Post-Mississippian Unconformity in Western Canada Basin: ABSTRACT

Abstract

One of the most important geological features of the Western Canada basin is the major unconformity of post-Mississippian age. Seventy-eight oil and gas fields in Alberta are situated either overlying or underlying this surface. Detailed knowledge of this fossil topography is obviously 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. In the eastern portion of the basin the unconformity is very pronounced; westward it decreases in magnitude and breaks into a series of disconformities and diastems; over the present site of the Rocky Mountains the unconformity probably disappeared entirely, deposition having been continuous throughout late Paleozoic and Mesozoic time. Beneath the uncon ormity beds ranging in age from upper Mississippian to Precambrian subcrop successively from west to east. Outliers, inliers, and other erosional complexities are present along all the truncated edges. The lithologies, thicknesses, and attitudes of the beds overlying the unconformity are to a considerable extent influenced by the configuration of the surface. An over-all 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-oriented 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 described by text writers for streams developing on an emergent coastal plain composed of gently dipping strata. Valley gradients, slopes of the upland surface, and local relief 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 represe ted on the surface. Of the 76 oil and gas fields in Alberta at the unconformity, 59% produce gas, 34% oil, and 7% both. Approximately two-thirds of them 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 been proved dry or are untested to date. All fields underlying the unconformity are on noses or closures. It is not entirely clear whether any true structural anomalies 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 with 38° API for non-unconformity oils. Various observations bear on the problem of time of migration and accumulation of u conformity hydrocarbons. End_of_Article - Last_Page 418------------