General Geology of the Illinois Basin
The Illinois Basin, which covers approximately 53,000 square miles, is a broad elliptical basin covering most of Illinois, southwestern Indiana, and western Kentucky. Its main axis trends from the northwest to the southeast. It is basically bland structurally with the exception of the LaSalle Anticline which trends to the southeast across a major portion of the basin and some fault systems, which are primarily located in the southeastern part of Illinois.
The Illinois Basin is located in the central lowland province, which also included north central Texas, central Oklahoma, eastern Kansas, and Michigan. The subsurface geology and paleo-environment of the province appear to be correlative throughout with commercial production in the deeper intervals in other parts of the province that are not currently being produced in the Illinois Basin.
Oil Fields in Illinois
As of this date the Illinois Basin has produced approximately 4.2 billion barrels of reserves.
Illinois has about 650 oil fields, primarily in the southern half of the state. The porous rock reservoirs or pay zones typically lie about ½ mile deep.
Drilling for oil has always been a risky financial venture because fewer than half of the holes drilled in Illinois actually strike enough oil to repay the drilling costs. Economically non-viable wells or dry holes are filled with cement and plugged to protect the groundwater.
Illinois' greatest drilling boom was in the 1940s and 1950s when the state was one of the nation's leading producers. In 1996, Illinois produced over 15 million barrels (630 million gallons) of oil. That year about 500 new wells were drilled, mostly to develop known fields. In 1998, the average daily oil production from approximately 30,000 active wells was only 1 to 2 barrels (42 to 84 gallons).
Production History
Oil or gas production in Illinois began in 1853 when marsh or drift gas was produced from two wells drilled near Champaign. This gas came from rotting vegetation buried in the glacial deposits. At the time, people knew little about where gas or oil came from, or how to search for it. In the early 1860s, several holes drilled in Clark County produced enough oil for the name "Oilfield" to be given to a small town there, even though commercial-scale production in the area did not begin until 40 years later. The search for oil and gas began in earnest there in 1866 when the Clark County Petroleum and Mining Company established its headquarters at Marshall. Natural gas seeps near Oilfield led the company's owners to believe that commercial quantities of oil and gas were there. However, because well casing technology did not yet exist, water from drilled-through upper layers of earth flowed into the wells and prevented most of the oil in deeper layers from seeping out of the rocks. Farther to the west near Litchfield, holes drilled in the late 1860s to search for coal leaked oil and water into the workings of a mine, and for several years people skimmed oil off the water and sold the oil. By the early 1880s, natural gas had been discovered in the area and was being piped to Litchfield for domestic use. Continued drilling in the area eventually established oil production, and in 1889 wells produced 1,460 barrels. By 1902, when production ceased, the wells had produced only 6,576 barrels of oil. (The barrel equals 42 U.S. gallons and is a standard unit of volume measurement in the petroleum industry.)
Anticlinal Theory and Well Casing
By the turn of the century, new well casing technologies solved the problem of water flowing into oil wells. Producers also now recognized that oil and gas collected at anticlines, or the crests of upwarps in the rock layers. From 1904 to 1910, numerous shallow oil and gas reservoirs were discovered in the many anticlines in a large structure now known as the La Salle Anticlinorium, which lies beneath eastern Illinois. With these discoveries, Illinois leaped to third among states as its annual petroleum production rose from 181,000 barrels in 1905 to 33 million barrels by 1910. Yet by 1913, with all the easily discovered anticlines drilled, the heyday of the oil fields in Clark, Cumberland, Edgar, Crawford, and Lawrence Counties was essentially over. By 1936, with few new discoveries to replace fields already pumped dry, the state's total oil production had dropped to less than 4.5 million barrels.
Seismic Exploration
In the late 1930s, a new technology called seismic exploration allowed geologists to find hidden anticlines—structures too deeply buried or too subtle to be found otherwise. Still used today, this technique uses sensitive microphones called "geophones" to record sound waves from ground-level dynamite blasts as they "echo" off the tops of the successive rock layers below. The echo data are then used to form a picture of the rock layers below. With seismic exploration, hundreds of new anticlines and other types of oil traps were found and drilled in many areas of southern Illinois. In 1940, the state's total oil production rose to 147.6 million barrels, the largest in the state's history. Some of the largest oil fields—in area and volume of oil produced—were discovered in this period. These include the Clay City Field, which covers parts of Clay, Richland, and Jasper Counties, the Salem Field in Marion County, and the Louden Field in Fayette County. Although intensive exploration with the seismic technique continued during World War II, production declined after the 1940 peak as the size and number of new fields found each year fell. All the large and easily found targets had already been drilled, and even with the new technologies, the new fields being discovered did not hold enough new oil to replace the production from increasingly depleted old fields.
