--> Abstract: Controls on Reservoir Quality in Two Aux Vases Sandstone Fields in the Illinois Basin, by B. Seyler, D. S. Beaty, and J. P. Fagan, Jr.; #90987 (1993).

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SEYLER, BEVERLY, D. S. BEATY, and J. P. FAGAN, Jr., Illinois Stat Geological Survey, Champaign, IL

ABSTRACT: Controls on Reservoir Quality in Two Aux Vases Sandstone Fields in the Illinois Basin

Many Aux Vases Sandstone reservoirs in the Illinois basin have been interpreted as shallow marine bars deposited in a tidally dominated mixed siliciclastic-carbonate environment. The Aux Vases represents a regressive sequence in which sandstone bars overlie and are adjacent to carbonate ooid shoals and grainstone banks. Studies of Aux Vases reservoirs have shown depositional styles to be similar across the southern portion of the Illinois basin.

The depositional pattern has caused varying degrees of compartmentalization in Aux Vases reservoirs at Zeigler and Dale Consolidated fields. Reservoirs are typically 10-25 ft thick, 0.5-1.5 mi long and 0.25-0.5 mi wide. Sandstone bars may either coalesce to form a single reservoir, or shingle to form individual compartments within a field. Varying degrees of diagenetic alteration affect reservoir quality within individual bars, leading to further heterogeneity in reservoir architecture. Diagenetic alterations are controlled by localized depositional conditions such as shale drapes and other fine grained tidal deposits that act as barriers to fluid flow. Petrographic evidence suggests that many Aux Vases reservoirs were cemented by calcite that has subsequently been dissolved. In Dale onsolidated, limestone beds, which are proximate to the bars without an intervening shale bed, may have buffered diagenetic reactions within the sandstone leading to preservation of early calcite cement. In Zeigler field, where low-permeability units of shale, siltstone, and fine grained sandstone separate sandstone bars from limestone units, early calcite cement has been preserved only at outer edges of sandstone bars. Because present reservoir quality is significantly influenced by the amount of carbonate remaining in sandstone bars, it is necessary to assess the localized depositional conditions and subsequent diagenetic alteration of each field when designing drilling, completion and recovery treatments and strategies.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.