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A basin-wide subsurface and outcrop study of the Aux Vases Sandstone was undertaken to determine the source of the sandstone, the regional extent of producing zones, the nature of its stratigraphic traps, and reservoir characteristics. The Aux Vases marks the transition from the predominantly carbonate deposits of the Valmeyeran to the clastic dominated Chesterian.
The Aux Vases in southwestern Illinois is composed of fine-grained, subangular to rounded sandstone commonly occurring in massive sand bodies 50 to 200 ft (15 to 60 m) thick. Many of these sand bodies are fluid-saturated and porous, but are not petroleum reservoirs. The eastern and central parts of the Illinois basin, where the Aux Vases is usually less than 30 ft (9 m) thick, are the major producing areas.
The overlying Renault Limestone separates the Aux Vases from lower Chesterian sands, forming a cap for many stratigraphic traps in the Aux Vases. The most common type of stratigraphic trap in the Aux Vases occurs in thin, shaly and silty sands overlying the Joppa Member of the Ste. Genevieve Limestone ("Aux Vases Lime"). Previous work has indicated that these are tidally influenced sands deposited on a platform of Ste. Genevieve oolitic limestone or grainstone. These Aux Vases platform sandstones grade laterally into either oolite, grainstone, or silty, shallow-marine shale. The best production comes from "permeability pods," where good porosity (15 to 20%) coincides with permeabilities in excess of 100 md.
Another type of stratigraphic trap occurs in a 4 to 10-ft (1 to 3 m) thick oolitic zone, in the lower part of the Joppa Member, usually separated from the Aux Vases by less than 10 ft (3 m) of dense limestone. When this oolite is loosely cemented, permeabilities are in excess of 200 md, resulting in excellent initial production (sometimes in excess of 1,000 bbl of oil/day).
Study results indicate a western source for the Aux Vases. The thickest accumulations of sand occur in the southwestern part of Illinois and display western cross-bedding. The amount of sand decreases and the amount of limestone increases in the producing part of the basin.
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