Devonian sandstone channels at Springfield East Field, Sangamon County, Illinois; predictor of oil productive zones in underlying Silurian rocks

Springfield East oil field in Sangamon County, Illinois is one of approximately two dozen fields that make up the Mt. Auburn (or Reef) trend in central Illinois. The field has produced over 688,000 bbl from 32 wells completed in brown and gray Niagran (Silurian) dolomite at a depth of approximately 1600’. Two wells attribute production to Hibbard (Devonian) Sandstone but, as the production is comingled with Silurian zones, the contribution is unknown. The Devonian New Albany Shale unconformably overlies the Silurian dolomite and Hibbard Sandstone, sealing and sourcing the field. The surface on the top of the Silurian is erosional and irregular. The productive zone in the Silurian is karsted dolomite. Core from the field contains large dissolution vugs and collapse features confirming the nature of the reservoir. The Devonian Hibbard Sandstone is a gray-green-brown fine grained sandstone with a dolomite matrix. Cross sections and isopach maps show that this sandstone occurs in shallow channels cut into the erosional surface of the underlying carbonates. When isopach maps are overlain on well maps 96% of the oil wells in the field are coincident with the occurrence of a Devonian sandstone channel Banks of the channels and areas void of Devonian sand are sharply defined by dry holes. This coincidence is most likely explained by fluids in the channels forming and enhancing porosity and permeability in the rocks below. Mapping these channels in other areas of the Mt. Auburn Trend may prove to be a useful tool in exploring for new production.