Pyrolysis results of shales from the South Cambay basin, India: Implications for gas generation potential

Abstract

Shale, an abundant organic-rich sedimentary rock of extremely low porosity, is of lately being realized as a significant energy source, owing to the possibility of huge amount of natural gas which may be stored in it. Instigated by the enormous production of natural gas from the shale formations of Barnett, Marcellus and several other plays in USA, the Indian sedimentary basins are being looked assertively for their shale gas prospects. The petroliferous Cambay basin in western India with interbedded carbonaceous shales in its thick Tertiary sequence forms a potential prospect for the shale gas. Fine grained, clastic and organic-rich Cambay, Tarapur and other Tertiary shales have sourced the oil and gas for the basin. The quality, quantity and type of organic matter play an important role in the generation of gas in shale horizons. Rock-Eval pyrolysis is one of the most basic organic geochemical methods to study these parameters. In the present study, the interbedded shale formations within the middle Eocene lignite sequences, referred to as Cambay Formation, (Nagori et al., 2013), of the Tadkeshwar and Rajpardi mines in Surat and Bharuch districts, respectively, have been sampled to study the organic matter properties using Rock-Eval pyrolysis. The sedimentary sequences exposed in the mines show the shales to be high in Total Organic Carbon (TOC) content, ranging between 0.2% to 47.3%. The S1 (free hydrocarbons) and S2 (hydrocarbons from cracking of kerogen) values range between 0.04 to 7.12 and 0.08 to 190.11 mg HC/g rock, respectively. The T<inf>max</inf> (temperature at highest yield of S2) varies between 342°C to 450°C, and the hydrogen index (HI) ranges between 32 to 754 mg HC/ g TOC. The variation of HI vs. T<inf>max</inf> suggests an immature to mature stage for the hydrocarbons. The organic matter in shales is characterized by Type II / III kerogen, suitable for the generation of gas.