Pore characteristics and controlling factors of coal reservoirs from Jixi Basin

Pore structure is a core issue of coalbed methane of untraditional reservoirs, which can control preservation properties and development results. The pores and fractures of coalbeds from the Jixi Basin were quantitatively characterized by means of low temperature nitrogen adsorption, argon ion milling scanning electron microscopy and nuclear magnetic resonance. The controlling factors of organic pores and mineral pores were discussed based on spectral analysis, mineral composition and coal component analyses. The results showed that the organic pores were mainly gas pores, usually bottle shaped, and some cylinder pores open at both ends and parallel plate pores open on four sides were also observed. The NMR T₂ spectrum has two or three peaks, indicating favorable porosity and permeability. High R_o and vitrinite content enhanced organic pore development, while water blocked pore surfaces and throats. Mineral pores mainly include intergranular pores of clay minerals and dissolution pores. The pores of monomer or aggregates of kaolinite, illite, and illite/smectite provided a large amount of adsorption space. The content of kaolinite has a positive correlation with the porosity of clay minerals, and the content of illite/smectite was negative.