New Findings on Silurian Petroleum Reservoirs Generate Interest in the Sangamon Arch in West-Central Illinois

Geologist Yaghoob Lasemi started working on the reservoir characterization of the Mt. Auburn trend in late 2007, in conjunction with a long-term Illinois State Geological Survey (ISGS) research project on the Silurian through Devonian succession of rock units in the Sangamon Arch and surrounding areas. The Silurian succession of the Mt. Auburn trend in west-central Illinois is composed of hydrocarbon-bearing carbonate rocks. Production is chiefly from dolomitized carbonate reservoirs in the upper part of the Niagaran deposits. Detailed studies using available subsurface data, including well cuttings, cores, and geophysical logs, have revealed that the uppermost Niagaran Racine Formation (equivalent to Moccasin Springs Formation of southern Illinois) is the only petroleum producer in the Mt. Auburn trend. A prominent unconformity subdivides the Racine Formation into two depositional sequences comprising several producing dolomite horizons. The reservoirs include mainly dolomitized skeletal carbonate facies in the upper sequence and coral patch reef/reef-related facies in the lower sequence. These facies were deposited along a southwest-trending, gently sloping ramp margin roughly parallel to the Mt. Auburn trend and graded basinward into muddy open marine carbonates.

Reports of initial petroleum production in the area indicate that the rocks of the lower sequence may have greater hydrocarbon reservoir potential than the upper sequence. However, in much of the Mt. Auburn trend, few wells have tested the lower sequence, and hydrocarbon production to date has been chiefly from the carbonate reservoirs in the uppermost part of the upper sequence of the Racine Formation. Although most of the wells drilled thus far in the Mt. Auburn trend have tested only the upper sequence, this study indicates that the lower Racine sequence also has excellent potential for additional discoveries.

Presentations in workshops and geological conferences, a geological report on the Silurian reservoirs for the U.S. Department of Energy Lower Paleozoic Final Contract Report, and publication in the August 3 issue of the Oil and Gas Journal entitled "Oil Potential Seen in Silurian Reef-related Reservoirs in Illinois' Sangamon Arch" have generated renewed interest for exploration and development in the unexplored areas of the Sangamon Arch and should encourage secondary recovery enhancement of the producing horizons in the area. A number of petroleum geologists and operators from Illinois and other states have contacted or visited the ISGS in the past few weeks for discussions or consultations and have expressed interest in developing prospects for further exploration in the Sangamon Arch area.