Location: T10-15, R27W1-6W2
Strata: Triassic / Jurassic / Cretaceous
Year of Study: 1987
A well log (over 600 wells), core, and geophysical evaluation of the Mesozoic succession in the northeastern Williston Basin (Townships 10-15, Ranges 27W1M-6W2M) documents the major controls on existing Mesozoic oil accumulations and points to additional prospective areas. The major stratigraphic units examined are the Watrous, Shaunavon-Gravelbourg, Vanguard, Cantuar and Viking-equivalent (Ashville).
Several stratigraphic unconformities and pinchouts are present within the area. Major unconformities are found upon the Mississippian, Watrous, Shaunavon-Gravelbourg and Vanguard intervals. The limit of preserved Lower Watrous sediment is located south of the Wapella Field and displays a northwest-southeast trend. All the units examined thicken towards the southwest, and several display thinner values over the Mississippian oil pools located in the southern half of the study area.
The Mesozoic succession reflects an overall transgressive phase, beginning with red bed and evaporite deposition (Watrous), and ending with the dominantly open marine shales of the Colorado Group. Marine deposition was interrupted by sea level falls, with the associated formation of linear paleovalleys, immediately preceding Shaunavon-Gravelbourg and Cantuar deposition. Subsequent alluviation and valley filling was triggered by a return to transgressive conditions. The valley-fill sediments of the Shaunavon-Gravelbourg interval are characterized by the development of fluvial deposits capped by lenticular sandstones associated with a marine fauna. These latter deposits represent tidal reworking during transgression of the youngest valley-fill sediments. The thickest Cantuar valley fills are characterized by massive fluvial quartz sandstones juxtaposed against Middle Jurassic escarpments formed of shales and rnudstone. Both Jurassic and Lower Cretaceous valleys were cut by rivers draining the Precambrian Shield located to the northeast. The Cantuar valley fills make up subsidiary drainage networks, which join a master valley system oriented northeast-southwest located farther to the west. Ashville (Viking) Sand deposition also represents coarse clastic sedimentation associated with a drop in sea level. However, unlike the older valley-fill deposits, Ashville sandstones formed predominantly within marine environments.
Sedimentation within the study area was largely influenced by a prominent tectonic hinge line oriented northwest-southeast and located within an approximately central position within the study area. Evidence for the existence of the hinge line is seen in the coincidence of: the limit of preserved Lower Watrous sediments, the northernmost limit of preserved intra-Vanguard stratigraphic markers, and the area south of which thickening of all Mesozoic units occurs. Besides the hinge line, the major structural elements of the study area comprise several northeast-southwest linear antiformal structures trending through the entire study area. These structures are post-Favel (Second White Specks) in age, and are possibly related to Laramide stresses. Other potential influences on structural development, such as Precambrian basement structures located to the south, are also possible.
All significant Mesozoic oil accumulations in the study area are found northeast of the Lower Watrous edge, where significant stratigraphic variations (Jurassic escarpments, lenticular oolitic sandstones) are traversed by regional structural lineaments. The Lower Watrous Formation appears to be the vertical seal for Mississippian oil pooled in the south. The absence of this seal in the Wapella-Red Jacket area allowed Mississippian oil to migrate to the north, enter and migrate along permeability fairways such as porous valley fills or structural lineaments, and get trapped by stratigraphic variations such as up-dip escarpments. Detailed study of Cantuar reservoirs within the Wapella Field displays clear relationships between the distribution of porous sandstone, the presence of oil, the location of a steep escarpment within Jurassic rocks, and structure. The relatively long distance of secondary migration explains why Mesozoic reservoirs contain heavier oil relative to their Paleozoic counterparts.
Several prospective areas remain; the best of these are associated with valley fills traversed by lineaments found north of the major hinge line. Three stratigraphic situations relating to oil entrapment were modeled geophysically for two frequency situations (15 ms and 25 ms), and the results are discussed with respect to exploration practices.For more information contact:
Petrel Robertson Consulting Ltd.
500, 736- 8th Avenue S.W.
Phone: (403) 218-1618
Fax: (403) 262-9135