Unconformity-Related Exploration Plays Within the Eastern Portion of the Cretaceous Western Interior Seaway—Subtle Tectonic Control of Reservoirs

Per Kent Pedersen
Apache Canada Ltd, Calgary, AB

Several major unconformities have been mapped within shale dominated Cretaceous strata of the eastern portion of the Western Interior Seaway. These unconformities are characterized by an increase in depth of erosion towards the Precambrian Shield to the east and northeast. Identification of these unconformities greatly impact the perception of the eastern basin margin as being dominated by deep water shales that has no more than source rock potential and limited exploration potential.

The unconformity at the base of the Turonian Second White Specks Shale (Greenhorn limestone) is characterized by deep erosion in central Saskatchewan where Second White Specks Shale directly overly the Fish Scales Formation. Erosion has been recognized as far west as southwesternmost Saskatchewan, where reservoir sandstones of the Senate Pool unconformably overly western derived shelf sandstones of the Belle Fourche Formation. Detailed core and well log correlations show the location of the transgressive sandstones of the Senate Pool was controlled by a subtle structural controlled paleohigh to the south onto which the sandstone onlap. Tight calcareous shales and marlstones of the overlying Second White Specks Formation form the vertical seal. The Senate Pool is one of the most prolific Belle Fourche gas pools in Saskatchewan, with initial production exceeding 1.8 mmcf/day/well. As the Greenhorn unconformity cuts deeper towards the northeast and east, similar unconformity related plays are likely present here and therefore not only limited to southwestern Saskatchewan and southeastern Alberta. A younger unconformity between the Carlile and Niobrara formations is poorly understood and might have significant untested exploration potential in the eastern margin of the Interior Seaway.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005