SANGAMON ARCH REEF PLAY: AN IMPORTANT EXPLORATION TARGET IN CENTRAL ILLINOIS

The Sangamon Arch is a broad southwest-trending structure in west-central Illinois which was formed as a result of upward warping during Late Silurian and Early Devonian times. It is located in the northwestern part of the Illinois Basin and underlies an area approximately 17,000 square miles. The Silurian rocks of the Sangamon Arch have been producing hydrocarbons for over 70 years, chiefly from dolomitized carbonate reservoirs located in the upper part of the Silurian succession. To date, there has been no detailed reservoir characterization study on either a local or regional scale of the Silurian (Niagaran) deposits in the Sangamon arch area; there is a general lack of understanding of the geometry, facies distribution, porosity development, petroleum entrapment and their controls.

Preliminary studies along the southern flank of the arch in Macon, Christian and Sangamon Counties, using available core chips, cuttings and wire line logs, have revealed the presence of permeability pinch outs at several horizons. In the lower part of the upper Niagaran succession, a dolomitized coral reef/rudstone facies has been recognized. The reefs are composed mainly of coral skeletons, but their internal structure is poorly preserved as a result of pervasive dolomitization. They occur as patch reefs of limited lateral extent that grade laterally and vertically into an impermeable, bioturbated and bioclastic mudstone to wackestone or to a very finely crystalline, argillaceous dolomite inter-reef facies. The reef or reef rudstone reservoir facies display a shallowing-upward cycle and may occur in 1 to 3 horizons.

In the past, most wells were drilled into the uppermost part of the Niagaran succession. The majority of wells do not test the lower part of the upper Niagaran deposits that include the newly-recognized reefs so the potential of these prolific lower horizons has been mostly overlooked. Examination of a few highly productive wells (with initial production of 1,100 to 3,000 barrels of oil per day) indicates that the highest production is associated with this type of reservoir. As most of the wells drilled thus far have only tested the uppermost non-reef interval, there is an excellent possibility for finding more productive Silurian reefs and associated reservoirs along a vast area of the Sangamon Arch.