Hydraulic Fracturing
Two new technologies developed in the 1950s let producers force more oil out of newly discovered and existing fields. In the first process, called hydraulic fracturing, powerful pumps at the surface inject a fluid, commonly with the consistency of a milkshake, into the oil-producing reservoir rocks. The pressure exerted by the fluid, is great enough to fracture the rocks around the well, and sand grains injected with the fluid keep the cracks propped open once the pumping stops. The newly opened fractures make the reservoir rocks more porous, and oil can flow more easily into the well.
Secondary Recovery and Waterflooding
A familiar oil-well image is an uncontrolled "gusher" spewing oil high into the sky to rain down on the excited drillers. Oil gushes because it is under pressure in the ground. In a reservoir, oil and water fill the open pores between the grains of the reservoir rock, much as a drink fills the spaces between ice cubes in a glass. The weight of all the rock on top of a reservoir applies pressure on the reservoir rock, as well as on the oil, water, and gas within it. When a well penetrates a reservoir, the pressure in the reservoir drives the oil and gas toward the lower pressure of the open well. As the oil or gas is withdrawn, reservoir pressure falls as the volume of oil between the grains of the reservoir rock is reduced. Somewhat like a balloon whose neck is opened, the reservoir gradually "deflates." In some very large oil fields, this deflation has actually caused the land surface to subside. In some areas of Long Beach, California, for example, subsidence due to oil withdrawal exceeds 10 feet.
As the pressure in the reservoir falls, the flow of oil into the well slows to a trickle, and, eventually, to nothing. Oil produced under this natural driving pressure is the "primary recovery" or "primary production" of a field.
In another technology introduced in the 1950s, repressuring the reservoir through waterflooding was introduced. Water is injected into the reservoir rocks to maintain reservoir pressure as the oil is withdrawn, and to sweep the oil out of the reservoir rocks and toward the well. In the most commonly used technique, the five-spot pattern, water is pumped into the reservoir rocks at four wells arrayed around a central producing well in an arrangement like the five dots on the face of a domino or die.
Using techniques to maintain reservoir pressure and drive the oil out of the reservoir rocks is called "secondary" recovery or production. With hydraulic fracturing and waterflooding, Illinois' total oil production rose to about 82.3 million barrels in 1956, a peak from which it has been declining almost continuously ever since.
Commercial Petroleum Accumulations
There are three elements necessary in order to have a commercial petroleum accumulation. These are a: Source Rock; Reservoir Rock; Trap
Source Rock
The source rock is generally layers of shale rich in organic matter, including the remains of microscopic plants and animals, mostly zooplankton, that lived in a marine environment. As the plants and animals died, their remains settled into the mud at the bottom of the ocean. Because the sediments and surrounding water contained almost no oxygen, the organic matter was preserved. This mud was buried and compressed beneath successive layers of sediment to form a sedimentary rock called shale. Because the earth gets warmer with increasing depth, the shale eventually reached a temperature of about 200°F, as the heat began to mature the organic material into a progenitor of hydrocarbon or crude oil. As this maturation process continued the hydrocarbon became mobile and migrated from the source rock. Ideally, migration leads to a reservoir where the petroleum is trapped, discovered, and produced.
One source rock identified in the Illinois Basin produced from a temperature-pressure zone called the “hydrocarbon kitchen.” Here the important organic-rich, oil-generating black shale in Illinois, called the New Albany Shale (approximately 360 million years old), was “cooked” enough to convert organic matter into oil. Oil was then expelled from the shale and migrated upward along cracks, fractures, or through porous and permeable-strata to oil reservoirs. This migration is usually nearly vertical, and thus most of Illinois’ oil fields occur in the area outlined by the “kitchen” boundary. Notice that oil is also found in reservoirs many tens of miles outside the kitchen. The oil reached these reservoirs through porous and permeable layers that acted like slightly inclined pipes that transported the oil far from the kitchen. Geologists say that the oil “migrated laterally” to these reservoirs. The oil fields in western Illinois may be good examples of fields that required lateral migration.
Reservoir Rock
Porosity and permeability are the two most important properties for a reservoir rock. Porosity is the percentage of void space available to contain fluids. Permeability is the degree to which the pore space is interconnected and allows fluids to be transmitted through the pore space. These two properties then directly influence the storage capacity and transmissivity of the reservoir rock.
