Mechanisms of accumulation and coexistence of coal and oil shale in typical basins

The accumulation of coal and the formation of oil shale occur in basin conditions that are unique for each of these energy resources. In particularly special conditions, they can coexist in the same basin. Many studies have been carried out on theories and models explaining coal accumulation, as well as on the metallogenic model of oil shale. This paper attempts to address the key theoretical issue of the coexistence of coal and oil shale. We study different types of basins in order to investigate the geotectonic basis of such coexistence. We conclude that the basins’ tectonic activity controls the overall characteristics of basin filling, and thus represents the external conditions for the coexistence of coal and oil shale. Episodic tectonic activity and the tectonic evolution of the basins leads to changes of the water system and material supply, resulting in various cyclic patterns of coal and oil shale formations. Past climatic conditions determine whether there were suitable materials to form coal and oil shale. We identified six main types of coexisting coal and oil shale. Our research shows that all six types can occur in the system tract of water expansion. Periodic lake flooding is conducive for the development of coal and oil shale in multiple thin layers. This paper demonstrates that there are some similarities and differences between the conditions suitable for the formation of coal and oil shale. We establish a model for the coexistence of coal and oil shale through a comprehensive analysis of basin background, basin spatial framework, stratigraphic framework, dynamic field of deposition and its evolution in the coexisting space, and the controlling mechanism of the formation, together with the coexistence process, distribution and occurrence.