Limestone and dolomite
Limestone and dolomite are the most widely quarried rocks in the Illinois Basin. The most common and important product from quarrying these carbonate rocks is crushed rock. Crushed stone is used for construction aggregate, road surfacing material, lime, and agricultural limestone. High-calcium limestone is used as a major ingredient of cement — the binding agent used in construction projects, and as a scrubbing agent for pollution control in power plants and incinerators. Limestone and dolomite quarries are located where thick stone deposits occur near the surface, mainly in the northern, western, and southern parts of the Illinois Basin. In some areas, these deposits are mined underground. Other products from these quarries include building, decorative, and landscaping stone.
Sand and gravel
Sand and gravel deposits are widely distributed in select locations across the state; they are most abundant and of highest quality in northeastern Illinois, but less abundant and lower quality elsewhere. Sand and gravel in much of Illinois was deposited by water from the melting glaciers. The huge ice lobes of continental ice sheets that moved into Illinois from Canada carried enormous amounts of rock debris, much of which was washed and sorted by meltwaters into various sand and gravel deposits. Sand deposits of more recent origin are found in larger streams and rivers, where they are recovered by dredging. Many deposits of sand and gravel are used as construction aggregate in asphalt and concrete pavements and in commercial and residential structures, especially in the northern half of the state.
Clay is the very fine-grained material that is usually formed by the weathering of different kinds of rocks. In the Illinois Basin, clay deposits are either unconsolidated surficial clays or consolidated bedrock clays and shales. Clay has many uses, including the manufacture of brick, tile, plaster, and cement. Clays make excellent barriers to fluid transport and are used as pond liners, landfill covers, and liners for waste disposal or isolation. Near the southern tip of Illinois, a unique deposit of clay is mined for use as kitty litter and a commercial absorbent product.
Commercial silica (quartz) sand is produced mostly from the Ordovician St. Peter Sandstone in LaSalle County in northern Illinois. This sand is famous for its high purity and is widely used in making high-quality glass. Silica sand is also used as molding sand because it can withstand the high temperatures used in casting metals. It is also used as a component in paint fillers, pottery glazes, enamel, and in petroleum reservoir fracturing ("frac sand").
Tripoli is a very porous rock that is composed of tiny particles of quartz. These deposits occur in the extreme southernmost part of Illinois. Tripoli is used in the ceramic industry, in polishing optical lenses, as paint filler, and as a fine abrasive.
Peat deposits are scattered across northern Illinois and are used mainly as a soil conditioner. This material is debris from the rich growth of plants that accumulated in low bog areas on the land surface after the glaciers melted away. Peat is harvested New Morrison in Whiteside County, Illinois.
Oil production in Illinois began in the early 1860s in Clark County when drilled wells produced enough oil for a nearby town to be called "Oilfield." Early oil discoveries were mostly guided by the occurrence of natural oil seeps at the surface. In the early 1900s, many shallow oil and gas reservoirs were discovered in structural traps (anticlines). In the late 1930s, a second oil boom resulted from the application of a new technology. Seismic exploration allowed geologists to locate structural traps that were deeply buried or too subtle to be found otherwise.
In many reservoirs, oil and water fill the open pores between the grains of the reservoir rock, much as a soda fills the spaces between ice cubes in a glass. Some reservoirs in carbonate rocks produce oil from void spaces that resulted from ancient biologic life (e.g. Silurian reef deposits).
In the Illinois Basin, oil has been produced from the Middle Ordovician Galena Limestone, Upper Silurian carbonates, Middle Devonian limestones, Middle Mississippian limestones, Upper Mississippian sandstones, and Lower and Middle Pennsylvanian sandstones.
Natural gas is another hydrocarbon fuel, but it is much less abundant than crude oil in Illinois. Few wells produce just natural gas in the Basin. Generally, gas in Illinois is a by-product that bubbles out of the oil when the oil is brought to the surface. The natural gas is separated in a gas separator and may either be prepared for a gas pipeline, used to power the motor that runs the pump, or flared (burned) at the site to safely dispose of small quantities of this combustible material.
Natural gas also occurs within the coal layers in the Illinois Basin. Methane has been produced from Pennsylvanian coalbeds in Crawford and Franklin Counties in Illinois and coalbeds in southwestern Indiana.
Layers of coal are very abundant under the surface of a very large portion of the Illinois Basin. These coal deposits began to form more than 300 million years ago during the Pennsylvanian Period, when the land that is now the Basin was near the equator. Peat, the precursor to coal, formed in dense, tropical swamps, where the ancestors of modern ferns and scouring rushes grew as tall as trees and thick deposits of plant debris accumulated under the water. These deposits were buried by sediments and, over millions of years, the layers of peat were compressed into coal. This process created more than seventy-five layers, or "seams," of coal in the Basin that ranged from a few inches up to 10-feet thick. Some of the coal seams cover thousands of square miles in Illinois, Kentucky, and Indiana, but most occur in small, more limited areas. Depending on where you live in the Basin, a coal seam may lie close to the land surface or it may be buried more than 2,000 feet down. About thirty of the coal seams are considered to be thick enough to mine economically.
Coal is the energy source for about half of the electricity produced in the United States. The burning of coal provides heat that is used to convert water into steam, which is used to drive the electrical generators. Coal mined in the Illinois Basin is consumed in Illinois, Kentucky, Indiana, and other states in the country. Illinois Basin coal is also used to provide electrical power and heat at large manufacturing and food processing plants.
Coal is mined by surface or underground methods. In most of the Basin, the most accessible of the near-surface deposits have already been mined out. For the most part, future mining is expected to be underground.
Water is a basic resource for all forms of life. Potable water is essential for humans. In the Illinois Basin, much of the potable water is derived from aquifers. Surface water (rain and snow melt) is the other water source. Aquifers in the Basin occur either in bedrock or in the surficial deposits lying over the bedrock. Bedrock aquifers range in depth from near the surface to depths greater than 1,000 feet (in northwestern Illinois). Surficial aquifers usually occur as sand or sand and gravel deposits within either the widespread Pleistocene glacial deposits or modern and ancient river valleys. Some of the aquifers within ancient river valleys (e.g. the Mahomet Aquifer in east-central Illinois) contain wells that are prolific producers of freshwater. There are also prolific well fields located in the floodplains of the Mississippi, Ohio, Illinois, and other rivers.