Potential for Prolific Reef-Related Reservoirs in the Sangamon Arch, West Central Illinois

Yaghoob Lasemi and Beverly Seyler
Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, IL 61820; [email protected]; [email protected]

Hydrocarbon production along the Sangamon Arch, a broad southwest-trending structure in west-central Illinois, thus far has been chiefly from the non-reef carbonate reservoirs in the uppermost part of the Niagaran (Silurian) succession. To date, there have been no documented reports of any reef reservoirs in the Sangamon Arch area and there is a general lack of understanding of the occurrence, geometry and facies distribution of these potentially important reservoirs.

Preliminary studies along the southern flank of the arch have revealed the presence of a reef interval in the lower part of the Niagaran succession. 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 impermeable, bioturbated and bioclastic mudstone to wackestone or to a very finely crystalline, argillaceous dolomite inter-reef facies. The reefs and associated facies display a shallowing-upward cycle and may occur in 1 to 3 horizons.

The majority of wells in the study area have not tested the lower part of the Niagaran deposits that include the newly-recognized patch reefs, so the potential of these prolific lower horizons has been mostly overlooked. Examination of a few highly productive wells indicates that the highest production is normally associated with this type of reservoir. The results of our study suggest that, there is an excellent possibility for finding more productive Niagaran patch reefs along a vast area of the Sangamon Arch.

Presented AAPG Eastern Section Meeting, Pittsburgh, Pennsylvania 2008 © AAPG Eastern Section