Diagenesis of ammonium during hydrocarbon maturation and migration, Wilcox Group, Louisiana, U.S.A.

As organic matter matures, N is released in the reduced form as NH₄⁺ and substitutes for K⁺ in clay minerals. As oil forms, an abundance of N is generated, coincident with the formation of authigenic illite. This study suggests that NH₄⁺ released from organic matter during maturation migrates through sandstones together with the pore fluids and volatile phases generated from the migrating crude oil. This causes anomalously high NH₄⁺ substitution in authigenic illites. Fixed-NH₄ concentrations in illite are a product of both hydrocarbon maturation and migration. Concentrations of fixed-NH₄ from organic-rich mudstones in two Wilcox Group oil fields show a general increase with depth, coincident with increasing organic maturation. Reservoir sandstones containing crude oils have fixed-NH₄ concentrations as much as two times those of the mudstones, when values are normalized to the quantity of clay minerals. Fixed-NH₄ concentrations in the gaseous portion of the reservoir are three times as high. Two out of five sandstones from a dry hole adjacent to a producing oil field contain anomalously high (>2σ above mean) concentrations of fixed-NH₄ with respect to total organic centent. This, together with a high Production Index and Hydrogen Index in those units, suggests that migrating hydrocarbons may have influenced the fixed-NH₄ concentrations.