Abstract

The Pembina Oil Field, discovered 30 years ago, is Canada’s largest. Its original proven recoverable reserves amount to 1.81 billion barrels (0.29 billion m³) from 66 separate pools. The Cardium sandstone is the principal producing reservoir and contributes 1.56 billion barrels (0.25 billion m³) with the remainder from deeper pools.

The geological significance of this giant discovery made it unique amongst the some 200 other giant oil fields discovered to date. The extent of Pembina’s Cardium reservoirs encompasses an area in excess of 900 square miles (2,331 square km) making it second in areal extent to the leading supergiant Ghwar Field in Saudi Arabia. The stratified Cardium oil producing sands and conglomerates are completely encapsulated reservoirs whose bounding surfaces are unrelated to any major unconformities. The field is neither structurally complicated by compressional, extensional or geostatic tectonics. No gas cap or active water drive is present and there has been no biodegradation of the crude by meteoric water invasion.

The up-dip limit of the field is defined by the “shale-out” or termination of the reservoir sands. However, the basinward limit appears related to a subtle structural hinge which evolved in response to synorogenic loading of the foreland basin. A marked increase in sediment loading with attendant burial diagenesis has resulted in the loss of Cardium primary porosity. These processes appear to have post-dated the migration of petroleum into the Cardium trap. This secondary or diagenetic seal parallels the structural hinge.

The Cardium reservoirs consist of thin discrete sands and conglomerates characterized by relatively poor porosities and permeabilities and separated by shale beds. The overpressured gas solution drive mechanism suggests isostatic rebounding as a result of post-Eocene denudation. The paleoburial compaction pressures expelled most of the original formation water from the reservoir to a level of irreducibility. Pembina’s Cardium Field is nature’s example of a replete oil trap.