Ja. Curiale et Bw. Bromley, MIGRATION INDUCED COMPOSITIONAL CHANGES IN OILS AND CONDENSATES OF A SINGLE FIELD, Organic geochemistry, 24(12), 1996, pp. 1097-1113
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 c
auses of compositional differences among them. Compositional variabili
ty in elemental concentration, carbon isotope ratios and molecular dis
tributions suggests that at least two sub-groups occur in the sample s
et. The sub-groups are characterized by differences in n-C-15 content
(relative to n-C-15+), pristane/phytane ratio, delta(13)C, sulfur and
nitrogen concentration, and S/N ratio. Although the separation into th
ese two sub-groups is superficially indicative of distinct sources and
/or variable levels of thermal maturity, interpretation of the data se
t as a whole indicates that source influences, maturity differences, a
nd biodegradation do not control observed variations in composition. R
eliable source (e.g., alpha alpha alpha R-C-27-29 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/conden
sates. The remaining geochemical differences, including various molecu
lar (pristane/phytane), isotopic (delta(13)C(oil)) 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-C-
15 in each sample. The migration-fractionation process in the Vermilio
n 39 field is facilitated by fault-initialed pressure releases within
reservoirs that originally contained full-range crudes. Vertical migra
tion of the released saturated hydrocarbon gases has led to a concentr
ation of lighter components in the most fractionated samples. This geo
chemical 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 pot
ential for misinterpretation of geochemical parameters previously cons
idered to be influenced solely by source influence or thermal maturity
. Copyright (C) 1996 Elsevier Science Ltd.