Paleozoic Hydrocarbon Reservoirs In Saskatchewan And Their Relationship to Basement Lineaments

Abstract

Abstract In the northern Williston Basin, 94 per cent of the oil produced from Paleozoic rocks comes from stratigraphic traps formed in Mississippian strata at the pre-Jurassic unconformity. A comparison of initial recoverable reserves and the volume of sediment in the Williston Basin and in the Illinois and Michigan basins suggests that about 800 million barrels of oil remain to be found in Paleozoic rocks of the northern Williston Basin. About 500 million barrels of that total are anticipated to be present in pre- Mississippian strata. In the pre-Mississippian rocks, the Silurian reservoirs are probably analogous to those of the Mississippian; that is, they are likely to be stratigraphic traps at the pre-Devonian unconformity. On the other hand, the best reservoir prospects in Ordovician and Devonian rocks probably will be associated with flexures and multi-stage salt solution features, respectively both of which may be underlain by basement structures. To the present time, most pre-Mississippian oil in the northern Williston Basin has been found in a roughly triangular area bounded in part by the Nesson anticline to the east and the Cedar Creek anticline to the southwest. Both these structures are thought to be related to zones of weakness in the Precambrian basement rocks. The distribution of the oil-bearing flexures and multi-stage salt solution structures in the area between the two zones of weakness, and the general subparallelism of the zones of weakness and the flexures and solution features, suggests that the latter overlie subsidiary structures formed in the basement rocks in response to regional shear couples exerted on the major zones of weakness. If this conclusion is valid, then it is reasonable to assume that many more flexures and solution features remain to be found in southeastern Saskatchewan. Exploration for the flexures and solution features can be assisted by detailed seismic information and by the preparation of a series of isopach maps of various pre Mississippian stratigraphic units. The isopach maps are useful inasmuch as the structures controlling the formation of the flexures and solution features appear to have been periodically rejuvenated. Thus, the flexures may be identified by anomalously thin Lower Paleozoic rocks, and the multi-stage salt solution features by anomalously thick strata younger in age than the Middle Devonian Prairie Evaporite. INTRODUCTION FRIOR to the discovery of oil in 1947 in Upper Devonian rocks of the Leduc area of Alberta only a limited number of exploratory wells had been drilled for oil and gas in the Province of Saskatchewan. The majority of the wells drilled in the pre-1947 period were shallow tests that generally penetrated Cenozoic and Mesozoic strata. However, in the period after 1947 the number of wells penetrating Paleozoic rocks increased sharply, and this resulted in the discovery of oil in Mississippian strata in the Ratcliffe area of southwestern Saskatchewan in 1952. Intensive ex-ploration of the Mississippian since that discovery yielded one or more new- pool discoveries each year up to the end of 1969.