Middle Devonian Winnipegosis Reefs of Tableland Area, Southeast Saskatchewan

Bill Martindale, Neil Orr

In 1986, Home Oil drilled the first of several hydrocarbon-bearing Winnipegosis reefs in the southeastern part of the Elk Point basin in Saskatchewan. The discovery well, 8A-22-2-9W2, initially flowed 37° API oil at 223 m³/day (1,400 bbl/day) from a dolomitized buildup estimated to be 70 m thick. This well was located using conventional two-dimensional and seismic modeling techniques. A pseudo 3-D survey, using the existing grid of 2-D data, was used to locate subsequent wells.

The reef consists of a basal fossiliferous mound with steep cemented flanks and directly overlies the lower Winnipegosis platform. This basal mound is overlain by a series of coalesced peloid and codiacean algal grainstone shoals, also with steep cemented margins. The shoals are capped by a framestone unit of branched corals and stromatoporoids in a "lagoonal" position and by a boundstone unit of blue-green and red algae forming a reef crest.

Dolomitic laminites of the Ratner Member (upper Winnipegosis-lower Prairie Evaporite) overlie the reef flank and occur between reef complexes in basinal areas. These laminites, in turn, are overlain by anhydrites and halites of the Prairie Evaporite Formation.

The reef in the Tableland area is characterized by a fauna of high abundance and low diversity, both indicative of growth in a stressed, probably hypersaline, environment.

Reservoir quality of the Winnipegosis depends both on original depositional facies and subsequent diagenetic events. Although extensively dolomitized, both shallow and deep diagenetic fabrics can be recognized.

Oil source rock correlations prove that the hydrocarbons in the Tableland reef are locally sourced from bituminous mudstones of the Ratner Member.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988