Prospects of shale gas exploitation in Lower Gondwana of Raniganj Coalfield (West Bengal), India

Abstract

Geochemical analyses such as proximate, pyrolysis, TOC and FTIR, and other analyses like surface area, pore size, pore volume (using low–pressure N2 physisorption measurements) and SEM were performed on the shale samples derived from Early Permian Barakar and Late Permian Barren Measures formations of the Raniganj Coalfield, West Bengal. Rock–Eval pyrolysis and TOC data indicated that the heterogeneity of Barren Measures and Barakar shales is laterally varying, but in general, factors which support the occurrence of shale gas accumulations include a moderate to high TOC content (3.38–7.87 wt.%) with sufficient thermal maturity and type III–IV organic matters (kerogens). FTIR spectra indicate the presence of quartz and kaolinite with absorbance bandwidth between 1200–800 cm−1 and 3750–3400 cm−1, respectively. Abundance of quartz, as compared to clay, points towards the brittle characteristics of shales favourable for good fracability. Besides, mesopores and macropores are well–developed and the capacity of gas generation and adsorption are significant. On the basis of SEM, the pores are classified into four types– (i) inter–granular pores, (ii) dissolve pores, (iii) composite inter–granular pores, and (iv) hair line micro–fractures. The BET multipoint surface area varies from 8.104 to 16.937 m2/g and 17.376 to 29.675 m2/g for Barakar and Barren Measures shales, respectively. Size of the pores varies from 3.072 to 3.728 nm for the Barakar shales and 2.984 to 3.521 nm for the Barren Measures shales, as measured by BJH method. Overall, it is observed that mesopores, macropores, micro–fractures and micropores are adequate in the samples and the studied shales are having interconnected networks of natural cracks and pores system, which may control the storage and migration of shale gas in the reservoir.