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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Contrary to the findings of other investigations in other petroleum
provinces, sulfur isotopic changes occurred in the Wind River basin during
the nonbiological (thermal) maturation of petroleum. The d
S34
and d C13 of Pennsylvanian-Permian
crude oils of the Wind River basin of Wyoming change abruptly from approximately
-5.0 to +5.0 and from approximately -28.7 to -27.7 per mil, respectively,
in the high-sulfur, lower API gravity, shallow crudes to the low-sulfur,
higher API gravity crude oils below present depths of 10,000 ft. Conversely,
the isotopic compositions of the low-sulfur, higher API gravity Cretaceous
oils do not seem to change with depth.
The d S34, d
C13, and percent sulfur of the Paleozoic crudes are distinguishable
from those of the Cretaceous crudes. These parameters also are compatible
with the hypothesis that crude oils in Triassic and Jurassic strata are
migratory Cretaceous or Paleozoic crudes rather than mixtures of the two.
Only petroleums that have not undergone significant sulfur isotopic
alteration during maturation should be used to calculate the sulfur isotopic
difference between seawater sulfate and petroleum. When this is done, the
difference is about 20 per mil, rather than the 15 per mil suggested by
other authors.
Hydrogen sulfide associated with the Paleozoic oils has a sulfur isotopic
range of 15 per mil. The sulfur and carbon isotopic evidence suggests that
this hydrogen sulfide has a dual origin: (1) nonbiological thermal desulfurization
of crude oils below some threshold depth which exceeds 9,000 ft, and (2)
bacterial reduction of sulfate in shallow formation waters.
Instead of a concomitant enrichment in S32 as the associated
petroleums become enriched in S34, hydrogen sulfide also becomes
enriched in S34 with depth. This surprising phenomenon seems
to imply that a considerable amount of S32-enriched hydrogen
sulfide has been lost or isotopically modified. Although some of the present
S34-enriched hydrogen sulfide appears to have been converted
recently to formation water sulfate, no evidence remains that S32-enriched
hydrogen sulfide was converted to formation water sulfate. Some of the
apparently missing S32-enriched hydrogen sulfide may have been
partly responsible for the precipitation of pyrite in the neighboring Gas
Hills uranium district.
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