Source rock characteristics and pore characterization of Indian shale

Indian shale samples, collected from Permian Damodar valley and Late Oligocene Assam basin were collected and studied thoroughly to know their hydrocarbon generation potential and pore characteristics. Rock eval pyrolysis, low-pressure adsorption (N2), and x-ray diffraction analyses were performed on four Permian and three Late Oligocene shales considered as potential targets for shale gas exploration in India. The relationship between the source rock characteristics, mineralogy, BET surface area and pore volume were discussed. The study reveals a good to excellent hydrocarbon potential with high TOC content (4.8-37.3) of type III and type II+III kerogen. Thermal maturity data shows that Permian samples are mature (Tmax 440 °C-465 °C), whereas Late Oligocene samples are immature (Tmax 422 °C-434 °C). Mineralogy reveals a clay rich nature of the shale samples. BET surface area of the samples were between 0.7 and 13.6 m2/g and total pore volume ranged from 0.011 to 0.027 cc/g. Pore size distribution for all samples are found to be unimodal or bimodal (peaks at around pore radius 1.1 nm and at >3 nm). Kerogen type is observed to have a significant contribution on pore volume; type II+III shale samples (HI > 300) have higher average pore radius (6.62 nm) and higher average total pore volume (0.020 cc/g) than that of type III samples (4.2 nm and 0.018 cc/g respectively). Samples of higher surface area and total pore volume show smaller average pore radius. Clay minerals positively influence the BET surface area and total pore volumes of the samples. A negative relationship between quartz and TOC in the shale samples suggest a non-biogenic detrital source of quartz grains.