Pierre shale samples from a thin stratigraphic zone within the contact aureole of the lamprophyric Waisen 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 illite 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.
