Eastern Interior Basin
The Eastern Interior basin, in Illinois, Indiana, and Kentucky, has produced oil for more than 60 years and has been a major producing province for 45 years. During this time nearly 1⅔ billion barrels of oil have been obtained from the basin. Its reserve producible by current methods is about ⅔ billion barrels, and the use of water flooding and other secondary methods may add another ½ or ⅔ billion barrels to the recoverable reserve. About three-quarters of the current annual withdrawal of 80 million barrels of oil is being replaced by new drilling, largely in extensions to known pools. There are already more than 500 named pools and, as multiple pay zones and isolated lenses in the same zone are very common, there may be 3,000 or more separate reservoirs. About 60 new pools are being found each year, primarily in the same areas and beds as the present production and are therefore of little interest in this appraisal of those neglected beds and areas which may furnish future vertical or horizontal extensions to the province.
The rocks of the Eastern Interior basin are unmetamorphosed Paleozoic sedimentary rocks, except for a glacial veneer over the northwestern half of the basin, a thin wedge of Tertiary and Cretaceous rocks of the Mississippi embayment overlapping the southwestern margin, and quantitatively insignificant post-Paleozoic intrusives. The Permian is the only Paleozoic system not represented.
The basin is usually defined to include the area of 53,000 square miles that is covered by Pennsylvanian rocks