The Siluro-Devonian Play along the Southern Flank of the Sangamon Arch, Central Illinois: Recent Discoveries, Reservoir Facies, and Future Potential
One of the hottest petroleum plays in the Illinois Basin is the "Siluro-Devonian" in the basinward margin of the Mt. Auburn trend along the southern flank of the Sangamon Arch, central Illinois. The play includes multiple geologically related prospects that lie on the regional dip that gently slope to the southeast. It consists of laterally discontinuous reservoirs within the unconformity bounded Middle Silurian and Middle Devonian strata at an average depth of 2000 (610 m) feet. The Mt. Auburn trend is currently experiencing a third phase of development following the drilling booms of late 1940's-mid 1950s and late 1960s-early 1980s. A number of prolific wells have been completed in Macon and Christian Counties in 2011 and 2012, respectively. Some of these wells have produced over 75,000 barrels of oil in less than 9 months; in this part of the Illinois Basin, average primary oil production during the entire life of a well is less than 30,000 barrels.
The Middle Devonian reservoir is a porous dolomite, sandy dolomite or dolomitic limestone with well-rounded quartz grains. It may contain thin quartz sandstone lenses and occurs at or near the Middle-Upper Devonian unconformable boundary in the strata that wedge out against the southern flank of the Sangamon Arch. The Middle Silurian reservoir is a porous vugular, fine to coarsely crystalline dolomite that exists near, and parallels, the Middle Devonian isopach contour line. Both Devonian and Silurian reservoirs are commonly characterized by porous dolomitized lime mudstone to grainstone facies. They contain partially dolomitized bioclasts or molds of crinoids, brachiopods, and corals, suggesting deposition in a shallow marine environment. Reservoirs are lenticular bodies up to 10 feet thick and are generally interlayered with laterally extensive, impermeable limestone beds displaying cyclic reservoir- non-reservoir packages within highstand systems tracts. Potential for new discoveries in the Silurian appears to be excellent in the unexplored areas between the existing fields and in the deeper horizons considering that there are many oil fields with few or no tests in the deeper zones. The Devonian traps, on the other hand, appear to be concentrated in the southeastern margin of the Mt. Auburn trend oil fields where the Devonian strata pinch out against the arch. Production rates are high and yields per well are large in both Silurian and Devonian reservoirs making them excellent exploration targets.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013