Determination of basin-scale fluid flux to understand the processes controlling regional dolomitization in the Western Canada Sedimentary Basin

• Main Author: Stacey, Jack
• Format: Dissertation
• Language: English
• Published: University of Manchester 2021
• Subjects: Aquifers; Carbon Capture and Storage; Dolomitization; Faults; Fluid flow; Hydrocarbons

Dolomitization in the Western Canada Sedimentary Basin (WCSB) has been extensively researched, with previous studies primarily focussing on hydrocarbon reservoirs hosted within differentially dolomitized Middle to Upper Devonian strata, and on Mississippi-Valley-Type lead-zinc deposits within pervasively dolomitized Middle Cambrian strata. This has resulted in a conflicting combination of dolomitization models that developed over multiple decades, which produced a combination of poorly defined concepts regarding dolomitizing fluid sources and migration pathways, and uncertainty regarding the timing of dolomitization. This study aimed to address these uncertainties by re-evaluating existing dolomitization models to establish how the Cambrian and Devonian strata of the WCSB were dolomitized, and to determine the principal controls on porosity and permeability in dolomitized strata to inform future mature field development and repurposing for carbon capture and storage (CCS). The results of this thesis demonstrate that Middle to Upper Devonian strata were dolomitized by the circulation of modified Devonian seawater along faults and basal aquifers during shallow burial (L. Devonian - Mississippian). Saddle dolomite cement (SDC) precipitated during deep burial (L. Cretaceous - Palaeocene) from hydrothermal basinal fluids (residual evapo-concentrated Middle Devonian seawater) that interacted with basal clastic (West Shale Basin and Peace River Arch areas) and carbonate (East Shale Basin) aquifers prior to their emplacement along faults. Results indicate that the Middle Cambrian Cathedral Formation was hydrothermally dolomitized by Cambrian seawater along surface-penetrating faults and basal aquifers, which continued during shallow burial (Middle Cambrian). During burial, an increasing component of serpentinite-derived fluids mixed with latent seawater (convecting along faults to ~6 km depth), facilitating burial dolomitization and SDC precipitation. The high pressure of these fluids, and the presence of overlying sealing shales, resulted in the formation of SD cemented breccias at very shallow depths (< 1 km). Results indicate that the porosity and permeability of Middle to Upper Devonian dolomite is primarily controlled by depositional facies, with the best reservoir found in reefal and heavily bioturbated facies. Dolomitized intervals have significantly higher poroperm than undolomitized intervals. As porosity-occluding calcite and dolomite cements are abundant in fault-related breccias, it is recommended to drill away from faults, as these may also be susceptible to reactivation/induced seismicity. The findings of this project provide new insights into the processes controlling regional dolomitization, which has addressed many of the uncertainties commonly associated with dolomitization models. It has also determined the principal controls on dolomite poroperm in the Devonian reservoirs of the WCSB, which has implications for future hydrocarbon exploration/production and CCS.