WBPC19 Abstract #9:
The Torquay Formation in Saskatchewan is subdivided into six units, Unit 1 to Unit 6 in ascending order, which can be correlated across much of the Williston Basin. The subdivisions defined by examination of geophysical well logs and cores provided the stratigraphic framework for mapping all units in this study.
In Saskatchewan, the current Torquay oil wells are concentrated in two sweet spots: Torquay and North Portal areas, along the US border; and the Ryerson area, near the Saskatchewan–Manitoba border.
In the area along the US border, a total of 450 oil wells have produced about 3 x 106 m3 (18.6 mmbbl) of oil up to the end of 2018, from depths of 2200 to 2400 m. Of these, 405 are fracked horizontal wells. The reservoir rocks are dominantly silty dolostone, with porosities ranging from 4 to 8% and very low permeabilities, ranging from 0.01 to 1 mD. Here, the Big Valley Formation is absent, placing the Torquay tight reservoirs directly beneath and in contact with Lower Bakken shale that, in this area, are mature, based on reported Tmax and vitrinite reflectance (%Ro) values up to 435oC and 0.68, respectively. Oil generated from the Lower Bakken source rock has most likely migrated vertically down from the shales into the top (Unit 6) of the Torquay Formation.
In the area near the Manitoba border, a total of 270 oil wells produce from the Torquay, of which 130 are fracked horizontal wells producing from depths of 1000 to 1200 metres. These wells have produced a total of 1.4 x 106 m3 (8.7 mmbbl) of oil up to the end of 2018. Absent from this area are most of the Middle Bakken, all of the Lower Bakken shale, the Big Valley Formation, and units 5 and 6 of the Torquay Formation so that the Unit 4 of the Torquay are directly overlain by the upper Middle Bakken. Oil is produced from the dolomitic siltstone to very fine sandstone interbedded with dolostone. The reservoirs have good porosities, mostly from 10 to 18%, and low permeabilities, from 0.1 to 10 mD. Because the Bakken shale is not mature in this area, lateral and up-dip oil migration from the south was necessary to accumulate oil at the Torquay subcrop, where long-term exposure improved porosity. The subcrop was sealed by the Upper Bakken shale in up-dipping direction to form the effective trap.