Methane adsorption effected by pore structure of overmature continental shale: Lower Cretaceous Shahezi Formation, Xujiaweizi Fault, Songliao Basin

Abstract

Overmature continental shale is commonly developed, but few studies have given insight into its pore structure and sorption capacity. Various techniques, including SEM, helium porosity and permeability, N₂/CO₂ adsorption, MICP, and NMR, were used to detect the pore structure of shale from the Shahezi Formation, Xujiaweizi Fault, Songliao Basin. The excess methane adsorption volumes were measured by the volumetric method and modeled by the Langmuir model. Based on the findings, the most developed pores are intraparticle pores in clay minerals, followed by the dissolution pores in feldspar, but organic pores are uncommon. The selected shales have low helium porosity (mean 1.66%) and ultralow permeability (mean 0.0498 × 10⁻³µm²). The pore throats are at the nanoscale, and the pore-throat size distributions are unimodal, with most less than 50 nm. The studied shales are characterized by the lower specific surface area (SSA) and pore volume (PV) but the larger average pore diameter. The total SSA is contributed by the micro- and mesopores, while the PV is dominated by meso- and macropores. The pore structures are more complex and controlled by multiple factors, such as mineral compositions and diagenesis, but organic matter is not critical. The maximum absolute adsorption methane volume (V_L) is 0.97–3.58 cm³/g (mean 1.90 cm³/g), correlating well with the total SSA, SSA, and pore volume of micropores, which indicates that methane is mainly adsorbed and stored in micropores, followed by mesopores.