Source rock properties and kerogen decomposition kinetics of Eocene shales from petroliferous Barmer basin, western Rajasthan, India

Abstract

Hydrocarbon exploration and production are going on in Barmer basin (Rajasthan, India) for more than a decade. The potential source rocks are of Paleocene – Eocene age, and Mesozoic siltstones form the reservoirs. The western and central portions of the Rajasthan basin are characterised by extensive lignite formations, which can be promising for artificial transformation to oil and gas. We study the source rock properties, depositional environments and hydrocarbon generation potential of the Paleogene lignitic shales of the Giral lignite mine and the Cretaceous Sarnu siltstones for their source and reservoir rock potentiality. The total organic carbon content (TOC) of the Giral samples range between 0.76 and 49.83 wt% and the thermal maturity, as reflected by the pyrolysis Tmax, lies between 412 and 468 °C. Sarnoo siltstones, on the other hand, have a very low TOC ranging from 0.02 to 0.08 wt% and a Tmax of 320–608 °C. The higher TOC and a lower oxygen index (OI) of Giral lignites and shaly lignites indicate the prevalence of a reducing depositional environment. Bulk organic geochemical parameters involving kerogen pyrolysis and thermal degradation kinetics indicate a more promising hydrocarbon generation potential in the lignite than shales, which, however exhibits higher thermal maturity of organic matter. Giral lignites as well as shales show dominantly Type-III heterogeneous kerogen, which is sourced from terrestrial organic matter. This is also corroborated by a broader distribution of activation energy derived from the thermal decomposition of the kerogen. The kerogen transformation ratio (KTR) and the hydrocarbon generation rate (HGR) suggest a considerably earlier and quicker kerogen transformation. Samples from the Sarnoo area offer no significant information on the source rock characteristics, due to their lean organic nature. However, lignites and shaly lignites of the Giral mine are identified as excellent candidates for their suitability towards easy conversion into hydrocarbon products through artificial techniques.