Effect of thermal events on maturation and methane adsorption of Silurian black shales (Checa, Spain)

The present work is devoted to the study of the relationship among composition, properties and methane adsorption of selected Silurian black shales from the Bádenas Formation, located near Checa town (Spain). These outcropping samples were collected in a formation, which is not considered as target for shale gas exploration. The clay minerals are predominant in the whole-rock samples and mostly composed of illite (both potassium and ammonium phases), rectorite and pyrophyllite with subordinated kaolinite and chlorite. This peculiar mineralogical assemblage is related with burial and thermal events having affected the shales since their deposition. Differences in TOC (1.92-7.62 wt%), BET specific surface (27-34 m2/g) and thermal maturity (Tmax of 459-484 °C) were observed prior to experimental adsorption. Excess adsorption isotherms for methane were measured on dry bulk samples at 30 °C, 50 °C and 80 °C, up to 3.5 (MPa) by the use of a combined manometric-calorimetric device. The maximum methane excess adsorption capacities within this pressure range varied from 0.021 to 0.168 mmol/g at 30 °C. TOC seems to be the primary factor affecting methane adsorption capacity but nonlinear regression of adsorption capacity with TOC content was observed. A modified Langmuir-type adsorption function, with a term taking into account the volume of the adsorbed phase, provide a good representation of the measured excess adsorption isotherms. Additionally, the heats of adsorption were found nearly constant, with an approximate value of 30 kJ·mol− 1, indicating that physical adsorption process controls the adsorption of methane molecule to the shale samples.