Oxidation of organic matter to carbon dioxide by microorganisms in the sedimentary environment is a process which alters the chemical composition of crude oils by sequential removal of hydrocarbon types. Differences in the chemical composition of Early Cretaceous Windalia crude oil (aromatic-naphthenic) and Late Jurassic crude oil (high wax) from the Barrow Island field of the Northwest Shelf, Western Australia, are consistent with the removal of n-alkanes from the former by microbiologic oxidation. However, the carbon dioxide so formed could combine with metal ions in solution to produce 12C-enriched secondary biogenic carbonate rocks in which the 13C/12C ratio of the parent organic matter would be preserved. Such carbonate rocks would differ markedly in isotopic composition from those normal primary marine carbonate rocks in which the 13C/12C ratio averages zero PDB (Pee Dee Belemnite) and which result from equilibration with atmospheric carbon dioxide. A method for differentiation between the two carbonate rocks is thus provided.

The concentration and isotopic composition of total reactive carbonate material in core and ditch-cutting samples from nine exploration wells in the Barrow subbasin were determined. Extensive sedimentary secondary biogenic carbonate rocks having 13C/12C ratios as low as −27 parts per thousand PDB were found in association with the main oil-producing Windalia zone (Early Cretaceous). The deeper, Late Jurassic high-wax oil-producing beds contained relatively minor amounts of secondary biogenic carbonate material. Carbonate rocks from other than these two oil-producing intervals were entirely or largely of primary marine type.

Evidently normal diagenetic or geothermal maturation processes are not responsible for the high concentration of secondary biogenic carbonate rocks associated with the upper oil-rich zone; otherwise similar or greater contents of such biogenic carbonate material might be expected to accompany the oil found at greater depth.

The extensive occurrence of significant secondary biogenic carbonate material in the Windalia zone and the virtual absence of n-alkanes in that interval suggest that the Windalia oil has been microbiologically modified. Possibly the traces of paraffinic-naphthenic oil which occur in the Windalia zone at the Flag 1 well, which are similar in composition to the Jurassic crude oils, are representative of the bacterially unaltered parent crude.

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