Radiolysis as a source of 13C depleted natural gases in the geosphere

Abstract Is natural radioactivity a significant agent of hydrocarbon gas generation from sedimentary organic matter? Laboratory gamma radiation of dead crude oil (no solution gas) yields wet hydrocarbon gases depleted in 13C: δ13C CH4 (-75‰ to -65‰), δ13C C2H6 (-52‰ to -45‰), δ13C C3H8 (-42‰ to -37‰) and δ13C C4H10 (-35‰ to -32‰). Although laboratory irradiation dose rates are orders of magnitude higher than those in geological settings, radiolytic transformations occur at total radiation doses equivalent to those in natural geological settings over many millions of years. Radiolysis generates wet gases with isotopic signatures that might be interpreted as “biogenic” if only the methane carbon isotope ratio is considered. We examine situations where such gases may be quantitatively significant.