ABSTRACT

In the Canadian portion of the Williston basin, oil exploration has been based on the concept of the subcrop stratigraphic trap. The truncation of porous Mississippian strata at the Paleozoic angular unconformity, combined with either erosional or Mississippian structure, defines the play. In such a hydrocarbon trap, the updip reservoir seal is created by a zone of porosity destruction due to diagenetic processes associated with the unconformity and the onlap of impermeable red beds of the Jurassic lower Amaranth (Spearfish) Formation onto the unconformity surface.

In 1980, the traditional play concept was challenged at Waskada, Manitoba, with the discovery of significant oil reserves in the top seal. Oil at Waskada field is obtained from 3 stratigraphic intervals in the Mississippian carbonates. Oil migration was not halted at the Paleozoic unconformity, but continued through the unconformity zone until trapped by permeability barriers within the siltstones and fine-grained sandstones of the lower Amaranth Formation.

This discovery has led to a reexamination of the traditional subcrop play and has added a new dimension to exploration in the region. The key to such a subcrop-supracrop play lies in the identification of major paleotectonic structural disturbances in underlying Paleozoic rocks occurring in conjunction with favorable reservoir facies in the overlying top seal. Such traps may presently exist as bypassed pay in other subcrop stratigraphic pools.

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