Sequence stratigraphy of the Aux Vases Sandstone: A major oil producer in the Illinois basin

The Aux Vases Sandstone (Mississippian:) has contributed between 10 and 25% of all the oil produced in Illinois. The Aux Vases is not only an importan t oil reservoir but is also an important source of groundwater, quarrying s tone, and fluorspar. Using sequence stratigraphy, a more accurate stratigra phic interpretation of this economically important formation can be discern ed and thereby enable more effective exploration for the resources containe d therein. Previous studies have assumed that the underlying Spar Mountain, Karnak, an d Joppa formations interfingered with the Aux Vases, as did the overlying R enault Limestone. This study demonstrates that these formations instead are separated by sequence boundaries; therefore, they are not genetically rela ted to each other. A result of this sequence stratigraphic approach is the identification of incised valleys, paleotopography, and potential new hydro carbon reservoirs in the Spar Mountain and Aux Vases. In eastern Illinois, the Aux Vases is bounded by sequence boundaries with 2 0 ft (6 m) of relief. The Aux Vases oil reservoir facies was deposited as a tidally influenced siliciclastic wedge that prograded over underlying carb onate-rich sediments. The Aux Vases sedimentary succession consists of offs hore sediment overlain by intertidal and supratidal sediments. Low-permeabi lity shales and carbonates typically surround the Aux Vases reservoir sands tone and thereby form numerous bypassed compartments from which additional oil can be recovered. The potential for new significant oil fields within t he Aux Vases is great, as is the potential for undrained reservoir com part ments within existing Aux Vases fields.