Where is Oil Found?

The oil reservoir is typically composed of layers of sandstone, limestone, or dolomite. The reservoir rock has tiny pores, and, 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.

View of Oil and Sand from Microscope

Grains of coarse sand from an oil reservoir, magnified 50 times by a scanning electron microscope. The oil-water fluid fills the spaces between the grains.

Above an oil reservoir is a layer of shale or other fine-grained rock through which water or oil cannot pass. This layer acts like a seal or cap over the actual reservoir. Some Illinois reservoirs are found in dolomite. This rock is similar to limestone but has been chemically altered. The limestone has been recrystalized to form fine-grained dolomite, a process that may dissolve fossils and limestone to form large and small holes (pores) that can hold oil.

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.

Where did oil come from?

Most of the oil in Illinois reservoirs came from layers of shale rich in organic matter, the remains of microscopic plants and animals that lived in a shallow ocean that covered Illinois about 360 million years ago. 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 rock called shale. Because the earth gets warmer with increasing depth, the shale eventually reached a temperature of about 200°F, when the heat began to break down the organic material— “cooking” the organic matter in the sediment and forming oil, which later migrated from the shale into the reservoir rock.

The Hydrocarbon Kitchen

Hydrocarbon Kitchen

A Map of Illinois Counties Showing the Location of the Hydrocarbon Kitchen

Oil in the Illinois Basin is produced from a temperature-pressure zone called the “hydrocarbon kitchen.” Here the most important organic-rich, oil-generating black shale in Illinois, called the New Albany Shale, 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 the 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 are good examples of fields that required lateral migration.

Strata of Earth Showing Oil

The folded rock layers trap the oil (black), which is lighter than water and floats at the top of the reservoir.

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 they explore for oil, geologists 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. Some of the biggest oil fields in Illinois have over 2,000 oil wells and contain over 200,000,000 barrels of oil; that is more than 8.4 billion gallons of oil.

*An anticline is an upward fold in the layers of rock, much like an arch in a building. Petroleum migrates into the highest part of the fold, and its escape is prevented by an overlying bed of impermeable rock.


Up Next: Extracting Oil.