--> Abstract: Sequence Stratigraphy and a Mixed Carbonate-Siliciclastic System in Illinois, by H. E. Leetaru; #90937 (1998).

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Abstract: Sequence Stratigraphy and a Mixed Carbonate-Siliciclastic System in Illinois.

LEETARU, HANNES E., Illinois State Geological Survey

The Aux Vases Sandstone (Mississippian Age) is the reservoir for 10 to 25 % of all of the oil produced in the Illinois Basin and it is also an important quarrying stone. The formation is a mixed siliciclastic-carbonate succession that is bounded at its base and top by sequence boundaries.

The lower portion of the Aux Vases (also known as the Joppa Limestone) is an interbedded sandstone, shale, and limestone unit that transgressed over a sequence boundary at the top of the Ste. Genevieve Limestone. The upper Aux Vases Sandstone is a tidally influenced siliciclastic wedge that prograded over the lower Aux Vases during the highstand. The sedimentary succession is typified by offshore cross-bedded sandstone that gradually is covered by intertidal mixed flat and supratidal sediment and is capped by a sequence boundary.

The carbonate sediment occurs as both thin laminae and as thicker beds (10 feet or more) within the sandstone. Carbonate interbeds occur as thin cross-bedded calcarinites which are interpreted to be storm deposits. These thin carbonate beds can vertically compartmentalize the Aux Vases reservoir. Thicker carbonate intervals form lateral seals and are interpreted to be residual remnants of a more laterally continuous carbonate shelf succession that has been scoured during the siliciclastic progradation.

The mixture of the two lithologies is economically important. In southeast Illinois the Aux Vases is an important quarrying stone because the interlaminated carbonate and siliciclastic sediment gives the rock excellent abrasive properties for road construction. In the central part of the basin, the carbonate intervals compartmentalize the reservoir sandstone and create numerous bypassed oil zones from which additional oil can be recovered.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah