Bacterial alteration of subsurface reservoired hydrocarbon accumulations extends into the range of the C 1 -C 4 gaseous hydrocarbon, preferentially attacking the wet gas components. During the initial stages of bacterial gas alteration, propane seems to be selectively attacked, resulting in anomalous propane carbon isotopic compositions as well as unusual carbon isotopic distributions among the wet gas components. Extensive bacterial alteration appears to be capable of removing all but traces of the wet gas components, resulting in a dry gas that could be confused with overmature dry gas whose wet components have undergone extensive thermal cracking. Recognition of microbial alteration in subsurface reservoired hydrocarbon gas accumulation is important for both correlation purposes and maturity estimates.--Modified journal abstract.

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