The oil reservoir is typically composed of layers of sandstone, limestone, or dolomite. The reservoir rock has tiny pores, and/or, in many cases, cracks or fissures, that are filled with oil, gas, and water. The pores and cracks are connected, so that when a well is drilled into the reservoir, the fluids in the pores can drain into the well.
The top of an oil reservoir is covered by a layer of shale or other fine-grained rock through which water or oil cannot pass. This layer acts as a seal or cap over the reservoir. Without adequate seals on the reservoir the petroleum continues to migrate until it is trapped or escapes at the surface or disseminates.
Reservoir sandstone has individual sand grains that are slightly cemented together. Several sand grains could fit on the head of a pin, but there are still many pores or spaces between the grains that can hold oil. These sand grains were originally deposited in river channels and deltas or as sandbars and beaches in a shallow sea. Limestone reservoir rock may consist of sand-sized or larger fragments of corals, sponges, snails, clams, and other marine animals. Many ancient limestone reef deposits in Illinois contain oil.
Trap
Because oil is lighter than water, it tends to rise through the layers of the earth. If its way is blocked by a layer of shale or other impervious rock layer, the oil can move sideways through the pores and cracks in the rock layers until it is finally trapped in an upwardly arched bed called an “anticline” or against a fault plane; if the oil can reach the earth’s surface, it forms a tar seep. One of the most famous seeps in the world is at the La Brea Tar Pits, which are located in Los Angeles, California.
When geologists explore for oil they look for anticlines and other traps. Some of the anticlines in Illinois may be a mile or more across and several miles long. More than one layer of rocks in these folded strata might trap oil. These are referred to as multiple pay zones and are common in Illinois oil fields. Some of the biggest oil fields in Illinois have over 2,000 oil wells and contain over 200,000,000 barrels of oil (more than 8.4 billion gallons of oil).
Other types of traps are associated with changes in the nature of the rock itself and include stratigraphic traps. Porous and permeable reservoir rocks may be surrounded by tight rocks that trap petroleum in the reservoir. Reefs that have good reservoir characteristics may be encased by tight rocks to form a commercially substantial hydrocarbon accumulation. There are many possibilities for these subtle traps to be discovered.
Natural Gas
Natural gas is another hydrocarbon fuel, but it is much less abundant than crude oil in Illinois. Few wells produce just natural gas here. Generally, gas in Illinois is a by-product that bubbles out of the oil when the oil is brought to the surface, where there is little confining pressure to keep the gas dissolved. This pressure relief is somewhat similar to opening a can of soda and having bubbles of carbon dioxide released. Natural gas is separated in a gas separator and may be prepared for a gas pipeline, used to power the motor that runs the pump jack, or flared (burned) at the site to safely dispose of small quantities of this combustible material. Small yellow flames with dark smoke at the ends of pipes several feet off the ground near the separator and tanks mark an oil field where gas is flared.
Producing Oil
Deep in the well is a pump that is connected to the surface by long steel rods. When the pump jack at the surface is rocking up and down, it opens and closes valves in the submerged pump. Each stroke brings a cup or two of fluid up to the surface. This fluid, generally a mixture of oil and water, must be separated by an oil-water separator. The separator takes advantage of the fact that oil floats on water. Once separated, the oil is stored in large tanks before it is transferred by pipeline or truck to the refinery and the water is safely pumped back into the ground.
Oil Prices
Since 1973, Illinois oil producers have confronted wide swings in oil prices due to world and national events. The price of oil in the United States nearly doubled during the 1973–74 Arab states oil embargo. To try to protect the economy from this shock, the government imposed price controls that lasted until 1978. When the controls were lifted, the price of crude oil more than doubled again to an inflation-adjusted peak of nearly $55 per barrel in 1981.
In response to these high oil prices, Illinois' producers drilled many new wells, and production rose slightly in Illinois in the early 1980s. In 1981, however, an economic recession began, and increased energy costs caused a large decrease in energy consumption through efficiency improvements and conservation techniques. A major factor in this decrease in oil demand was the dramatic improvement in the fuel efficiency of U.S. automobiles. By July 1986, reduced oil demand and major additions in supply, such as the North Sea oil field, had driven the price of crude oil down to about $19 per barrel.
In 1998, the price of Illinois crude oil sank to less than $9 per barrel, a level where many of Illinois' small, independent producers could not make a profit.
The price of oil is now around $70 per barrel, a level where many of Illinois’ small, independent producers can make a profit. The more sophisticated technology, once available only to the majors, is now more readily available and more affordable for independents. In this climate of demand the promise of deeper reserves in Illinois is most attractive.
