Evolution and Evaluation of Coal-bed Methane in Cambay Basin, Western India: Insights from Stable Isotopic and Molecular Composition

Abstract

Owing to a rapid rate of depletion of conventional energy resources and to keep pace with the ever increasing energy demand, scientists all over the world are looking for some alternate source. Coal bed methane (CBM) is one of them. With the discovery of coalbed gas from an oil and gas producing basin- the San Juan basin in Colorado and New Mexico by AMOCO in early 1980’s, the attention of world-wide hydrocarbon industry shifted to evaluation of CBM potential of Tertiary basins. The Tertiary Cambay basin in Gujarat is a known onland hydrocarbon province of India and has been producing oil since 1958. During drilling for petroleum, thick Eocene coal seams are encountered at varied depths in Mehsana area and further north. In 2004, few coalbed methane (CBM) blocks were awarded in northern Cambay basin and contagious Barmer Basin. The present paper entails the results of investigation carried out for CBM in the deep-seated coals of Sanchore and Tharad area in north Cambay basin. The methane content of the desorbed gases varies from 20–68%. The stable carbon isotope study (δ¹³C) shows that the desorbed gas is dominantly biogenic but the compositional analysis of desorbed gas reveals significant fractions of ethane (C₂) and butane (C₃) indicating the gases to be of mixed origin. Further, presence of high CO₂ content is observed which varies from 20–45% of the total volume of desorbed gas. The estimated mean CBM resource of the study area is ∼ 46 billion cubic meters.