Abstract:

Multiple thick coal beds occur in Jurassic, Ordos basin. They have good coal qualities, huge reserves, simple structures, and relatively shallow burial depth. As a result, they possess large exploitation values. The characteristics of coal petrology are specific in Jurassic coal sequences in Ordos basin. Their genetic swamp environment was mainly a series of high-stand swamps on the upper fluvial system that was related with the development of typical inland lakes. The sand bodies were developed. The peat, after the  burial, continued to undergo oxidation conditions with different degrees. As a result, the content of inert components is widely higher than normal values. The vitreous coal component had extensively undergone specific-degree fusainization and became semi vitrinite.The study shows that in Ordos basin, the coal petrology in Jurassic is mainly semidull coal and dull coal, and semilustrous coal and lustrous coal as follows. It is also found that the contents of semidull coal and dull coal have higher percentages in the basin margin, especially in the area near the provenance of northern basin; the semilustrous coal and lustrous coal become more and more southwards towards the central basin. This result explains that the depositional environment and coal-generating swamp environment have the major controlling effects on the coal components.Another result is that in these coal sequences, the fusinite content is higher in coal macerals with thicker sandstone and coarser grain sizes, moreover, its texture is complete and clear, especially in lower coal beds. The semi vitrinite has the same regularity. The cell texture of plants becomes more complete and regular. After the burial, the lithology of roof and floor rocks can continue to affect the evolution of coal petrology. The sand bodies in the roof and the floor of coal beds have good physical conditions so that the pore water can still be in the state of oxidizing, circulating and connecting in a long term. It makes the coal components stay in the oxidation phase for a long time, which is favorable for the process of fusinitization. On the contrary, the development of lacustrine facies and peat rapidly covered by mudstone after the burial swiftly, make the coal beds in reduction and anoxic environment. As a result, the gelatification can be done abundantly and the vitrinite content increases. Exinite often accumulates in a specific position of coal bed, and its average content is not high in the whole coal bed, but it has great contribution to the total hydrocarbon generated capacity of coal beds.In the past research results, the exinite content is often underestimated, especially amorphous bitumen in fluid state, for the reason that it is easiest to flow into the fusinite with strong rigidity or several fissures. The exinite content deserves to be emphasized in the further research.

Key words: Jurassic, coal-generating environment, coal maceral, Ordos basin