Coupled fluid flow, heat and mass transport, and erosion in the Alberta basin: implications for the origin of the Athabasca oil sands

Abstract

Regional topography-driven flow systems related to Laramide tectonic rebound were simulated using two-dimensional, coupled fluid-flow, heat transport, and solute transport numerical models to replicate present formation water salinity and temperature distributions and investigate the accumulation of the Athabasca oil sands. Previous modelling of this system was replicated, and it predicted repeated replacement of all basin formation water with freshwater during deposition of the oil sands due to high permeabilities. To match present Alberta basin temperature and salinity distributions, model hydrostratigraphy, permeabilities, and heat fluxes were adjusted. This revised model conducts fluids along the Mannville aquifer, rather than the Upper Devonian aquifer, and replicates present salinity distributions, assuming instantaneous uplift around 60 Ma. Fluid fluxes in principal aquifers decrease by two-orders of magnitude using new permeabilities, resulting in primarily conductive heat transport. Thus, genesis of the Athabasca oil sands cannot be explained by dissolved-phase petroleum transport due to low simulated fluxes. Model simulations representing constant erosion of a higher topographic gradient produce similar flow patterns, but fluid fluxes, temperatures and hydraulic heads uniformly decrease over 58 million years. Increased erosion rates in the last stage of simulations produce sub-hydrostatic pressures near the uplift, which trigger a flow reversal in the basin. Thinning of the capping Cretaceous aquitard and Mannville permeability distribution causes discharge in the vicinity of the Peace River, coincident with Peace River oil sands and solonetzic soil zones. Regional topography-driven flow gradually decays via diminishing fluid fluxes, underpressuring near the disturbed belt, and development of local flow sub-systems driven by small-scale relief.