Abstract:

In order to clarify the deep coal-forming environment and its controls on the microsopic pore structure and reservoir properties of coal rocks, the deep No.5 coal rock of Shanxi formation in southwestern Ordos Basin was selected for investigating the facies, pore structure and reservoir properties of deep coal rocks through macroscopic observations, coal quality measurements, scanning electron microscope (SEM), and gas adsorption tests. The results show that the No.5 coal rock features extra-low water yield, moderate ash yield, extra-low volatile yield, and moderate-high fixed carbon content, with the average vitrinite reflectance up to 2.38%. The content of vitrinite ranges from 42.09% to 72.49%, with an average of 60.60%, and the content of inertinite reaches 27.34% averagely, while exinite is rare in the coal. Desmocollinite, telocollinite and semifusinite are the dominant sub-macerals of the coal samples. The coal-forming environment was dominated by moist forest-swamp facies, with large overlying water depth and weak hydrodynamic force. The bedding fractures, gas pores and plant tissue pores serve as the dominant reservoir space types, and a small amount of intergranular pores and clay mineral intercrystalline pores are also observed. Micropores and mesopores with pore sizes less than 22 nm are the reservoir space, and the heterogeneity of pore structure containing larger mesopores is more significant. The coal-forming environment with strong water overburden and weak flow is conducive to the development of vitrinite, which also determines that micropores are the main reservoir space of the deep coal. Under the action of gelation, the adsorption and adhesion of terrigenous detritus by coal organic matters led to strong heterogeneity of pore structure containing larger mesopores.

Key words: Ordos Basin, Shanxi formation, deep coal rock, coalbed methane, pore structure, reservoir property, influence factor