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Organic matter (OM) is the primary gas occurrence carrier in shale reservoirs due to their abundant nanopores. To reveal the OM pore structure, adsorption capacity and evolution during thermal maturation, this study collected data from samples spanning the entire evolution stage, from immature to over-mature. Scanning Electron Microscope (SEM) observation and low temperature gases adsorption experiments were used to qualitatively-semi-quantitatively and quantitatively analyze OM pore structure evolution, and CH 4 isothermal adsorption experiments were used to reveal the adsorption capacity evolution. Then, the influence and mechanism of maturity and hydrocarbon generation on pore development and adsorption capacity were quantitatively reviewed based on the experimental data. The results show that OM pores are poorly developed in the immature stage due to weak hydrocarbon generation, although micro-fractures are occasionally found at the edges of OM particles. In the low maturity stage, OM pores are partially developed due to liquid hydrocarbon generation, with liquid hydrocarbons also filling some OM pores. The contribution of total organic carbon content (TOC) to adsorption extent is not significant in these two stages. From high to high-over maturity stages, massive gaseous hydrocarbons are generated, significantly improving the surface porosity of OM. Clear positive linear correlations are observed between TOC and adsorption amount. However, the development of OM pores significantly declines when thermal maturity ( R o ) exceeds 3.5% due to excessive aromatization. The accuracy of research on the evolution of pore structure and adsorption capacity is limited by several factors: (i) errors caused by sample specification, calculation processes, parameter settings, and kerogen models in isothermal adsorption experiments and molecular simulations; (ii) difficulty in achieving control variables due to the strong heterogeneity of natural maturation shale samples; and (iii) the need to enhance compatibility between thermal simulation experiments and natural thermal evolution. Therefore, isothermal adsorption experiments on bulk shale and molecular simulations of intact shale model are necessary, taking into account the dynamic temperature and pressure of in-situ reservoirs. Moreover, shale samples with varying maturity, influenced by their distance from the paleo-thermal source, may provide significant verification for thermal simulation experiments.
Xie et al. (Wed,) studied this question.