TY - JOUR
T1 - Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration
AU - Williams, Lynda
AU - Ferrell, Ray E.
AU - Hutcheon, Ian
AU - Bakel, Allen J.
AU - Walsh, Maud M.
AU - Krouse, H. Roy
N1 - Funding Information:
Acknowledgments-This work was funded by a grant (DE FG02 9lERl4218) from the U.S. Dept. of Energy, but this support does not constitute an endorsement by DOE of the views expressed in the article. Additional support was provided by the Basin Research Institute (Louisiana State University) and the Dept. of Geology (Arizona State University). LBW thanks John Echols for samples, and James Cooper, Jeff Hanor, and Steve Larter for useful discussions. Isotope methods (ANL) were developed with Ben Holt and Greg Arehart. Sample preparation and analysis was aided by the efforts of Tobias Fischer ( ASU); John Cody and Nenita Lozano (Univ. Calgary ); Wanda LeBlanc and Rick Young (LSU). We appreciate the helpful comments of GCA reviewers Kenneth Peters, Maowen Li, and one annonymous reviewer.
PY - 1995/2
Y1 - 1995/2
N2 - 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 δ 15N in kerogen, bitumen, oil, formation water, and fixed-NH4 extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because 14N is released preferentially to 15N from organic molecules during thermal maturation. Released 14N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in 15N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH4+ 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 δ 15N depends on the timing of authigenic mineral formation relative to fluid migration. The average δ 15N of kerogen (3.2 ± 0.3‰) and fixed-NH4 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 δ 15N of crude oil averages +5.2 ± 0.4‰, similar to the δ 15N of bitumen in the proposed source rocks. Formation waters are 14N-enriched with an average δ 15N of -2.2 ± 2.6‰. Fixed-NH4 δ 15N values lie between that of the oil and water. The average δ 15N of fixed-NH4 is 3.0 ± 1.2‰ in productive sandstones, and 0.2 ± 2.4‰ innonproductive sandstones. In the shallower reservoir sandstones (<90°C) fixed-NH4 is apparently not influenced by the presently associated fluids. Productive and nonproductive sandstones have distinctly low average δ 15N values (-1.2 ± 0.8‰), yet crude oil (+11.1 ± 0.3‰) and water (+3.8 ± 0.1‰) have been 15N-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 15N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.
AB - 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 δ 15N in kerogen, bitumen, oil, formation water, and fixed-NH4 extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because 14N is released preferentially to 15N from organic molecules during thermal maturation. Released 14N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in 15N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH4+ 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 δ 15N depends on the timing of authigenic mineral formation relative to fluid migration. The average δ 15N of kerogen (3.2 ± 0.3‰) and fixed-NH4 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 δ 15N of crude oil averages +5.2 ± 0.4‰, similar to the δ 15N of bitumen in the proposed source rocks. Formation waters are 14N-enriched with an average δ 15N of -2.2 ± 2.6‰. Fixed-NH4 δ 15N values lie between that of the oil and water. The average δ 15N of fixed-NH4 is 3.0 ± 1.2‰ in productive sandstones, and 0.2 ± 2.4‰ innonproductive sandstones. In the shallower reservoir sandstones (<90°C) fixed-NH4 is apparently not influenced by the presently associated fluids. Productive and nonproductive sandstones have distinctly low average δ 15N values (-1.2 ± 0.8‰), yet crude oil (+11.1 ± 0.3‰) and water (+3.8 ± 0.1‰) have been 15N-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 15N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.
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U2 - 10.1016/0016-7037(95)00005-K
DO - 10.1016/0016-7037(95)00005-K
M3 - Article
AN - SCOPUS:58149212536
SN - 0016-7037
VL - 59
SP - 765
EP - 779
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 4
ER -