Peace River Arch: ABSTRACT

Abstract

The Peace River arch is a pre-Devonian structure 250 miles long which was modified by post-Devonian tectonic events. It is situated in the west-central part of the Alberta basin in Western Canada. The structure consists of Precambrian and Cambrian rocks which were tectonically elevated and formed an island in the Devonian seas. During the time of Devonian sedimentation it contributed arkosic quartz sandstones to the surrounding area and provided a platform on which a major barrier reef complex developed. The island was enveloped in sedimentary rock by the close of Devonian time or very soon thereafter. The arched area became slightly negative during Mississippian time and in late Mississippian time subsided rapidly. Structural failure in the crestal part resulted in a horst and graben complex. Downfaulted areas are filled mainly by late Mississippian and younger clastic rocks beneath a pre-Mesozoic unconformity. The configurations of deeply buried rocks were altered by these tectonic movements which depressed the pre-Devonian surface. A very moderate negative condition prevailed in the area of the arch until about Middle Cretaceous time, after which the rate of subsidence was common to most of the Alberta basin. Thick upper Cretaceous and Tertiary beds were deposited. The Laramide orogeny resulted in uplift and differential warping. Large quantities of Cretaceous and Tertiary rock were subsequently removed by erosion. The Alberta basin tilted westward and the area of the arch upwarded in such a manner that the ridge on the pre-Devonian basement nearly achieve its original amplitude. It formed a westward-plunging nose on the basement surface. The present westward dip results in large stratigraphic and structural traps in sandstones and reefal carbonates along the east (updip) side of the arch. On their discovery, major hydrocarbon accumulations were anticipated but to date only small pools have been found despite intensive search. The lack of major accumulations may be due to one of three factors or combinations of them: (1) lack of source rocks surrounding the most important reservoir systems, due to a particular pattern of sedimentation; (2) loss of hydrocarbons up the slope of the island during a lengthy period of non-deposition during Upper Devonian time; (3) lateral and vertical dispersal of hydrocarbons into numerous small pools in many reservoir systems and structural complexes during times of tectonic activity. The failure to find large oil reserves on this great paleotectonic structure has been a costly disappointment to the Canadian oil industry. End_of_Article - Last_Page 381------------