CO2 Storage Potential of Coaly Shales of the Barakar Formation in the Rajmahal Basin, India

Abstract

Coaly shale layers are frequently found in most basins around the world, but less is known about their CO2 sequestration and shale gas potential. To fill this knowledge gap, the mineralogy, geochemical, and pore structure properties of 12 coaly shale samples collected from the Barakar Formation within the Rajmahal Basin in India were examined by XRD analysis, programmed pyrolysis, and low-pressure nitrogen (N2) and carbon dioxide (CO2) adsorption, respectively. The results showed that the samples are organic rich, with 3.05 to 33.92 wt % total organic carbon (TOC). Moreover, Rock-Eval pyrolysis revealed the abundance of gas-prone kerogen type III in these coaly shales, with temperature of maximum pyrolysis yield (Tmax) values between 422 and 446 °C, indicative of an immature to early maturity stage. N2 adsorption experiments showed that samples’ BET-specific surface areas (BET-SSA) vary from 8.47 to 32.4 m2/g, and pores are generally between 20 and 23 nm. However, the CO2 adsorption measurements suggested that the samples’ DFT surface area is 23.064 to 108.3 m2/g. The D-A pore diameter was found to range from 1.31 to 1.71 nm, and the DFT micro-PSD indicated substantial peaks for pores smaller than 0.6 nm in all samples. The adsorbed CO2 volume also ranged between 2.06 and 10.30 cm3/g. According to the fractal analysis, a higher pore structure complexity of the samples compared to their pore surface roughness was observed. Overall, although the investigated coaly shales had favorable geochemical and petrophysical characteristics as shale gas resources, the thermal maturity of the organic matter suggests low volumes of generated gas. On the contrary, the ideal microporous characteristics, including the high micropore surface areas and the existence of pores below 1 nm, as well as large CO2 adsorption quantities that originate from high amounts of organic matter content and their coaly nature, could recommend their great potential for storing CO2. Accordingly, further and supplementary investigations in the region are recommended to gain more information regarding the CO2 storage capacity in the Barakar Formation.