Upper Devonian Duperow Sedimentary Rocks in Southeastern Saskatchewan--Why No Oil Yet?: ABSTRACT

Abstract

Although oil shows are common in the 500 to 700 ft thick Duperow Formation (Frasnian) of southernmost Saskatchewan, no commercially viable accumulations have been found. However, a few miles south of the Canadian border producing fields are present in Montana. The sedimentary rocks comprise more than 20 carbonate/evaporite cycles with potentially good reservoir rocks--sucrosic dolomites, oolites, and vuggy carbonates. The inhibiting factors to hydrocarbon accumulation are: (1) extensive anhydritization plugging pore spaces; (2) halite plugging vuggy carbonates in the upper Duperow; (3) many vertical fractures, now plugged by anhydrite, but good potential escape routes for fluids in the past; (4) absence of oolite developments more than 2 ft thick. These factors explain the lack of success to date. However, the Duperow still is considered a good prospect because: (1) good porosity and oil staining are common in two zones near the middle of the Duperow section; (2) only 180 wells have penetrated the Duperow over an area of 15,000 sq mi--35% of these have terminated above the porous zones, sometimes on seismic highs where the prospect was obviously the overlying Birdbear Formation, a known producer; (3) vuggy carbonate rocks may not always be halite plugged in an area southwest of a major halite body; (4) isopach maps show that extensive solution of Middle Devonian Prairie Evaporite salts took place in Duperow deposition causing locally anomalous accumulations of Duperow sediments. Structure contour maps indicate northwest and nor heast trends of the anomalies. These trends are strongly emphasized in a computer-plotted third-degree residual trend surface analysis map. A major northeast lineament lines up with the Duperow fields in north Montana. Conjugate sets of weakness in the Precambrian basement may have caused these trends, and their intersections are the most likely prospects for Duperow oil. End_of_Article - Last_Page 907------------