Sedimentology, Stratigraphy and Reservoir Characterization of the Middle Jurassic Upper Shaunavon Member in Southwestern Saskatchewan
The Upper Shaunavon Member in southwestern Saskatchewan has once again become an area of a new interest with advances in drilling and completion techniques. The member has been a medium oil producer since the 1950’s and has some of the most prolific oil production within the province. In southwestern Saskatchewan the Upper Shaunavon Member unconformably overlies the Lower Shaunavon Member. Detailed core descriptions and geophysical welllogs identified seven recurring lithofacies that include: 1) very fine to medium-grained peloidal quartz arenite (Facies 1); 2) sandy, bioclastic oolitic grainstone (Facies 2); 3) bioclastic, bioturbated sandstone (Facies 3); 4) well-cemented sandstone (Facies 4) ; 5) calcareous mudstone (Facies 5); 6) mixed sandstone and dolomitic shale (Facies 6); 7) shale, sandstone and coquina interlayers (Facies 7). These facies are grouped into 3 lithofacies associations within the Upper Shaunavon Member. Deposition of the Upper Shaunavon Member primarily occurred within a marginal marine environment under tidal and brackish water influences. Facies Association 1 has tidal indicators such as inclined heterolithic stratification, channel lags and coals. Brackish influences are indicated by low-diversity, diminutive trace fossils as well as synaresis cracks. Facies Association 2 was deposited in inner shelf fully marine wave dominated conditions indicated by Cruziana Ichnofacies. Facies Association 3 formed as the result of a base level drop, resulting in erosion and the creation of incised valleys. The subsequent rise in sea level caused these valleys to be filled by tidal inlet channel sediments. Geologic mapping of stratigraphic units underlying Upper Shaunavon Member reveal they have a significant amount of control on deposition and the distribution of oil. II The best oil production is found within Facies Association 3 tidal inlet channels deposited on structural lows in the study area. Tidal flats deposits are the least productive reservoirs due to a primarily muddy lithology, a relatively thin reservoir and low permeability restricting oil migration. Oil within the Upper Shaunavon Member is trapped both hydrodynamically and stratigraphically by impermeable shales. Previously missed pay zones and reservoirs found west of the main oil field trend offer new potential explorations targets.
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Faculty of Graduate Studies and Research, University of Regina