Loss of paraffin wax constituents from crude oil has been attributed by some investigators to microbiologic activity in the reservoir, but until now no significant evidence supporting this hypothesis has been reported. Gas chromatographic and mass spectral data now indicate that a highly selective loss of n-paraffins has occurred in some oils. A classic example of selective n-paraffin removal is found in the Cretaceous Muddy reservoir at Bell Creek field, Montana. Occurring within the same reservoir system are (1) oil showing no loss of n-paraffins, (2) oil with all n-paraffins missing up to about n-C₁₇, but unaffected above n-C₁₇, and (3) oil with the entire range of n-paraffins missing. A microbiologic process seems to be the only logical explanati n for such high selectivity. Surface recharge waters apparently transport aerobic microorganisms along with the necessary oxygen to accessible reservoirs. Other evidence for microbial activity at Bell Creek includes increase in nitrogen content and optical rotation disproportionate to n-paraffin loss, suggestive of addition of microbially produced materials to an altered oil. Optical rotation, carbon isotope, and mass spectral data obtained on distillation fractions of Muddy oil from Whitetail field, near Bell Creek, provide direct evidence that components are being added to the oil by microbial activity. In East and West Hackberry fields, Louisiana, there is evidence that some branched as well as straight-chain hydrocarbons have been microbially destroyed. Examples of microbial alterati n have been found in most oil-producing areas of the world.

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