Date of Award

1-1-2009

Document Type

Campus Access Dissertation

Department

Earth and Ocean Sciences

Sub-Department

Geology

First Advisor

Alicia M. Wilson

Abstract

The origin and residence times of porefluids in the Alberta Basin have been debated for more than 40 years with conflicting conclusions reported by geochemical and hydrogeologic studies. In this study, brine migration and groundwater age were simulated over the last 100 million years, along a 700 Km, east-west cross-section of the basin, using salinity and Cl/Br ratios as geochemical constraints. The 2D finite element FORTRAN code COMPACT was used to simulate variable-density fluid flow, heat transport, solute transport, and sediment compaction in a tectonically evolving basin. COMPACT was modified to add effects of erosion and sediment decompaction, and dissolution of evaporites.

Fluid flow results indicate the development of compaction-driven, topography-driven and buoyancy-driven flows in the basin during the 100 million year history of basin deposition, uplift and erosion. Compaction-driven flow occurred during deposition of the foreland basin (100 Ma to 60 Ma). A regional fluid drain developed from west to east along permeable layers of the Viking and Mannville Formations below the thick Cretaceous shales of the Colorado Group of rocks by the end of maximum burial (60 Ma). Smaller regional fluid drains along permeable carbonate layers in the Paleozoic formations and the Cambrian Sandstone in the deeper parts of the basin were also found at the end of 60 Ma. A strong topography-driven flow system developed as a consequence of the uplift of the Rockies and waned with the onset of erosion. At present, there exists a shallow, topography-driven flow system in the post-Colorado group of rocks and a deeper, buoyancy driven flow system driven by thermo-haline effects in the Paleozoic formations.

Simulation results suggest that porefluids in this basin represent a mixture of four geochemical end-members: seawater, freshwater, brines formed by evaporation of seawater, and, contrary to prior interpretations of Cl/Br ratios, brines derived from halite dissolution. Sensitivity studies revealed that similar distributions of salinity and Cl/Br ratios could be obtained using extremely low permeabilities, a scenario that was ruled out based on field-based permeability data and prior simulations of petroleum migration in the basin. Geochemical data can provide important constraints for regional-scale models, but the presence of evaporites introduces significant uncertainty in using Cl/Br ratios for interpreting origin of brines in sedimentary basins.

Groundwater age and residence times vary across the basin. The maximum residence time of the deepest brines in the western most parts of the Alberta Basin was found to be about 210 million years. Results suggest that although old, residual brines still exist in the deepest parts of the basin, these waters have signatures of at least 14% and up to 26% meteoric flushing. Fresh, young waters exist only in shallow formations.

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