Petroleum source rocks characterisation and depositional environment of Kimmeridgian-Tithonian Sequences, Jaisalmer Basin, Western Rajasthan, India

Abstract

In hydrocarbon exploration, total organic carbon content (TOC) and Rock-eval pyrolysis are commonly employed geochemical techniques that offer concise insights into kerogen type, effective source rock identification, and thermal maturity. In the current study, the data obtained from Rock-eval pyrolysis has been used for defining the source rock quality, generative potential, kerogen type, maturity of the source sediments, and kerogen kinetics of the Baisakhi-Bhadesar Formation of Kimmeridgian -Tithonian (154.7−145.6 Ma) age. Basinal level HI, TOC, source rock maturity, transformation ratio, and heat flow maps have been generated by integrating the data from pyrolysis and the previously available data from wells drilled in the basin. The Total Organic Carbon (TOC) content of the Kimmeridgian-Tithonian sequence ranges from 0.03 to 12.71% in the studied samples, with an average TOC content of 1.28 %, indicating good source rock quality. The Hydrogen index (HI), in collaboration with T max and VRo data, demonstrates that the Baisakhi - Bhadesar Formation is characterised by type II, mixture of type II/ III and type III kerogen facies and exhibits good source quality and poor to good generative potential in the basin. The studied samples are marginally mature to mature in nature (Tmax 430-450 0 C, VRo % ranges from 0.52% to 0.72%). Maturity analysis of the basin suggests that, during the Late Jurassic, most areas were under the oil window zone, except the Bhakhari Tibba and Miajlar areas. The Transformation ratio overlay for the Kimmeridgian-Tithonian source sequences shows better transformations of the source rock in the area of the Shagarh sub-basin. Kerogen kinetics of the studied Baisakhi-Bhadesar Formation demonstrate that the activation energy ranges between 46-74 Kcal/mol with the significant distribution of activation energy 54 Kcal/mol (42.07 %), representing a strong heterogeneous type of organic matter in the sediments. Based on lithological, paleontological, and electrolog studies, a shallow marine to near-shore environment of deposition with sediment input direction from the southeast has been inferred for Kimmeridgian-Tithonian Sequences. The results of this study quantitatively establish the role of the Kimmeridgian-Tithonian sequence as source rock, ultimately contributing to the generation of hydrocarbons in the basin along with spatial changes in the quality of source sediments in different parts of the basin.