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Four alteration processes: biodegradation, evaporative fractionation, migration contamination, and extraction by water (aqueous fractionation), are recognized in a suite of 354 oils and gas-condensates from Tertiary and Quaternary reservoirs in the Outer Continental Shelf area, and are evaluated in conjunction with maturational changes.

Evaporative fractionation is the principal alteration process. Gas, migrating from deep accumulations, is believed to interact with reservoired oils resulting in vaporization, loss of light ends, and gas-condensate formation. Residual oils are characterized by reduced API gravity as well as enhanced levels of naphthenic and aromatic hydrocarbons in the remaining gasoline fraction.

Biodegradation likewise results in oils of low API gravity, light end loss, and enhancement of naphthenes, but is accompanied by depletion in light aromatics. This contrasting feature is utilized in a light hydrocarbon compositional index diagram to facilitate the recognition of fluids altered principally by one process or the other, or by both. Approximately 74 percent of the sample set are fractionated, 64 percent are biodegraded; many have undergone both processes. Merely 5 percent are relatively unaltered.

Migration contamination involves limited quantities of contaminants, but is a widespread phenomenon, particularly in gas-condensates. Contamination in petroleums, it is suggested, is revealed by anomalous pairs of values of related maturity indices. Such pairs are non-concordant with “generation curves” representing the evolution of the parameters in question in source rock bitumens at progressively higher levels of maturity. Supermature gas-condensates (11% of the entire suite) are grossly non-concordant in methylphenanthrene, ethylcholestane, and hopanoid parameters. Approximately 50% of analyzed oils and condensates are interpreted as significantly contaminated by extraneous biomarker compounds, including ααα20R-ethylcholestane and 18α-oleanane. Analogous statements appear to be relevant for oils in numerous worldwide Tertiary deltaic sequences.

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