LARGE-SCALE, LONG-TERM SOLUTE TRANSPORT IN DEEP SEDIMENTARY BASINS: NUMERICAL SIMULATIONS OF BRINE MIGRATION

The composition and distribution of brines in sedimentary basins can provide valuable clues to geochemical history and paleohydrogeology, but few studies have investigated brines from a hydrodynamic standpoint. Whereas geochemical studies have suggested that ancient evaporatively-concentrated brines may persist at depth for hundreds of millions of years, previous numerical simulations of topography-driven flushing of brines from foreland basins have suggested short residence times for solutes, < 5 My. Preliminary simulations are used here to address this contrast and investigate advective, dispersive, and diffusive controls on the modern distribution of salinity. Simulations are based on the Alberta foreland basin, which extends 500-700 km eastward from the Canadian Cordillera and contains a maximum of 4-5 km of Devonian through Tertiary sediments. Devonian evaporite beds lie within 1 km of the Precambrian crystalline bedrock throughout the eastern half to two-thirds of the basin. Previous hydrodynamic studies suggest that topography-driven flow drives fluids roughly parallel to deep evaporite beds.

The modern distribution of brines in the Alberta basin and similar basins containing topography-driven flow systems and evaporite beds reflects a balance between topography-driven flushing of brines and mixing with deep brines derived from dissolution of evaporites. Thus, at high flow rates, a brine plume derived from dissolution of evaporites should exist marginward (updip) of the evaporite beds and should spread vertically above and below the evaporites because of transverse mixing processes (dispersion). If flow is significantly restricted in the deep part of the basin, as suggested by observations of the Alberta basin, then solutes could reach the shallower topography-driven flow system via diffusive processes as well. The modern distribution of brines in sedimentary basins can be viewed as natural long-term, large-scale tracer tests that can be used to investigate large-scale permeability and dispersivity.