Characterization of methane adsorption on shale of a complex tectonic area in Northeast Guizhou, China: Experimental results and geological significance

Abstract

Complex tectonic settings are generally considered to be unfavorable for shale gas accumulation, but a favorable desorption-gas content has been detected in Lower Cambrian Lujiaping Shales in Dabashan fold-thrust belt in northeastern Chongqing, Southern China, which suggests a good exploration potential. High-pressure (~30 MPa) and high-temperature (~120 °C) (HP-HT) methane adsorption isotherms were conducted on seven samples collected at 2665–2707 m below the ground surface from the Lower Cambrian Niutitang Shale (equivalent to the Lujiaping Shale) from northeast Guizhou Province, Southern China to assess methane adsorption capacity and the influence of total organic carbon (TOC) content, thermal maturity, mineralogy, and temperature-pressure conditions on methane adsorption in over-mature shales in complex tectonic areas. The samples contain high TOC contents, ranging from 5.1% to 8.4%, and equivalent vitrinite reflectance Ro values ranging from 3.62% to 3.85%. Over the experimental pressure range up to 30 MPa, Langmuir volumes correlate positively with TOC contents, which may be related to the increase in specific surface area and micropores (<2 nm) volume with an increasing TOC content. Organic matter (OM)-hosted micropores and nanometer-sized intergranular pores in clay-OM nanocomposites mostly contribute to the adsorption capacity of this over-mature Niutitang Shale. In addition, the different behaviors of other minerals (quartz, carbonate and illite) under intensive tectonic compression also affect the adsorption capacity of shales. Due to the low porosity of the Niutitang Shale, the free gas content in these samples is relatively low, and the adsorbed gas always makes up the majority of gas-in-place (GIP), hence the “equivalent point” of adsorbed gas and free gas does not exist at depths of <6 km. This suggests that the shallow (<3 km) Niutitang Shale should be primarily targeted for adsorbed gas under low-temperature and high-pressure geological conditions.