Synthesis of findings from several detailed reservoir characterization studies of the Aux Vases sandstone (Middle Mississippian)in Illinois under a research grant from the USDOE, shows that production problems are caused by clay mineral grain coatings, reservoir compartmentalization, and hydraulic fracturing. Although hydraulic fracturing of Aux Vases reservoirs increased estimated reserves by 45% in 1953 alone, the practice reduced future waterflood potential. In one field we studied, hydraulic fracturing enlarged naturally occurring fractures, causing channelized permeability that led to an ineffective waterflood sweep.

Aux Vases reservoirs typically exhibit low resistivity (2 ohm/m), are highly porous (>20 percent), and permeable (50 md - 1,000 md.), fine-grained and are loosely cemented by clay minerals. Detailed petrographic studies and coreflood experiments have shown that Aux Vases reservoirs are highly susceptible to formation damage. An initial oil saturation model utilizing capillary pressure data and petrophysically derived flow units has been used to improve determination of oil saturation distribution of Aux Vases reservoirs.

Reservoirs are small (less than 600 acres), slightly elongate northeast-southwest sandstone bodies 1/2 mile wide and 1 mile long. These cross-bedded sandstones have been interpreted as off-shore tidal bars. Stacking overlapping and clustering of the tidal bars formed compartments in many Aux Vases reservoirs. In one field, pressure surveys identified a permeability barrier separating two reservoir compartments. Strategic placement of water injection wells for waterflooding improved recovery efficiencies to more than 45 percent. Recognizing compartments and their small scale permeability barriers, and analyzing and mitigating human-induced operation and completion problems, can improve recovery efficiency and provide new production opportunities in this prolific reservoir, particularly for the smaller independents.

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