Carbonate Sand Shoals in the Middle Mississippian (Valmeyeran) Salem-St. Louis-Ste. Genevieve Limestones, Illinois Basin

Abstract Upper Middle Mississippi an (Valmeyeran) sediments in the Illinois Basin consist of nearly 300 m (1000 ft) of shallow water carbonates. Several intervals of carbonate sand are interbedded with fine grained limestones and dolomites in shoaling upward sequences. A typical carbonate cycle consists of the following facies, from bottom to top: 1) highly bioturbated skeletal wackestones and mudstones, interpreted as open-marine fore-bar deposits; 2) cross-bedded oolitic-skeletal grainstones and mud-lean packstones, interpreted as high energy shoal deposits; 3) moderately bioturbated oolitic-skeletal packstones, commonly with hardgrounds and minor truncation surfaces, interpreted as back-bar deposits; and 4) bioturbated mudstones and peloidal packstones with fenestral vugs, calcispheres, ostracodes, thin shale parting, and anhydrite nodules, interpreted as very restricted lagoonal and tidal flat sediments. Carbonate sand shoals are best developed in the Salem Limestone in the central portion of the Illinois Basin, where three or more thick (15-50 m; 50-150 ft) cycles occur with all of the above facies types developed. Towards the margins of the basin the Salem grainstone cycles thin and grade into fine grained, restricted facies indicating lagoonal and intertidal deposition. The restricted environment eventually prograded across the carbonate shoal area, depositing a thick sequence of cherty lime mudstones and microsucrosic dolomites across the basin, most of which are assigned to the St. Louis Limestone. Few carbonate sands are found within the St. Louis. In latest Valmeyeran time carbonate shoals once again developed across wide areas of the basin. Oolitic sands in the Ste. Genevieve Limestone, commonly called McClosky sands, occur in a variety of complex depositional geometries including irregular banks, linear sand bars, and tidal bar belts. Shoaling upward cycles are not as apparent in the Ste. Genevieve, where most areas are characterized by oolitic bars separated by fine grained interbar skeletal wackestones. Nearly 20% of the oil and gas reserves in the Illinois Basin occur in Valmeyeran carbonate sands, and much of the current exploratory efforts in the basin are directed towards these reservoir sands. Rational exploration and exploitation of the predominantly strati graphic-type traps found in these units requires detailed understanding of the sand body characteristics and their geometry.