Fluid flow and water/rock interaction during the Early Triassic evolution of the western Canada sedimentary basin as revealed by carbonate diagenesis

Abstract

Diagenetic evolution of sedimentary basins, which is crucial to resource exploration and development, and carbon capture, utilization, and storage (CCUS), is mainly controlled by fluid influx associated with the regional tectonic and hydrogeologic setting of these basins. Therefore, petrographic and isotope geochemical studies integrated with geodynamics of sedimentary basins, fluid flow and transport hydrodynamics are insightful tools to reconstruct the regional history of paleofluid flow and their driving mechanisms. This study investigates carbonate diagenesis in the Lower Triassic Montney Formation of the Western Canadian Sedimentary Basin (WCSB) to constrain the origin of diagenetic fluids, their major flow systems, and their association with the tectonics and hydrogeology of the WCSB. Petrographic analysis shows various generations of early-to late-stage non-ferroan to ferroan calcite (C1–C3) and dolomite (D1-D4) cement. Bulk δ13C and δ18O values of carbonate cements range from −7.1 to +0.4‰ and −9.5 to −2.8‰ (VPDB), respectively, noticeably more depleted than those estimated for Early Triassic seawater. Their 87Sr/86Sr isotope ratio, 0.7108 to 0.7128, is significantly more radiogenic than Early Triassic seawater. These isotopic values confirm the extensive interaction of the Montney Formation pore water with hot basinal brines of Precambrian metasediments and siliciclastic strata, the influx of hydrothermal fluids, and high burial temperature in the basin. The 13C-depleted signature of these cements supports the contribution of organic carbon, from the breakdown and oxidation of organic matter and hydrocarbons via microbial and thermochemical sulfate reduction processes. The results of this study suggest that Montney Formation diagenesis was influenced by evolved Montney pore water interacting with fluids sourced from metasediments of the Cordillera thrust belt and topography-driven fluids from Columbian and Laramide tectonism. This study highlights the importance of an integrated approach to better understand changes in fluid compositions during diagenesis and reconstruction of basin-scale paleofluid flow systems in sedimentary basins.