsequently interrupted by transgression or terrigenous clastic deposition, eventually forming OS-CB-OS or CB-OS combinations. Through analyzing depositional evolution of the basin we conclude that lowstand system tracts tend to develop CB-OS associations; expanding system tracts tend to develop CB-OS, CB-M-OS and OS-CB-OS combinations; and highstand system tracts tend to develop OS-CB combinations.