The 1972 discovery of oil in the Salem Limestone (Valmeyeran Series) in Wayne County, Illinois, stimulated a resurgence of Salem and deeper exploration that continues to dominate Illinois basin activity.

Lower parts of the porous pay intervals in the Salem reservoirs are cross-bedded, oolitic grainstones and packstones, grading upward into highly bioturbated, mixed oolitic-skeletal grainstones. Hardground surfaces and fenestral vugs filled by anhydrite and sparry calcite are common in the uppermost parts of the pay zones. The reservoirs are capped by fine-grained, dolomitic, and argillaceous peloidal packstones and wackestones. Although most Salem reservoirs discovered occur at or near the crests of plunging anticlines, there commonly is no apparent structural closure over the pool, and the updip entrapment is entirely stratigraphic owing to thinning of the porous oolitic facies and thickening of the overlying packstone-wackestone facies. These variations in thicknesses probably refle t relict topography across oolite shoals.

Porosity and permeability in the Salem are closely related to depositional facies. Most Salem porosity is primary--depositional interparticle and intraparticle spaces, subsequently reduced in volume by pressure solution and cementation. Cementation of Salem grainstones was strongly influenced by the availability of suitable particle surfaces for nucleation of cement crystals. Sparry calcite cement is common on clean crystalline substrates (fossil fragments, especially monocrystalline echinoderms) and is rare on microcrystalline substrates (micritized fossils, peloids, oolites). Highest porosity and permeability occur in rocks with high percentages of oolitic coatings and micritized grains--most notably the oolitic grainstone facies.

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