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Regional potentiometric maps of the Lower Cretaceous Viking Formation in central Alberta indicate a low-pressure area or "sink" centered on the Joffre-Bentley-Gilby trend of oil fields, with formation waters in central Alberta flowing into the trend. According to integrated geologic and pressure studies, the "sink" actually consists of 6 separate, nearly static, pressure systems controlled by the environmental facies and subsequent structural deformation.
The Viking reservoirs are interpreted to be lenticular sandstone bodies deposited as en echelon offshore bars with a NW-SE trend. Postdepositional uplift and subsequent erosion have exposed the Viking sandstones at their lateral extremities; in places, these extremities are covered by a thin veneer of permeable glacial deposits. Five of the 6 pressure systems in the Viking appear to be controlled by the distribution of these bars and by the elevations of outcrops. The sixth system is characterized by pressures 1,200 psi below hydrostatic pressure
and by as much as 1,900 psi below the outcrop-controlled systems. This system is interpreted to be a sandstone lens completely enclosed in shale; its pressures were developed by geo-osmosis, as indicated by facies relations, shale analyses, and salinity maps.
The osmotic cell in this system consists of the low-salinity Viking sandstone and the deeper, high-salinity, Mannville sandstone and shale; the semipermeable membrane of the system is the intervening Joli Fou shale. Because the Viking sandstone system is isolated from the outcrop by shale and shaly rocks of very low permeability, the osmotic process produced a marked pressure anomaly.
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