Paleokarst in the Grosmont Formation and reservoir implications, Saleski, Alberta, Canada

We delineated a bitumen-rich paleokarsted carbonate reservoir of the Upper Devonian (Frasnian) Grosmont Formation with a high-resolution 3D seismic survey tied to core and petrophysical log data from 35 wells within a 34.2 km(2) study area in northern Alberta, Canada. There were two laterally continuous karst facies: a solution-enhanced vuggy dolostone that resulted from the carbonate dissolution of body fossils and a stratiform breccia that resulted from the dissolution of interbedded evaporites. Three laterally discontinuous karst facies were identified: sinkhole fills, collapsed paleocaves, and solution valley fills. We measured 368 subcircular features (sinkholes and collapsed paleocaves) having a median circle-equivalent diameter of 69 m and representing 5.5% of the total study area. Sinkhole fills include Cretaceous-aged sandstone, mudstone, and coal. Collapsed paleocaves were filled with matrix-supported breccia that had clasts of disoriented blocks of dolomite and a matrix of disaggregated dolomite and Cretaceous-aged mudstone. The paleocaves and sinkholes formed in the solution- enhanced karst facies of the Grosmont C at the interface of an interpreted ancient vadose-phreatic mixing zone. The marine deepwater deposition of the Clearwater Formation during the Albian filled the depressions created by the mechanical collapse of the paleocaves and provided a seal for thermal operations. The fracture density inferred from seismic amplitude variation with angle and azimuth analysis and corroborated by well data showed that fractures are ubiquitous and were enhanced during meteoric karst. The high-vertical permeability resulting from solution-enhanced fractures, the laterally predictable flow units, and a competent seal make this an ideal reservoir for thermal bitumen recovery.