Investigations on the methane sorption capacity of marine shales from Sichuan Basin, China

Abstract

High-pressure methane sorption isotherms measured at 30, 50 and 80°C and pressure up to 20MPa were measured on Lower Silurian marine shales from Sichuan Basin of China. The effect of total organic carbon content, temperature, thermal maturity, mineral composition, and pore structure on methane sorption capacity has been investigated. A linear combination approach has been developed to predict the Langmuir sorption capacity of shales based on the mass fractions of their organic and inorganic components. This information can be used to estimate the in situ sorption capacity of shale layers as a function of burial depth (formation pressure and temperature), and composition.Methane sorption capacity of the dry shales shows a positive relationship with TOC and specific surface area. The Langmuir pressure decreases with increasing maturity, but the TOC-normalized Langmuir sorption capacity of over-mature shales also decreases with increasing thermal maturity. This may be related to the carbonization of highly over-mature organic matter. For the samples investigated, approximately 16.3%–46.7% (average 28.6%) of the methane sorption can be attributed to clay minerals and 46.5%–81.5% to (average 67.6%) organic matter in these shales, respectively. Using the linear combination approach, the calculated Langmuir sorption capacity function matches the measured values reasonably. Methane sorption capacity computed as a function of depth (for a mean hydrostatic pressure gradient of 0.01MPa/m and a geothermal gradient of 0.03°C /m) shows a rapid increase and reaches a maximum between 900 and 1800m followed by a slow decrease with increasing depth. At shallow burial depths (<900–1800m), the sorption capacity is controlled by the combination of hydrostatic pressure and geothermal gradients. At deeper burial (>900–1800m) sorption capacity is controlled by the geothermal gradient. Higher geothermal gradients result in a steeper decline of sorption capacity.