LAUSR

Abstract The present study investigates the molecular ratio and stable carbon isotopic compositions of gas samples from the Lower to Middle Eocene lignite deposits of the Lower to Middle Tharad Formation in the Sanchor Sub-Basin, Gujarat, India, which were understudied, hitherto. The stable carbon isotopic composition of methane (-69.30 to -57.20 ‰ in BK-5 and from -67.40 to -64.50 ‰ in BK-7 well) may imply the generation of primary microbial gas through the hydrogenotrophic methanogenesis, which was mixed with the early mature thermogenic gas, and this is a novel finding from this study area. The relation between the gas molecular ratio [methane/(ethane + propane)] and the stable carbon isotopic composition of methane confirms the mixing trend (∼90 to ∼40 %) of the primary microbial methane with the early mature thermogenic gas sourced from hydrogen-rich macerals either in situ or at a higher depth that subsequently migrated upwards. Further, the studied gas samples comprise a large amount of carbon dioxide (16.95 to 32.32 vol.% and from 18.91 to 28.60 vol.% in BK-5 and BK-7 wells, respectively). The relation between the carbon dioxide content and its stable carbon isotopic composition (-21.30 to -11.80 ‰ and from -11.20 to -9.40 ‰ in BK-5 and BK-7 wells, respectively) may suggest the origin of carbon dioxide from volcanic degassing from the Deccan Trap basaltic magma as well as from coalification through decarboxylation and decarbonylation reactions. So, the mixed origins of carbon dioxide (inorganic + organic) and methane (primary microbial and early mature thermogenic) would have influenced the carbon isotopic difference between carbon dioxide and methane as well as the isotopic discrimination factor, which is also reported for the first time from this investigation. Finally, the relation between the gas molecular ratio and the carbon isotopic difference between carbon dioxide and methane compliments the gas mixing effects. Hence, the novelty of this investigation lies in elucidating the influences of the early mature thermogenic gas on the gas molecular and stable carbon isotopic signatures of primary microbial gas originated through the hydrogenotrophic pathway as depicted by the gas molecular ratio and stable carbon isotopic compositions of methane and carbon dioxide.