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Recognition and subsequent correlation of biodegraded and/or water-washed oils with their unaltered counterparts has been a long-recognized problem in petroleum geochemistry. Because water washing and biodegradation can occur concurrently, it is difficult to delineate the individual effects of these two oil-altering processes, especially on the C15+ hydrocarbon fraction. Both processes can cause an oil to become enriched in sulfur and to have a lower API gravity owing to loss of the more volatile components. Thus, the altered oil may appear to be a high-sulfur crude of low maturity. Altered and unaltered oil fractions extracted from cores of a water-washed carbonate reservoir offshore northwest Palawan were studied to observe the effect of alteration on correla ion parameters.
Water washing resulted in the following changes: (1) depletion of 13C in the C15+ branched-cyclic alkane fraction, whereas virtually no change occurred in isotope composition of the aromatic fraction; (2) selective loss of dibenzothiophenes followed by loss of aromatic hydrocarbons; and (3) virtually no change in steranes or tricyclic and pentacyclic terpanes, with the exception of enhancement of 17^agr(H) norhopane and decrease of 18^agr(H) trisnorhopane. In situations of severe water washing, mild biodegradation also occurred, as indicated by the loss of n-alkanes (and possible selective loss of n-alkanes having less than 25 carbon atoms) relative to branched alkanes, cycloalkanes, and aromatic components.
Because cycloalkanes and aromatic hydrocarbons are not generally preferentially utilized by bacteria as long as n-alkanes are present, the changes observed in these two fractions most likely result from water washing. Because of the potential for extreme compositional changes in the n-alkane and aromatic fractions, the most useful correlation parameters between the altered and unaltered oils are sterane and terpane distributions. Stable carbon isotope ratios are also useful; however, more studies of oils in which water washing is known to have occurred need to be made to explain the general response (or various responses) of the isotopic composition of oils to water washing.
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