Goose Lake field is located on a north-northwest-oriented structural nose on the western side of the Williston basin and produces from carbonate island facies in the Mississippian Ratcliffe zone. Island facies are concentrated on the structural nose and include low-energy, fenestral, pisolite-algal lime boundstones (supratidal) and high-energy, evenly laminated, pellet-oolite lime grainstones (beach and storm deposits). These beach grainstones occur preferentially along a narrow zone on the western side of the field, suggesting that major wave and wind approach was from the west.

Development of distinct porosity systems within these rocks and in encasing offshore facies accounts for variations in permeability, oil and water production ratios, and overall cumulative production. Both reservoir facies contain porosity as high as 20%, whereas permeability is highly variable. In wells with high cumulative production (i.e., above 200,000 bbl), permeability is normally 10-20 md. Grainstones are typified by primary intergranular, oomoldic, and microvuggy porosity, whereas algal boundstones have micro-intercrystalline, fenestral, and microvuggy to mesovuggy porosity. Porosity occlusion in these facies is due to a variety of early calcite cements, late-stage anhydrite, and baroque dolomite. The encasing offshore marine facies may contain high micro-intercrystalline poro ity but lack effective permeability and serve as the updip seal. Porosity in these fine-grained facies and supratidal units is probably due to an early freshwater leaching episode that affected islands situated on paleohighs during rising sea level and less arid climatic conditions.

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