Illitizing fluids as precursors of hydrocarbon migration along transfer and boundary faults of the Jeanne d’Arc Basin offshore Newfoundland, Canada

Illite content in mixed-layer illite/smectite (I/S) and the degree of ordering in the fine fraction (<0.1 μm) of argillaceous rocks invariably increase with burial depth and temperature in 22 deep exploration wells in the Jeanne d’Arc Basin, offshore Newfoundland, Canada. The illitization of I/S mixed-layer clays, however, displays significant anomalies with respect to organic-matter maturation in the Trans-Basinal Fault (TBF) area, which is one of the major transfer zones of the Late Triassic–Early Cretaceous rift basin. Illitization is particularly rapid in the 30–70% illite range causing step-shaped %I/depth profiles. In comparison, illitization trends in the Southern and Northern Jeanne d’Arc Basin show a more or less linear increase in illite content with depth lacking the stepwise illite profiles. In the anomalous TBF zone and in the boundary fault zone on the western flank of the Outer Ridge Complex (ORC), the transition from random to (R1)-ordered I/S is advanced compared to the main parts of the basin as it occurs at lower vitrinite reflectance values. This indicates that factors other than temperature-controlled I/S evolution, possibly the chemistry of the pore fluids. Advection of deep K+-bearing fluids along faults could have accelerated I/S diagenesis. The concentration of all major oil discoveries in the TBF zone and ORC suggests that the upward migration of hydrocarbons to the reservoirs located up to 2.5 km above the source rocks may have followed the same pathways as the illitizing fluids. The regional pattern of I/S diagenesis thus supports the concept of fault-assisted fluid flow for hydrocarbon migration.