Reservoir Characteristics and Gas Occurrence Patterns of Deep Coalbed Methane in the Benxi Formation, Eastern Margin of the Ordos Basin

Since 2017, China has actively promoted the exploration of deep coal seams as unconventional natural gas reservoirs. However, the petrographic composition, pore–fracture structure, and gas occurrence characteristics of deep coal reservoirs remain poorly constrained, particularly under in-situ formation depth conditions. The deep coal seams of the Carboniferous Benxi Formation along the eastern margin of the Ordos Basin exhibit favorable reservoir development conditions and high gas productivity, making them an ideal target for investigating reservoir characteristics and gas-bearing mechanisms of deep coalbed methane (CBM). In this study, the petrographic composition, physical properties, pore–fracture structure, and gas occurrence states of coal reservoirs were systematically investigated through macroscopic observation and classification of coal lithotypes, combined with scanning electron microscopy (SEM), multi-component gas adsorption experiments, overburden pressure porosity–permeability measurements, and variable temperature–pressure adsorption tests. The reservoir characteristics and gas occurrence patterns of deep coal reservoirs were thereby elucidated. The results indicate that the macroscopic coal lithotypes of the Benxi Formation include bright coal (types I、II、III), semi-bright coal (types I、II、III), semi-dull coal (types I–II), and dull coal (types I–II), with bright coal being dominant. The average vitrinite contents of vitrain, clarain, and durain are 94.6%, 85.26%, and 58.8%, respectively, while the corresponding average fixed carbon contents (air-dried basis) are 79.6%, 72.6%, and 34.15%. The average maximum vitrinite reflectance ranges between 1.75% and 2.56%, indicating a high- to over-mature coal rank. Reservoir space is primarily composed of gas pores, cellular pores, and cleat fractures, with micropores and microfractures of 0.5–1.2 nm contributing the dominant pore volume. The average total pore volume of clarain and durain ranges from 0.0299–0.034 cm³/g and 0.014–0.025 cm³/g, respectively, with clarain II exhibiting the largest pore volume. The average permeability and porosity of clarain and durain are 0.623 mD and 5.23%, respectively. Natural fractures significantly enhance permeability under overburden pressure conditions, whereas artificial fractures exert negligible influence. Gas in deep CBM reservoirs occurs in both adsorbed and free states. With increasing burial depth, free gas content increases, adsorbed gas content first increases and then decreases, and total gas content either increases or remains relatively stable. Coal with higher vitrinite and inertinite contents, as well as a higher proportion of clarain, exhibits greater gas content. Under in-situ formation depth conditions, the measured total gas content of the Benxi Formation coal is lower than the theoretical maximum gas adsorption capacity.

Keywords: Ordos Basin; coalbed methane; in-situ formation; Benxi Formation; coal lithotype; reservoir space; pore–fracture structure; gas occurrence characteristics