Methane adsorption characteristics of overmature Lower Cambrian shales of deepwater shelf facies in Southwest China

The Lower Cambrian shales of deepwater shelf facies comprise an important part of the marine black shales in southern China. In this study, twenty-four Lower Cambrian shale samples from southwest China were subjected to analysis of geochemistry, pore structure and methane adsorption capacity. The samples have total organic carbon (TOC) contents ranging from 0.25% to 11.3% and equivalent vitrinite reflectance (EqVRo) values in the range of 2.51-4.23%. Specific surface areas determined from N-2 adsorption at 77.4 K and specific micropore volumes from CO2 adsorption at 273.15 K are in the range of 3.0-38.6 m(2)/g rock and 2.5-11.4 cm(3)/kg rock, respectively, and both of them have positive relationships with TOC values. The Langmuir volumes of these samples at 60 degrees C range from 0.47 cm(3)/g rock to 5.96 cm(3)/g rock and are positively correlated with TOC values, which on the one hand confirms the TOC control on methane adsorption capacity and on the other hand also suggests a weak control of thermal maturity on methane adsorption capacity at overmature stages. Although both specific surface area and specific micropore volume are positively correlated with methane adsorption capacity, the latter seems to have a stronger effect and the specific micropore volume determined from subcritical CO2 adsorption may be considered as a proxy for the volume of adsorbed methane in dry shales. For the extrapolation of measured methane excess adsorption isotherms to geological conditions, the use of a constant or varying Langmuir volume makes no significant difference with respect to the methane storage capacity of shales when the density of the adsorbed methane is allowed to change with temperature. However, the absolute methane adsorption quantity at subsurface is obviously dependent on the use of a constant or varying Langmuir volume, which subsequently affects the estimation of the relative percentages of adsorbed and free methane.