One of the most important geological features of the Western Canada basin is the major unconformity of post-Mississippian age. Ninety-three oil and gas fields in Alberta are situated either immediately above or immediately below this surface. Detailed knowledge of this fossil topography is important for successful exploration for oil and gas in the basin.
Structurally, the Western Canada basin is a large asymmetrical feature with gentle southwest dips over most of its extent. It is bounded on the east by the Canadian shield and on the west by the thrust belt of the frontal Cordillera. The stratigraphic section consists of a lower, Paleozoic, carbonate portion, and an upper, Mesozoic, clastic portion; the two are separated by the post-Mississippian unconformity, the subject of this paper.
A stream pattern is discernible on the unconformity throughout southern and central Alberta. It consists of three sets of streams, a southwest-flowing consequent set (the master streams), a northwest-southeast set of subsequent streams, and a poorly developed set of third-order, obsequent and resequent, streams aligned northeast-southwest. This pattern is similar to that on an emergent coastal plain composed of gently dipping strata. Valley gradients, slopes of the upland surface, and local relief, all corrected for regional tilt, are very low. These facts, plus the existence of numerous flat upland surfaces and streams with broad upper valleys and steep lower valleys, suggest that at least two cycles of erosion are represented.
Of the 93 oil and gas fields at the unconformity, 59 per cent produce gas, 34 per cent oil, and 7 per cent both. Approximately two-thirds are Cretaceous and one-third Mississippian. Most of the Cretaceous fields lie over at least some minor nose or closure on the unconformity. However, many similar anomalies have proved dry or are untested to date. All fields underlying the unconformity are on noses or closures. It is not entirely clear whether true structural anomalies exist, and/or textural changes in the carbonates occur where the truncated edges are productive. Oils present in reservoirs at the unconformity have an average gravity of 31° API compared to 38° API for non-unconformity oils. Various observations bear on the problem of time of migration and accumulation of unconformity hydrocarbons.
Figures & Tables
The first article in this book is the address that introduced the technical program of the 46th Annual Meeting of the AAPG. The organization and presentation of this symposium volume was developed in an orderly geographic continuity. Modern concepts of structural form and the sequence of tectonic events are carefully reported all along the mountainous western margins of the American continents. The relationship of this structural knowledge to the accumulation of oil and gas is constantly emphasized in the 26 papers contained herein.