The Upper Shaunovan Formation, Instow Unit, SW Saskatchewan: An
example of geological importance in reservoir modeling
Mark Urban
Department of
Geology
University of
Regina
The Mid-Jurassic Upper Shaunavon Formation in SW SK consists of interbedded siltstones, sandstones, limestones and shales. The high variation in rock type, as well as in lithofacies, suggests a wide range in depositional settings that was mainly controlled by fluctuations in sea level over a broad shelf. At the Instow Unit, core descriptions and cross sections show the Upper Shaunavon has been partially incised by a fluvial channel, creating a valley that was later filled by sands and limestones of an estuarine environment. Channel incision was controlled by a relative sea level fall, whereas the location of channel incision was controlled by Prairie Evaporite dissolution and possibly by tectonics as well. The estuarine deposits provide the main reservoir, and have produced more than 58 million bbls of oil since Instow¡¯s discovery in the early 1950¡¯s.
The Instow Unit is a 1- to
16-meter thick stratigraphically-controlled reservoir. It is comprised of a
complicated mixing of sandstone and limestone lithofacies, as controlled by 5th(?)
order relative sea level changes. Each lithofacies, 3 sandstones and 2
limestone/coquinas, has unique reservoir characteristics, which were mapped to
provide the geological framework by which the Instow reservoir can be modeled.
Previous modeling has implemented arbitrary divisions (2-meter slices) to
predict oil recovery, but is not accurate in describing the overall reservoir
heterogeneity. With the new geological information, prediction of 1) oil
recovery and 2) movement of a future-planned tertiary flood, becomes more
predictable.