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

Joseph A. Curiale, Bruce W. Bromley

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 ^dgr13Coil values is greater than 1.0 ^pmil. Although these data may suggest source rock and/or thermal maturity differences, the carbon number distribution of 5 ^agr(H),14 ^agr(H),17 ^agr(H)-C0-2-cholestanes, and the 20S/(20S+20R)-5^agr(H),14 (H),17^agr(H)-24-ethylcholestane ratios, are 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-C15/n-C15-30) correlates with many other geochemical parameters (e.g., ^dgr13Coil, 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 nd molecular) results will be presented to support this proposal.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France