Geology & History of the Illinois Basin

The Illinois basin is an asymmetrical spoon shaped structural depression that trends northwest-southeast and is filled with more than 14,000 feet of paleozoic sediment at its deepest point. These sediments thin depositionally and by erosion to 2000-3000 feet on the arches and domes surrounding the basin. The basin is bounded to the north by the Wisconsin arch, to the east by the Cincinnati arch, to the southeast by the Nashville dome, to the southwest by the Ozark dome and to the northwest by the Mississippi River arch (Exhibit A). The layers of sedimentary strata dip gently from these boundaries toward the deepest and thickest part of the basin known as the Fairfield Basin (Exhibit B & Exhibit C).

Since Precambrian times igneous and sedimentary strata have been subjected to repeated uplifts and down-wrappings. These drastophic movements have resulted in widespread folding and faulting of the basin strata.

The Illinois Basin is a shallow intracratonic depression characterized by rock layers that dip gently inward from the crests of surrounding arches. Present-day regional dip rates typically range from 30 to 70 ft./mi. During most of Paleozoic time, sediments accumulated in the slowly subsiding basin. The Paleozoic depositional basin differed from today’s structural basin in that it was open to the south, in the area now occupied by the Pascal Arch. Highly generalized lithologies are shown on the cross section: sandstone in the Mount Simon and mixed terrigenous clastic rocks in the Eau Claire; predominantly carbonates in the interval extending from Knox through the Valmeyeran, interrupted by one major sand and two major shale units; both carbonate and terrigenous clastic rocks in the Chesterian; and mostly sandstones and shales in the Pennsylvanian (please see Exhibit C & Exhibit D).

There are five major structural features that have proven to be the most significant as to the accumulation and production of oil in the Illinois Basin. These are the La Salle Anticline, the Clay City Anticline, the Wabash Valley Fault System, the Salem-Louden Anticline and the Rough Creek Fault System. These major structures (please see Exhibit A & Exhibit B).

The La Salle Anticlinal Belt is the most prominent anticlinal feature in the Illinois Basin. It is actually a complex structure of en echelon folds, asymmetrical anticlines and monoclines. The term "La Salle Anticline" has been loosely used in the petroleum industry to refer to the main oil producing portion of the belt which extends from Coles and Edgar Counties to Lawrence County. Along this trend, many large, multi-pay fields are located on sizable structural closures at the crest of some of the component anticlines. Through 1998, combined cumulative production from the larger fields totaled more than 750,806,000 BO.

The Wabash Valley Fault System in southeastern Illinois, southwestern Indiana, and adjacent corner of Kentucky extends about 60 miles north-northeastward from just north of the Shawneetown and Rough Creek Fault Zones. The fault system consists of subparallel, high-angle normal faults that have vertical displacements as great as 480 ft. The faults bound horsts and grabens, and commonly overlap one another. Major fault plains dip at angles ranging from 50 to 85 degrees. Individual fault blocks are only slightly tilted, and drag is generally absent or weakly expressed.

The Salem-Louden Anticline is more properly recognized as the Salem Anticline and the Louden Anticline, two distinct positive structures separated by a structural saddle. However, because the two structures align to form a single larger, more regionally prominent structure, they are commonly referred to as the Salem-Louden Anticline. Superimposed along the Salem-Louden Anticline are localized domes that form the traps for some prolific multi-reservoir oil fields. Combined cumulative production from only those fields on the anticline is over 850,000 BO through 1996.

The Rough Creek Fault System extends approximately 100 miles across western Kentucky. This fault crosses the Ohio River into Illinois where it is known as the Shawneetown Fault Zone. Three different types of faulting and recurring movement are evident, and together they interact to form an extremely complex fault pattern. Faulting in this system probably began with wrench faulting in Precambrian time. Subsequent faulting included both normal and reverse movement, which produced primarily high-angle to vertical faults. Evidence does not support significant strike-slip movement at the surface, or in the shallow subsurface.

Brief History of Petroleum Development in the Illinois Basin

The before mentioned major structural features along with the infinite number of subtle folds and faults caused by basin subsidence have played a key role in petroleum accumulation within the basin. Originally the exploration for petroleum reserves was keyed directly to the drilling of the large structural features within the basin. Over 90 years and 200,000 plus wells later, drilling has given up a tremendous amounts of geological information. With this and other information geologists are attempting to search for more subtle stratigraphic traps such as sand lenses within the Aux Vases or oolitic lenses of porosity with the upper Valmeyeran carbonates.

To date, we have only developed the top 3,300 feet of 14,000 + feet of basin sediment. These shallow Pennsylvanian and Upper Mississippian age sedimentary rocks have produced the bulk of this basins total oil production and with the search and exploration for their subtle stratigraphic traps they will continue to produce an economical amount of oil. Within the last 10 years new significant discoveries have been made in the deeper Middle and Lower Mississippian age as well as the deeper Devonian age rocks. These zones of variable porosity and permeability are respectively known as the St. Louis, Salem, Warsaw, and Devonian-Dutch Creek. The ability to further develop and explore for these deeper zones has added important geological information and new field discoveries are providing added incentive for exploration in new areas and greater depths.