Multi-scale pore characterization of Barakar shale in the Mand-Raigarh Basin, India: scientific upshots from geochemical approaches and imaging techniques

Multi-scale pores, spatial distributions, and their connectivity have been investigated in shale deposits of the Barakar Formation in the Mand-Raigarh Basin, India, through imaging skills, i.e., x-ray tomography (μ-CT), scanning electron microscopy (SEM) images, and sample photographs. Additionally, geochemical analyses, ash yield, total organic carbon (TOC), X-ray diffraction (XRD), low-pressure nitrogen gas adsorption (N2), and porosity analyses were executed to account for the organo-inorganic controls on the development of pores in shale. Shale lithology, X-ray tomography, and SEM images spell out various types of multi-scale pores categorized as grain edge-pores, fracture-pores, interlayered, inter-particle, and inter-crystalline pores predominantly controlled by fissile nature, stress conditions, and organo-inorganic mineral compositions. The results of geochemical analyses, viz, ash yield, TOC, and mean vitrinite reflectance (RO%) ranging from 69.33 to 89.26 wt%, 1.88 to 6.99 wt%, and 0.64 to 0.96%, respectively, assign fair to good source rock budding for hydrocarbon generation. XRD analysis displays clay mineral richness (30.50 to 71.10 wt%) comprised mainly of kaolinite and illite with non-clays (26.30 to 60.50 wt%) influencing pore shapes and spatial distributions owing to the alteration and fissile nature. The pore sizes through Barrett-Joyner-Halenda (BJH) and density functional theory (DFT) range 2.98 to 4.19 nm and 0.79 to 12.79 nm and points at mesoscale pores standing out in fine-grained shale deposits carrying an ample space for gas storage. The cylindrical pores and bottle-neck pores were distinguished through the vicissitudes in the hysteresis curves. The clay minerals, non-clay minerals, and the total organic carbon content participate significantly in the development of various pores in shale formations.