Effects of igneous intrusions on coal petrology, pore-fracture and coalbed methane characteristics in Hongyang, Handan and Huaibei coalfields, North China

The effects of igneous intrusions have proved to be important for the exploration and development of coalbed methane in many coal basins. However, the studies of the influences of localized intrusions on coalbed methane reservoirs are still insufficient. In the context of five typical dike/sill intrusion patterns (i.e., dike cut-through, dike cut-in, floor intrusion by sill, roof intrusion by sill, and dual intrusions of roof and floor by sills), this study investigates the changes of coal organic composition and pores/fractures resulting from igneous intrusions. The influences of igneous intrusions on coal composition mainly include the decrease of vitrinites (especially telocollinite) and the increase of inertinites contents, as well as the formation of secondary devolatilization vacuoles and of exogenous desiccation microfractures. The vacuoles have size of 0.1–10 μm, and the microfractures are commonly developed with a wide space and a significantly short length. The microfractures typically have irregular dendritic, filamentous, and turtleback textures, and densities of hundreds per 9 cm2. The influences of igneous intrusions on coal pores and fractures vary significantly depending on the intrusion patterns, the coal ranks after the intrusion, and the nature of the adjacent formation surrounding the intrusion. For the coals with a VRr > 4.2%, the intrusion significantly reduced the coal pore development. For the coals with a VRr from 2.1%–4.2%, the development of the coal pores improved with decreasing distance to the contact. For the coals with a VRr < 2.1%, the intrusion resulted in a slight improvement of the coal pore development. A possible explanation for the increasing gas content near the intrusion is that the secondary vacuoles and microfractures induced an increase in the gas storage capacity and in the permeability of the coal reservoir. The detailed intrusion patterns of sills and dikes contribute to the formations of the coalbed methane accumulation in coal seams, and to the coal-generated gas accumulation in the reservoir of the highly permeable surrounding-rocks.

► We compared the influences of igneous intrusions on coal organic composition and pore/fracture.
► Intrusions result in decrease in vitrinites and increases in inertinites, vacuoles (0.1–10 μm) and microfractures.
► Contact-metamorphism slightly improves pore development of bituminous coals (< 2.1% VRr).
► Pores development of altered anthracites (2.1–4.2% VRr) improves with decreasing distance to contact.
► Contact-metamorphism significantly reduces pore development of meta-anthracites (> 4.2% VRr).