Chemical structure and hydrocarbon generation characteristics of tectonic coal with different metamorphic degrees: Implications for gas adsorption capacity

Coalbed methane (CBM) is mainly stored in coal by adsorption and tectonism is an important factor affecting the storage state of CBM. In order to gain a deeper understanding of the mechanism of the influence of tectonism on CBM formation, the chemical structure and hydrocarbon properties of tectonic coal and their effects on gas adsorption capacity were investigated. The results showed that the A (CH2)/A (CH3) values of tectonic coal decreased by 4.0%–64.1% after tectonism, and the aromaticity (I) and condensation degree of aromatic rings (DOC) increased up to 4.0 and 2.58 times, respectively. The interlayer spacing (d002) and number of stacked layers (Nave) of aromatic ring layers decreased slightly, but the extension height (La) increased by a maximum of 6.0%. The tectonism not only promoted the shedding of aliphatic side chains and reduction of aliphatic carbon content, but also promoted the aromatic structure of coal developed in an orderly direction. The increase in the polycondensation of aromatic structure of tectonic coal indicated a higher gas adsorption capacity than intact coal. The hydrocarbon generation potential of tectonic coal is lower than that of intact coal, which indicates that tectonic coal produces more hydrocarbons during geological formation and increases the coalification degree of coal. The chemical structural of coal has also been shown to promote gas adsorption.