ABSTRACT

Since the discovery of significant quantities of oil in Winnipegosis (Givetian) buildups in southern Saskatchewan in 1986, intense drilling activity has proven that this play extends well beyond the area of the initial discovery at Tableland (Twp. 2, Rge. 9 W2M). This case study describes the sedimentology, diagenetic history, seismic configuration, geochemistry and productivity of four separate Winnipegosis buildups in the Hitchcock area (Twp. 3, Rges. 8 and 9W2M).

The Hitchcock buildups are founded on a regionally extensive Lower Winnipegosis platform. They are totally dolomitized, vary from 57 to 63 m in thickness and average 25 hectares (63 acres) in area. Buildups were located using 2-D seismic but their overall configuration and the well locations were determined by a subsequent 3-D survey.

The lower 40 to 50 m of each buildup consist of steeply dipping (>30° locally), porous peloid and codiacean algal packstones and grainstones. The overlying unit is dominated by chaetetids and is locally bound by "stromatolitic" algae and fenestrate bryozoans. An in-situ framework, 3 to 4 m thick, of red algae, colonial corals, fenestrate bryozoa and stromatolitic algae represents the final phase of organic reef growth. Buildups are capped by planar laminated or highly contorted calcretes. Laminar, nodular and brecciated dolomitic anhydrite (Whitkow Anhydrite) overlies the buildups and acts as a seal to entrapped hydrocarbons.

Buildups were initiated as shoals stabilized by early marine fibrous cements. Emergence and subaerial erosion terminated reef growth and produced a capping calcrete crust and a system of solution pores and cavities. The pores and cavities were subsequently infilled by mudstones of probable vadose origin. Dolomitization occurred by seepage reflux of marine waters during shallow burial.

Euxinic mudstones occur in basinal areas between buildups. They are probably coeval with the buildups and are organic rich and thermally mature. Their geochemical "signature" matches that of Winnipegosis oils in the area.

To January 1, 1991, after 7 to 15 months of production, four wells in the Hitchcock area flowed 40,120 m3 (252,346 bbls) of oil.