The hydrogen and carbon isotopic composition of methane from natural gases of various origins

Abstract

The deuterium concentrations (δD vs SMOW) of biogenic methanes from world-wide occurrences range from -180 to -280%. and were found to be depleted in deuterium by approx. 160%. compared to the deuterium concentration of their associated waters. Theoretical considerations support this relationship to be the result of bacterial transformation of CO2 to methane and is therefore indicative of the biogenic origin of methane. Thermogenic gases with high C2+ concentrations (wet gases associated with crude oil) have D/H ratios from -260 to -150%. with deuterium contents tending to increase with decreasing wetness. Dry gases which are not associated with petroleum are more enriched in deuterium (-180 to -130%.) and show an increase in deuterium with increasing rank of the source beds as it is similarly known for carbon-13. Many dry gases in young sedimentary basins were found to contain significant amounts of C2+ hydrocarbons. These gases cannot be grouped with either the biogenic or thermogenic gases and their methane is concluded to be of mixed biogenic and thermochemical origin. Using a δ13C δD diagrammatic display of the isotope data of methanes the various genetic groups of natural gases can be defined more clearly. © 1980.