How the States Rate
Source:US Energy Information Administration's US
Crude Oil, Natural Gas, and Natural Gas Liquids Reserves, November 2001
In western Ohio, and extending into central and eastern Indiana, is a pocket of oil known as the Lima-Indiana Field. Here, 1,400 feet (~425 meters) beneath the surface, is a layer of rock known as the Trenton Formation. 100,000 wells have been drilled in the Lima-Indiana field alone! The Trenton Formation is Ordovician limestone that formed more than 450 million years ago. At the end of the Ordovician Period, the rocks were exposed for a brief period of time, causing some of the limestone to dissolve away. This increased the porosity and permeability, and was then covered by shales, now acting as the trap rock, preventing the escape of migrating hydrocarbons. Similar to the Albion-Scipio field discussed on the previous page, the rock also changed in many places from limestone to dolostone, increasing its porosity. The Lima-Indiana field (as of the year 2000) has produced 500 million barrels of oil and 1 trillion cubic feet of natural gas.
The Illinois Basin is a depression in the rocks over central and southern Illinois, southwestern Indiana, and northwestern Kentucky, and is the location of all the oil found thus far in Illinois. Over a period of 250 million years, the basin sank at a rate of 2/3" (1.7 cm) per 1,000 years, which is incredibly slow. The basin filled in with shallow salt-water (marine) sediments, which, in some places, are up to 3 miles (~4.82 km) thick. The sea was so shallow, that it often dried up due to minor changes in climate and landscape. The sea, coming from the southwest, advanced and retreated more than 50 times. As a result, many different types of sedimentary rocks corresponding with the changing water depths are found stacked on top of one another.
Pennsylvanian Aged Rocks (250-323 million years ago) and their structure in the Illinois Basin
The sediments of the Illinois Basin are Pennsylvanian to Cambrian in age, and most of the hydrocarbon traps are small anticlines. There are also ancient reefs that contain oil. The Silurian-aged Marine field in western Illinois has produced over 11 million barrels of oil in a reef that is 3 miles long and 3 miles wide (~4.82 km x ~4.82 km), and over 300 feet (~90 meters) thick. As of 1999, the Illinois Basin has produced 4.2 billion barrels of oil.
While the Illinois Basin is thought to be a "mature" field (meaning that most of the oil discoveries are thought to have already happened), new finds do still occur. Advances in seismic technology allowed geologists to find an unknown pinnacle reef in 1996. Oil from this 320-acre reef, 3850 feet (~1175 meters) beneath the ground in Marion County, Illinois, are likely to be more than 5 million barrels. However, this is less than half the oil contained in the largest reef structure ever found in the Illinois Basin. In 1943, the Marine Field was discovered in Madison County, Illinois. This Silurian age reef may ultimately yield 14 million barrels of oil.
The Paleontological Research Institution