Physics
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59..765w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 4, pp.765-779
Physics
9
Scientific paper
The magnitude of isotopic variations between organic and inorganic nitrogen was examined in samples from three stacked hydrocarbon reservoirs in the Fordoche Field (Louisiana Gulf Coast Basin, USA). Measurements were made of 15 N in kerogen, bitumen, oil, formation water, and fixed-NH 4 extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because 14 N is released preferentially to 15 N from organic molecules during thermal maturation. Released 14 N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in 15 N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH 4 + may be fixed in clay interlayers with an isotopic ratio similar to that of the migrating fluids. Results indicate that the influence of organic matter on mineral 15 N depends on the timing of authigenic mineral formation relative to fluid migration. The average 15 N of kerogen (3.2 ± 0.3 ) and fixed-NH 4 from mudstones (3.0 ± 1.4) is similar, while bitumen increases from +3.5 to +5.1 with depth. In deep reservoir sandstones (>100°C), the 15 N of crude oil averages +5.2 ± 0.4 , similar to the 15 N of bitumen in the proposed source rocks. Formation waters are 14 N-enriched with an average 15 N of -2.2 ± 2.6 . Fixed-NH 4 15 N values lie between that of the oil and water. The average 15 N of fixed-NH 4 is 3.0 ± 1.2 in productive sandstones, and 0.2 ± 2.4 innonproductive sandstones. In the shallower reservoir sandstones (<90°C) fixed-NH 4 is apparently not influenced by the presently associated fluids. Productive and nonproductive sandstones have distinctly low average 15 N values (-1.2 ± 0.8 ), yet crude oil (+11.1 ± 0.3 ) and water (+3.8 ± 0.1 ) have been 15 N-enriched by ~6 relative to the deeper reservoirs. This suggests that the present fluids migrated into the reservoir after authigenic illite had formed. Fluids become enriched in 15 N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.
Bakel Allen J.
Ferrell Ray E. Jr.
Hutcheon Ian
Krouse Roy H.
Walsh Maud M.
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