Abstract

Cores from Rosevear Field and surrounding areas were studied to determine controls on reservoir porosity and hydrocarbon distribution in the Swan Hills Formation (Middle to Late Devonian; Givetian to Frasnian) in west-central Alberta. Most reservoir porosity (3–15%) in the Rosevear area is in dolomites of the middle Swan Hills Formation. Dolomitization was facies-controlled, occurring mainly in peloid-bioclast and Amphipora-peloid grainstones. Porous dolomitized grainstones at Rosevear accumulated along the rims of a trough initiated by structural downwarp during Swan Hills time. The depositional trough is recognized by the distribution of facies as well as a 20 m thickening of the Swan Hills Formation within and on the flanks of the trough. Porous dolomitized grainstones also occur along the middle Swan Hills shelf margin. Lime mud-rich shelf-interior limestones and lime mud-rich slope to basinal limestones are generally not porous. Previously published geochemical and petrographic data suggest that most dolomite precipitated at relatively high temperatures during moderate to deep burial. The geochemical signature and stratigraphic distribution of dolomite may have been produced by (1) pervasive recrystallization of an early facies-controlled dolomite, or (2) dolomitizing fluids moving laterally and preferentially through porous shelf-margin and trough-margin grainstones after substantial burial.

The geometry of the Swan Hills dolomite is critical for the stratigraphic trapping of gas. In Rosevear’s southwestern pool, porous dolomitized grainstones pinch out updip (to the northeast) into nonporous slope to basinal limestones of the trough. In the northeastern pool, porous dolomitized grainstones pinch out updip (to the northeast) into nonporous shelf interior limestones. The gas–water contacts at Rosevear are at elevations that approximate the spill-point of Rosevear dolomites into a porous shelf margin dolomite trend. Many wells south and west of Rosevear have porous dolomite filled with water. The Rosevear dolomite bodies have enough of a north–south orientation to trap gas migrating from the south along the shelf margin.

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