ABSTRACT: The Isotopic Composition of Nitrogen in Clay Minerals and Associated Crude Oils

WILLIAMS, LYNDA B., Arizona State University, Tempe, AZ, RAY E. FERRELL, JR., Louisiana State University, Baton Rouge, LA, and IAN HUTCHEON and H. ROY KROUSE, University of Calgary, Calgary, Alberta, Canada

Nitrogen is released from kerogen during catagenesis and throughout petroleum migration. It is incorporated as NH4+ in diagenetic illite. The isotopic composition of "fixed-NH4" nitrogen in clay minerals was compared to the N-isotopic composition in associated crude oils to determine if correlations existed and could be used in tracking hydrocarbon migration. Twelve samples were examined from reservoir sandstones, nonproductive sandstones, and mudstones of the Eocene Wilcox Formation, Fordoche field, Louisiana Gulf Coast. The mudstones contain predominantly gas-prone type II/III kerogen, but the source rock is a more deeply buried oil-prone marine shale facies, and crude oils have migrated several kilometers updip. For comparison we examined ten mudstones from the Miocene Monterey For ation, Lost Hills field, San Joaquin basin, California. Crude oils were generated essentially "in situ" from oil-prone type II kerogen.

Fixed-NH4 nitrogen was extracted by Kjeldahl methods, and isotopic compositions were determined by mass spectrometry. Isotopic compositions of fixed-NH4 from nonproducing Wilcox sandstones average -0.3o/oo, reservoir sandstones average about or equal to 2.0o/oo, mudstones average about or equal to 3.0o/oo. In contrast, fixed-NH4 isotopic compositions from producing and nonproducing intervals of the Monterey samples were all about or equal to 6.0o/oo. In both cases, the associated crude oil is at least 3o/oo heavier than the fixed-NH4 nitrogen in the reservoir.

Isotopically light nitrogen is preferentially released from organic compounds and further fractionation may occur upon mineralization. Variations in nitrogen isotopic composition are observed in Wilcox reservoirs where migrated crude oils have interacted with the pore fluids. No isotopic variation is observed in the Monterey Fm where migration is minimal.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)