Diagenesis and paleofluid flow in the Devonian Southesk-Cairn carbonate complex in Alberta, Canada

Abstract

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 5000m in the vicinity of the disturbed belt of the Rocky Mountains and is rising to depths of 2000m in a northeasterly direction over a distance of 150km.The complex contains several natural gas pools, which contain up to about 30% H2S. The major focus of this study is to construct a detailed diagenetic history and concurrent pore fluid flow over time. The Southesk-Cairn complex reveals a complicated diagenetic history that consists of five phases of pore water evolution: (1) synsedimentary to shallow burial, (2) intermediate burial, (3) deep burial, (4) maximum burial, and (5) present depth. Each of these phases influenced the character and the quality of the reservoir rocks.Phase 4 is of special interest because the strontium isotope data of late diagenetic calcite cements suggest the influence of basin-external fluids. The 87Sr/86Sr-isotope ratios of these calcite cements are high (up to 0.7320) directly adjacent to the disturbed belt and they decrease with increasing distance towards the foreland basin (0.7100). The radiogenic strontium signal can be traced for about 100km 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.