Migration induced compositional changes in oils and condensates of a single field

Thirty-one oils and condensates from the Vermilion 39 field (offshore Louisiana, U.S. Gulf Coast) are examined using techniques of elemental, isotopic and molecular geochemistry, in an effort to determine the causes of compositional differences among them. Compositional variability in elemental concentration, carbon isotope ratios and molecular distributions suggests that at least two sub-groups occur in the sample set. The sub-groups are characterized by differences in n-C15 content (relative to n-C15+), pristane/phytane ratio, δ13C, sulfur and nitrogen concentration, and S/N ratio. Although the separation into these two sub-groups is superficially indicative of distinct sources and/or variable levels of thermal maturity, interpretation of the data set as a whole indicates that source influences, maturity differences, and biodegradation do not control observed variations in composition. Reliable source (e.g., αααR-C27–29 sterane distribution) and maturity (e.g., 20S/20S + 20R) parameters provide clear evidence of a common source and constant maturity level for all 31 oils/condensates. The remaining geochemical differences, including various molecular (pristane/phytane), isotopic (δ13Coil) and elemental (S/N) parameters, correlate with the extent of migration-fractionation that each sample has undergone, as measured by the relative amount of n-C15 in each sample. The migration-fractionation process in the Vermilion 39 field is facilitated by fault-initiated pressure releases within reservoirs that originally contained full-range crudes. Vertical migration of the released saturated hydrocarbon gases has led to a concentration of lighter components in the most fractionated samples. This geochemical interpretation is supported by geologic data, inasmuch as the extent of fractionation is least in the deepest samples and greatest in the shallowest samples. The results presented here indicate the potential for misinterpretation of geochemical parameters previously considered to be influenced solely by source influence or thermal maturity.