Geological controls on methane adsorption capacity of Lower Permian transitional black shales in the Southern North China Basin, Central China: Experimental results and geological implications

Experimental results indicate that the total organic carbon content (TOC) of shales in the present study ranges from 0.49% to 2.37% with an average value of 1.14%. The thermal maturity of these samples, as reflected by their vitrinite reflectance (Ro) values, demonstrates that all involved shale samples have extremely high thermal maturities with an average value of 3.23%. The levels of kerogen stable carbon isotopes (δ13CPDB) range from −25.00‰ to −24.10‰, thereby indicating the presence of primarily type III organic matter within these shale samples. Mineral compositions mainly comprise clay minerals and quartz with average values of 57.8% and 37.6%, respectively. Within clay mineral populations, illite, kaolinite, I-S mixed layer clays, and chlorite are the dominant clay minerals. BET (Brunauer-Emmett-Teller) surface area and total pore volumes of the studied shale samples range from 4.7085 m2/g to 9.1883 m2/g and 0.01069 cm3/g to 0.01802 cm3/g, respectively. Under the measured pressure range, methane adsorption capacity correlates positively with TOC content, likely due to the increase in surface area and total pore volume with increasing TOC content. Nanometer-scale pores ranging from 2 to 10 nm, as revealed by low-pressure nitrogen adsorption, are the main contributors to surface area and total pore volume within these shales. Although the TOC-normalized surface area increases with thermal maturity, a negative correlation between thermal maturity and methane adsorption capacity was observed in the present study, most likely due to the decrease in TOC during thermal maturation. A weak positive quadratic correlation was also observed between clay mineral contents and methane adsorption capacity, with the Langmuir volume initially decreasing before subsequently increasing with increasing total clay content, thereby indicating that clay minerals have a weaker effect than TOC on methane adsorption capacity. Geologically applying the methane adsorption capacity of shales as a function of burial depth, which was established based on the Langmuir model, indicates that the methane adsorption capacity of transitional shales initially increases before ultimately decreasing as the depth increases due to the joint effects of temperature and pressure.