Geology, diagenesis and paleofluid flow in the Devonian Southesk-Cairn carbonate complex in west-central Alberta, Canada

One of the largest geological features in the deep part of the Alberta Basin in western Canada is the Middle to Upper Devonian Southesk-Cairn carbonate complex (SCCC). The SCCC lies at depths of about 5000 m adjacent to the disturbed belt of the Rocky Mountains and is rising to depths of 2000 m in a northeasterly direction over a distance of 150 km.; The complex consists of four stacked carbonate platforms with associated reefs, which are intervened by marls and evaporites. The carbonates adjacent to the deformation front build a contiguous aquifer because the intervening aquitards are missing. Also, a carbonate-rich unit exists in the Wild River Basin, which has been named “Berland Carbonate”.; The SCCC contains several gas pools, which have up to about 30% H ₂S. This study focused on the diagenesis and concurrent pore fluid flow in the SCCC. The complex diagenetic sequence could be grouped into 5 stages: (1) synsedimentary to shallow burial, (2) intermediate burial, (3) deep burial, (4) maximum burial, and (5) present depth. Each of these stages influenced the quality of the reservoir rocks.; Stage4 is of special interest because the strontium isotopes of late diagenetic calcite cements suggest the influence of basin-external fluids. The 87Sr/86Sr-isotope ratios of these calcite cements are high directly adjacent to the disturbed belt and they decrease with increasing distance towards the foreland basin. This radiogenic strontium signal can be traced for about 100 km into the foreland basin. Circumstantial evidence suggests that the radiogenic strontium was carried by fluids that were expelled during the Laramide orogeny from the Rocky Mountain Main Ranges and/or the underlying basement. Geochemical data indicate that the pore fluids evolved from Devonian seawater that was subsequently changed by evaporation, gypsum dehydration, influx of metamorphic waters, and thermochemical sulfate reduction.; Fluid inclusion data and stable isotope data indicate distinct temperature ranges for early diagenetic, medium diagenetic, and late diagenetic mineral phases. The temperatures may have been as high as 220°C at the maximum burial during the Late Cretaceous/Early Tertiary.; Fluid inclusion salinities and chemistry data from drill stem tests indicate a low-salinity brine in the deepest part of the area, which is migrating updip, displacing a more saline brine. This migration pattern suggests a flow direction from the SW to the NE, from the peak of the Laramide orogeny to the present.