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

Abstract

The goal of this paper is to investigate the geological controls on the methane adsorption capacity of Lower Permian transitional black shales. To this end, a variety of methods, including quantifying organic matter richness, vitrinite reflectance, kerogen stable isotope analysis, X-ray diffraction (XRD), low-pressure nitrogen adsorption, and high-pressure methane adsorption were performed on black shale samples collected from the Mouye-1 well in the Southern North China Basin. Among them, high-pressure methane adsorption experiments were performed over a pressure range of 0–20MPa at temperatures of 30°C, 40°C, and 50°C. Then, the effects of organic matter, mineral compositions, pore properties, temperature, and pressure on methane adsorption capacity were analyzed, and a new formula for predicting the gas adsorption capacity and methane adsorption capacity of transitional shales as a function of burial depth in the Southern North China Basin was established.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 4.7085m2/g to 9.1883m2/g and 0.01069cm3/g to 0.01802cm3/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 10nm, 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.