Ammonium substitution in illite during maturation of organic matter

Pierre shale samples from a thin stratigraphic zone within the contact aureole of the lampro- phyric Walsen dike record changes due to thermal effects that are not influenced by detrital differences. Analyses of fixed-NH4, mineralogy, and Rock-Eval pyrolysis indicators of organic matter maturity provide new insights on the fixation process. Fixed-NH₄ increases with the quantity of authigenic iUite formed from illite/smectite, but the maximum fixation per unit of illite formed occurs within the "oil window" where thermal breakdown of organic matter is rapid. Extrapolation of these results to the burial diagenetic regime supports the potential use of fixed-NH₄ as an indicator of organic maturity and hydrocarbon migration pathways.