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Detailed geochemical analyses of twenty-three crude oils from various reservoirs in South Pass 61 field, offshore Louisiana, indicate that the oils belong to a single family and are derived from the same source. The large variation in bulk and chemical properties (e.g., API gravity, hydrocarbon composition) can be attributed to post-generative processes, most importantly biodegradation and evaporative fractionation. A third alteration phenomenon, which was noted only in a single sample, is gas deasphalting.

Triterpane and sterane biomarker distributions of the oils corroborate their common source. Maturity estimates based on sterane isomerization and alkyl phenanthrene distributions also indicate that the oils were generated at a similar level of maturity. However, the degree of sterane aromatization (% triaromatic sterane parameter), exhibits a wide range, and is not related to reservoir depth or other bulk properties of the oils. Hopane/sterane ratios also vary more than would be expected for a single family of oils. Both parameters are highly correlated with the absolute concentrations of 17α (H), 21β(H)-hopane and 5α(H),14α(H),17α(H)-ethyl cholestane (20R). The process(es) resulting in these biomarker correlations are uncertain.

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