Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration
- Arizona State Univ., Tempe, AZ (United States)
- Louisiana State Univ., Baton Rouge, LA (United States)
- Univ. of Calgary, Alberta (Canada)
- Argonne National Lab., IL (United States)
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 {delta}{sup 15}N in kerogen, bitumen, oil, formation water, and fixed-NH{sub 4} extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because {sup 14}N is released preferentially to {sup 15}N from organic molecules during thermal maturation. Released {sup 14}N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in {sup 15}N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH{sub 4}{sup +} 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 {delta}{sup 15}N depends on the timing of authigenic mineral formation relative to fluid migration. In deep reservoir sandstones (>100{degrees}C), the {delta}{sup 15}N of crude oil averages +5.2 {+-} 0.4%, similar to the {delta}{sup 15}N of bitumen in the proposed source rocks. Formation waters are {sup 14}N-enriched with an average {delta}{sup 15}N of 2.2 {+-} 2.6%. Fixed-NH{sub 4} {delta}{sup 15}N values like between that of the oil and water. Productive and nonproductive sandstones have distinctly low average {delta}{sup 15}N values (-1.2 {+-} 0.8%), yet crude oil (+11.1 {+-} 0.3%) and water (3.8 {+-} 0.1%) have been {sup 15}N-enriched by {approximately}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 {open_quotes}N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.
- DOE Contract Number:
- FG02-91ER14218
- OSTI ID:
- 56954
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 59, Issue 4; Other Information: PBD: Feb 1995
- Country of Publication:
- United States
- Language:
- English
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