The Aux Vases Sandstone, a prime exploration target in the Illinois basin, is a prolific but enigmatic reservoir that has produced nearly a billion bbl of oil. Detailed outcrop and subsurface study shows that much of this production is from diagenetically influenced stratigraphic traps in a complex tidal-sandbar system. The complexity of this system makes identification

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of traps difficult.

The sandbars are composed of fine to medium-grained, cross-bedded, clean, well-sorted quartz arenites. The Aux Vases typically possesses preserved, primary, interparticle porosities of 15-25%, and permeabilities of 40-250 md. Reduction of porosity and permeability in the tidal bars is caused by quartz overgrowths, pressure solution at quartz grain contacts, and authigenic chlorite cement.

The sandbars grade laterally and vertically into interbar and tidal flat sediments composed of fine-grained, bioturbated or ripple-bedded quartz wackes and arenites that typically have 15-20% porosity but are impermeable. The clay matrix of the interbar wacke sediments causes a reduction in permeability, as does the authigenic chlorite and illite cement in the fine-grained arenites associated with the wackes.

Exploration has been hampered because (1) bars of reservoir quality are not easily distinguished from interbar sediments on electric logs; (2) resistivity logs usually show excessively high water saturation; and (3) sandbar trends are difficult to predict. Study of electric logs has shown that a spontaneous potential (SP) of 75 mV or greater is a good indicator of relatively clean, well-sorted, sandbar sediments. An SP less than 75 is an indication of interbar sediments. SEM and x-ray analyses suggest that water adsorbed on mixed layer clays is interpreted on electric logs as free water, accounting for the high water saturation readings. Prediction of sandbar trends is difficult because there are two trend directions: the dominant trend is northwest-southeast, parallel to the shorelin ; however, many bars trend east-west, or roughly normal to shoreline. The two trend directions are commonly juxtaposed, thereby adding to the complexity.

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