Nearly 400 electric logs, using spontaneous potential and resistivity curves, were analyzed in a study of the Aux Vases Formation in the Buckner-Sesser-Valier fields of Franklin County, Illinois. Subsurface mapping procedures incorporated data from core descriptions, scout tickets, and electric logs in constructing structure maps on marker beds directly above and below the formation, and isopachs of producing and nonproducing sandstones within the formation.

Three lithofacies were recognized in the Aux Vases Formation; in ascending order they are: Facies A, a thin, nonproductive calcareous

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sandstone with a southwest-northeast sand body orientation; Facies B, a low permeability, rippled, 0 to 25 ft (8 m) thick, glauconitic sand also oriented in a southwest-northwest direction, and Facies C, a high-angle (20° to 30°) cross-bedded, permeable, producing sand oriented in a northwest-southeast direction ranging from 0 to 18 ft (5 m) thick.

Facies A sands may represent the remains of a thin progradational beach sequence ending in production of a thin shale. Facies B sands are interpreted as the erosional remnants of a "drowned" barrier-bar system with successive parallel bars, each 1 to 3 mi (2 to 5 km) wide, up to 20 mi (32 km) long and oriented southwest-northeast. The bars are incised by tidal channel-tidal delta deposits, approximately 0.25 mi (0.4 km) wide and 1 to 4 mi (1.6 to 6 km) long, oriented northwest-southeast. Between the barriers are sandy shales, shales, and limestones, representing back-barrier deposits, with very thin (< 5 ft, 1.5 m) facies B sands. No evidence of permeable facies C sands are recorded between bars. The sequence is analogous to modern barrier bar-tidal inlet sequences proposed by Kuma and Sanders for the Fire Island area of New York.

Oil producing tidal channel-tidal delta sands show evidence of migration to the north based on (1) a wedge-shaped isopach thickening to the north across the width of the channel, (2) a slight bowing to the northeast in reaction to longshore currents, and (3) isolated fossil-hash lime bodies paralleling the long direction of the channels to the north, representing fill in deposits of the last position of the channel that was inundated by a transgressive sea.

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