Abstract

The entire Late Jurassic–Early Cretaceous stratal package between the Fernie Formation and the Cadomin Formation, from Townships 62–74 in the subsurface of northwestern Alberta, is assigned to the Monteith Formation (Minnes Group). Three mappable units are present, informally termed Monteith A, B and C, from youngest to oldest. The Monteith C was deposited in a marginal marine–deltaic depositional setting. Upward, the transition to coastal plain (Monteith B) and fluvial (Monteith A) depositional settings records a continuous progradation of a long-lived depositional system. Mapping of these units is aided by identification of regionally extensive marine flooding surfaces in the Monteith C, which are used as stratigraphic datums.

The thickness of the Monteith Formation decreases from 450 m in the southwestern portion of the study area to a zero-edge in the northeast. The Monteith A, B and C are up to 160 m, 100 m and 200 m thick, respectively. Depositional thinning to the northeast is prevalent, but the primary reason for gross stratal thinning is substantial incision associated with the overlying sub-Cadomin unconformity.

The Monteith Formation in Alberta was deposited in a foredeep setting. Regional thickness trends suggest increased accommodation to the southwest and the strata are interpreted to have accumulated on the cratonward side of the foredeep axis. These observations, as well as evidence for basin axial channel flow to the northwest during deposition of the Monteith C, support an interpretation that the foreland basin had initiated prior to Monteith Formation deposition. Basin physiography, as well as uplift and denudation of the Cordillera, affected the depositional evolution of units in the study area. Ultimately, these factors strongly influenced the distribution of reservoir rocks (primarily sandstones of the Monteith A and C), which are part of the Deep Basin Gas System. The presence or absence of permeability enhancing fractures also impacts reservoir quality and well deliverability in the Monteith Formation.

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