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Experimental Study of Methane Adsorption on Organic Matter in Mudstones: Implications for Natural Gas Storage in Unconventional Reservoirs


Characterization of reservoir properties is critical for constraining gas-in-place predictions and estimated ultimate recovery in shale-gas systems. The heterogeneous nature of these shale lithologies; however, presents a challenge for analyses as key properties including porosity, permeability, and pore connectivity vary in scale from play to play. Previous work has shown that total organic carbon (TOC) content is the most significant control on gas sorption capacity in mudstones, and that kerogen type affects the affinity for gas sorption on organic matter, but not the sorption capacity. This study looks at a suite of artifically matured Woodford Shale samples to examine the effect of thermal maturation on gas sorption and organic-pore development in petroelum source rocks. One outcrop sample of Woodford Shale was treated by hydrous pyrolysis to achieve six different maturity levels ranging from immature to early-oil cracking stages, without attempting to simulate lithostatic loading. The residual source rock was then analyzed for gas sorption, pore-size distribution, and Brunauer–Emmett–Teller (BET) surface area. Methane adsorption capacity normalized to TOC directly correlates with the measured BET surface area and relates to thermal maturity as follows: maximum-oil > early-oil cracking > maximum-bitumen > early-bitumen > immature stages. The decrease in gas sorption from maximum-oil generation to early-oil cracking is interpreted to be the result of pyrobitumen formation and the associated occlusion of pore space. The porosity of the residual source rock shows a consistent increase with thermal maturity, and the most siginificant change occurs when the maximum-bitumen and oil-generation stages are reached. This is mainly attributed to the development of organic porosity related to kerogen transformation to petroleum, which has been quantified through mass-balance calculations of the generated bitumen, oil, and gas. Solvent-extracted residual source rocks follow the same trends as non-solvent extracted samples of the same rocks; however, the total gas sorption capacity is increased by bitumen removal (i.e. solvent extraction). Despite the possibility of some methane dissolution in bitumen, the reduced surface area of the kerogen that is due to the presence of bitumen, leads to a net decrease in gas sorption. These results have siginificant implications for gas storage in mudstones and resource exploration and assessment strategies.