Compositional Differences Among Oils of a Single Field -- Source-, Maturity-, or Migration-Influenced?: ABSTRACT

Abstract

The Vermilion 39 field, offshore Louisiana (USA), contains oils and condensates that show significant compositional variability with depth and stratigraphic location. Pristane/phytane ratios vary from less than 2.0 to greater than 4.5; sulfur/nitrogen ratios differ by more than an order of magnitude; the range of {delta}{sup 13}C{sub oil} values is greater than 1.0 %. Although these data may suggest source rock and/or thermal maturity differences, the carbon number distribution of 5{alpha}(H), l4{alpha}(H), l7.(H)-C{sub o-2} cholestanes, and the 2OS/(20S+20R)-5{alpha}(H), 14{alpha}(H), 17{alpha}(H)-24- ethylcholestane ratios, am similar for all of the oils/condensates. Detailed examination of the n-alkane carbon number distribution indicates that the `front- end bias` of this distribution (i.e., n-C{sub 15}/n-C{sub 15-30}) correlates with many other geochemical parameters (e.g., {delta}{sup 13} C{sub oil}, pristane/phytane ratio, etc.). Furthermore, the extent of this front-end loading correlates with depth: the shallowest reservoirs contain the most front-end loaded oils/condensates. We propose that the compositional differences among the Vermilion 39 field oils/condensates occur from the process of migration-fractionation, and are largely unrelated to differences in source rock(s), or to differences in the source`s thermal maturity level at the time the oil/condensate left the source. Geological and geochemical (elemental, isotopic and molecular) results will be presented to supportmore » this proposal.« less