Abstract

The sources of sorbed and free light hydrocarbon gases (methane through propane) in shallow sediment samples from seven locations in the Gulf of Mexico are characterized by the concentration and molecular composition of the gases, in conjunction with their stable carbon and hydrogen isotope ratios. The free gases in the sediment interstices have a bacterial source (δ13C1 = -87 ‰ to -108 ‰; δD1 = ca. -190 ‰), in contrast to the sorbed gases in the sediment which have thermogenic signatures (δ13C1 = -46 ‰ to -37 ‰; δD1 = ca. -210 ‰). Secondary alteration effects, such as bacterial oxidation and desorption fractionation, are observed in the free gases, but not in the sorbed gases. The maturity level of the source rocks, estimated from the isotope ratios of the thermogenic gases, ranges between 0.5 and 1.9% vitrinite reflectance and agree with maturity calculated by the Lopatin/Waples modelling of this Gulf region. The isotopic ratios of the surface gases are also in the range of the carbon isotope values of natural gases measured from this region of the Gulf of Mexico.

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