Geochemical characterization and methane adsorption capacity of overmature organic-rich Lower Cambrian shales in northeast Guizhou region, southwest China

The Niutitang Shale (Lower Cambrian) is another set of organic-rich shale in addition to the Lower Silurian Longmaxi Shale in southwest China. In this study, methane adsorption capacities of both whole rock and organic matter were investigated based on adsorption isotherms measured at various temperatures (40-120 °C) and pressures (up to 35.0 MPa) for eleven Niutitang Shale samples collected from northeast Guizhou Province. The investigated samples cover a wide range of TOC contents, varying from 1.8 to 11.3%, with an average equivalent vitrinite reflectance (EqVRo) value of approximately 4.0%; their BET specific surface areas and micropore volumes range from 11.1 to 26.2 m2/g rock and between 4.3 and 11.2 cm3/kg rock, respectively, both of which are positively correlated to TOC content. Model-fitted methane maximum absolute adsorption capacities (n∞) range from 1.80 to 5.85 cm3/g rock at 60 °C and are found to largely increase with increasing TOC; this suggests that the TOC control on methane adsorption capacity obtained by other authors for other shales with relative low thermal maturity levels may be extended to a very high thermal maturity level, e.g., EqVRo≈4.0%. The methane adsorption capacity of organic matter (OM) in our shale samples, however, is somehow reduced as compared to the Lower Silurian Longmaxi Shale and other shales that display EqVRo values smaller than 3.0%, indicating thermal maturity may adversely affect methane adsorption capacity of shales when it evolves to be EqVRo≈4.0%. Nevertheless, this reduction in adsorption capacity may be compensated by the very enrichment of OM in the Niutitang Shale. In addition, a higher thermal maturity level results in a lower Langmuir pressure value, which makes it more difficult for methane desorption at reservoirs conditions. The temperature dependency of adsorption parameters was also investigated, which could be used to predict the changes of methane adsorption with burial depth under geological conditions beyond experimental temperature and pressure ranges.