Nitrogen isotopic exchange during maturation of organic matter

Abstract

The definition of kerogen as the insoluble fraction of sedimentary organic matter leads to the assumption that all of its chemical constituents are autochthonous. Evidence is presented that some of the organic N in kerogen derives from allochthonous mobile and chemically reactive N species (e.g., ammonium, HCN and low molecular weight organic N containing compounds) that are delivered by migrating fluids to stationary kerogen. Reactions of humic compounds (e.g., protokerogen) with ammonium in soil and sediment are facilitated biologically. At higher temperatures, reactions between kerogen in rocks and N containing compounds in mobile fluids are driven geochemically. N isotopic exchange between kerogen and NH4+ or NH3 is evident from experiments where 15N enriched ammonium chloride solutions and source rocks containing kerogen types I, II, IIS and III were heated over 5 years at temperatures of 100, 144 and 196°C. The resulting data corroborate earlier results from hydrous pyrolysis experiments (330°C for up to 144h) and prove that ammonium is readily converted abiogenically to organic N in kerogen at temperatures that approach those in naturally maturing sediments. The propensity of different types of kerogen to react with NH4+ or NH3 increases in the order II<I<IIS<III. Ammonium ion mobility in formation fluids and the ability to isotopically interact with kerogen makes them an effective isotopic buffer for sedimentary organic N. Isotopic exchange tends to homogenize the isotopic compositions of participating pools of N and thus limits the isotopic ranges of organic and inorganic N. Recognition of N exchange in thermally matured sediments offers fresh perspectives and opportunities to constrain the extent and pathways of N containing geofluid migration, especially when synoptically characterizing organic and mineral bound N moieties across gradients of thermal maturity.