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Variations in Lithofacies in Upper Ordovician Herald and Yeoman Formations (Red River), North Dakota and Southeastern Saskatchewan
The Upper Ordovician Yeoman and Herald formations in Saskatchewan are equivalent to the Red River Formation in the United States and Manitoba portions of the Williston Basin. The Yeoman Formation generally consists of burrowed mudstones and skeletal wackestones to packstones with a marine fauna. In places, it also contains thin, organic-rich kukersites (kerogenites) that provided source beds for hydrocarbons. Oil and gas generated in these kukersites are common in structural traps and some stratigraphic traps in the southern and western parts of the Williston Basin, but only recently have Yeoman reservoirs become a major exploration target in the northern part of the basin.
The Herald Formation conformably overlies the Yeoman Formation and consists of several depositional cycles. In ascending stratigraphic order, these rocks are represented by the Lake Alma and Coronach members, and the Redvers Unit. The Lake Alma Member consists of laminated dolomudstones overlain by bedded to nodular anhydrite reflecting a increasingly hypersaline shallow subtidal environment. The Coronach Member has burrowed skeletal wackestones at the base, laminated dolomite in the middle, and, in the basin centre, bedded anhydrite at the top. It represents another brining-upward sequence. The Redvers Unit, with a lower limestone capped by laminated dolomite mudstones (and anhydrite in the basin centre), is a third brining-upward sequence.
Facies variations at and near the contact of the Yeoman and Herald formations are focal to this presentation. The nature of hydrocarbon source beds and reservoir facies are also documented. The cores on display are: 1) Total Petroleum #1-25 Alexander (Sec. 25, T129N-R105W; Bowman County, North Dakota) in South Horse Creek Field; 2) Mark Saskoil Minton 3-17-3-21W2 in Minton Field, Saskatchewan; 3) LVR et al Steelman 7-28-4-4W2, Saskatchewan; and 4) Cdn-Dev TW Langbank 15-28-12-2W2, Saskatchewan. The Alexander and Minton cores show oil-bearing dolomitized burrowed wackestone typical of reservoir rocks in the upper part of the Yeoman Formation. The Steelman well penetrated an unusual stromatoporoidal bank capped by thrombolitic (microbial) reefal dolostone in the lower Herald Formation. The Cdn-Dev TW Langbank 15-28-12-2W2 core reveals a distinctive shallow-water facies found towards the basin’s eastern flank near the limit of Lake Alma anhydrite deposition.
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