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Geothermal Reservoir Characterization of the South Swan Hills Reef Complex, Swan Hills, Alberta

Abstract

The Swan Hills Formation in central Alberta is composed of two units, a regionally extensive carbonate platform which provided the appropriate bathymetry for a patchy, overlying second unit - which comprises a series of atoll reef complexes. The South Swan Hills reef is one such complex and has historically been an extremely productive oil reservoir. Active wells in the field also annually produce over 200,000 m3 of hot ≥100° Celsius water. This heat energy of the water has potential for conversion to electrical power. Maximizing production of hot water from the aquifer is the impetus to characterize zones of high porosity and permeability.

Several controls affect the porosity and permeability of the reef: the lateral and vertical distribution of lithofacies, sedimentation stacking patterns and diagenetic cement cycles. Using core data to assemble E-W and N-S sections across the reef, key lithofacies are identified then linked to a depositional model to distinguish the lithofacies with high porosity and permeability. The highlighted environments are reef flats composed of mixed - stromatoporoid grainstones and reef margins represented by tabular stromatoporoid boundstones. Using a “rim-bounded reef” depositional model, a sequence stratigraphic approach is taken to incorporate trends of reef sedimentation and recognize sequences, mapping progradation and backstepping of the reef margin with respect to sea level. Three sequences are recognized within the platform and reef marked by exposure surfaces. Within sequences, autocyclic “megacycles” control reef intra-sequence sedimentation bound by flooding surfaces.

Especially within the reef interior, these megacycles are commonly divided into three diagenetic facies controlled by the position of a post-depositional paleo-watertable. Sediment below the paleo-watertable exhibits phreatic cementation occluding porosity. Above the paleo-watertable, pendant cements and dissolution occur but porosity is occluded by sustained cementation. At the water-table, phreatic cements occur but porosity is enhanced by strong dissolution.

Incorporating petrophysical data tied to core, A 3D geo-cellular model is created in Petrel in order to map the vertical and horizontal permeable facies. The model will be used to simulate the flow of water through the effective aquifer and lends itself to future work in modelling the complicated rock-water interactions of cool-water brine injection.