Regional-Scale Flow of Formation Waters and Terrestrial Heat in the Alberta Basin, Canada

Stefan Bachu

The heat and fluid transport in a basin may be coupled through buoyancy effects, caused by temperature variations, and through heat advection by flowing formation waters. However, there are cases when the two processes can be partially or totally decoupled when variations in formation water salinity play a greater role than temperature in establishing density differences, and when rock permeability is so small that flow velocity does not affect significantly the conduction of terrestrial heat through a sedimentary succession. The role of heat advection versus heat conduction can be established through either numerical or dimensional analysis, based on the geothermal and hydrodynamic characteristics of the basin or part thereof.

The Alberta basin in western Canada represents such a case where the two flow processes, of terrestrial heat and of formation fluids, can be decoupled. Several regional-scale flow systems, driven in different directions by past tectonic compression, erosional rebound in thick shales, and regional and local-scale topography, are currently active in the basin. The salinity increase with depth offsets the decrease in density due to temperature. On a regional scale, the permeability of aquifer rocks is low, leading to of formation waters velocities of the order of 10^-3 to 10^-2 m/yr. Dimensional analysis (geothermal Peclet number) shows that conduction dominates the terrestrial heat flow, As a result of erosion, the thermal conductivity of the sedimentary succession i creases generally eastward. The basement heat flow, calculated on the basis of rock lithologies, bottom-hole temperatures and thermal conductivity measurements, increases generally northward, corresponding on a basin scale with basement changes from old Archean rocks in the south to younger magmatic arcs in the north. Local-scale anomalies are superimposed over this general trend.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California