Permian Carbonaceous Shales From Raniganj Sub-Basin—A Potential Source of Hydrocarbons? Inferences Drawn From Kerogen Characteristics and Kinetics

Abstract

The present study reports a geochemical evaluation of Permian aged Raniganj, Barren Measure and Barakar shales from R1 and R4 boreholes from the NE part of the Raniganj sub-basin. We have investigated organic matter characteristics, stable isotopic composition, hydrocarbon potential and kerogen kinetics to decipher shale gas potential as well as artificial hydrocarbon generation prospect. The shales contain high total organic carbon (TOC) content ~2.5–26.5 wt%, mainly Type III kerogen with minor Type II/III kerogen. The provenance of organic matters has been determined using δ¹³C_org and δ¹⁵N_bulk composition and Total Nitrogen content to be predominantly terrestrial deposited primarily in an oxic to suboxic environment. Kinetic parameters for kerogen degradation reactions namely, activation energies (Ea) distribution, frequency factors are in the range of typical Type III kerogen. A narrow range of peak Ea distribution indicates relative homogeneity of organic matter which is also evident from FTIR spectra showing relative abundance of aromatic compounds. Maceral assemblage reveals substantial presence of Type I/Type II liptinite macerals namely, alginite, sporinite, cutinite along with Type III vitrinite macerals which likely elevated the kerogen type and hydrogen index (HI), and resulted in a narrow Ea distribution. The kerogen transformation ratio and the hydrocarbon generation rate indicate considerably earlier onset of kerogen transformation and quicker completion of hydrocarbon generation for R4 shales suggesting better potential for R4 shales relative to R1 shales. Furthermore, Rock-Eval parameters S2, TOC, Hydrogen Index, T_max summarise that the shales derived predominantly from terrestrial sources contain mainly Type III with some mixed Type II–III kerogen with potential for wet/dry gas generation. In the absence of any post-depositional igneous activity in the NE part of the Raniganj sub-basin, the shales are in an early matured state unlike shales from the NW part.