Origin of sulfate-rich fluids in the Early Triassic Montney Formation, Western Canadian Sedimentary Basin

This study investigates diagenetic and geochemical processes that control regional distribution and formation of sulfate minerals (i.e., anhydrite and barite) in the Early Triassic Montney Formation in the Western Canadian Sedimentary Basin. The generation of H2S in hydrocarbon reservoirs is often associated with the dissolution of sulfate minerals, as a major source of sulfate required for sulfate-reducing reactions. The formation of pervasive late diagenetic anhydrite and barite in the high H2S zone of the Montney Formation is therefore contrary to the normal paragenetic sequence of sour gas reservoirs. Petrographic observations revealed early and late anhydrite and barite cement. The early fine-crystalline anhydrite cement is dominant in northeastern British Columbia (low H2S zone), while the late-stage coarse-crystalline cement and fracture/vug-filling anhydrite are dominant in Alberta (high H2S zone). The bulk isotopic values (delta S-34: + 2.9 to +24.7 parts per thousand V-CDT, delta O-18: -11.2 to +15.7 parts per thousand V-SMOW) suggest that sulfate-rich fluids originated mainly from modified Triassic connate water was the origin of early anhydrite. In contrast, the SIMS isotopic values of late anhydrite (delta S-34: + 18.5 to + 37 parts per thousand V-CDT, delta O-18: +12 to +22 parts per thousand V-SMOW) and barite cement (delta S-34: +23.3 to +39 parts per thousand V-CDT, delta O-18: +13.2 to +18.7 parts per thousand V-SMOW) as well as fracture/vug-filling anhydrite (delta S-34: + 23.5 to +24.7 parts per thousand V-CDT, delta O-18: +13.3 to +14.7 parts per thousand V-SMOW) from Alberta represents a mixed isotopic signature of Triassic connate water and contribution of dissolved sulfate-rich fluids derived from dissolution of Devonian evaporites. The Sr-87/Sr-86 isotope ratios of the fracture/vug-filling anhydrite (0.7092-0.7102) are highly radiogenic suggesting extensive water/rock interactions between sulfate-rich fluids and siliciclastic and basement rocks. The similar isotopic composition of the late anhydrite/barite and fracture/vug-filling anhydrite in western Alberta with Devonian evaporites isotopic signature, and the highly radiogenic Sr-87/Sr-86 ratio further supports sulfate-bearing fluids were mainly originated from underlying Devonian evaporites and migrated upwards through deep-seated faults/fractures to the Montney Formation.