THE DEVONIAN SYSTEM IN THE ILLINOIS BASIN: THE PAST 159 YEARS OF GEOLOGIC RESEARCH AND FUTURE OBJECTIVES

Devonian rocks of the Illinois Basin were identified in the north-central United States during the 1850s and in southwestern Illinois in 1857. The Devonian System was completely identified in 1923 and has now been divided into three major chronostratigraphic series that are named, in ascending order, the Lower Devonian, Middle Devonian, and Upper Devonian. On the basis of conodont biozones, their relative ages range from Lochkovian to Famennian, equivalent to the Ulsterian to Chautauquan series in U.S. terminology. The Lower Devonian Series consists predominantly of marine carbonate strata and exhibits marked facies differentiation in basin and shelf areas. The basin area is composed of siliceous limestone, whereas the shelf area is composed of pure limestone. The thickness of this series exceeds 1,400 feet (427 m) in the extreme south of the basin and thins to a few feet along the Sangamon Arch. The Middle Devonian Series consists of marine limestone and dolostone strata, with several thin beds of organic-rich shale and siltstone. Restricted facies are exposed in the north part of the basin and contain 30 to 50 feet (9 to 15 m) of dolomitic limestone, dolomite, and beds of gypsum and anhydrite. The series ranges in thickness from 100 to 500 feet (30 to 152 m). The Upper Devonian Series consists of more than 400 feet (122 m) of thick green to gray shale and contains several organic-rich black shale zones (total organic carbon: 9–23 wt%) that are recognized as prolific hydrocarbon source rocks in the Illinois Basin. These shales, regrouped in the Devonian-Mississippian New Albany Continuous Assessment Unit, have produced around 66% in oil, with mean estimated total recoverable hydrocarbons of 214 million barrels of oil. They also remain an important resource for gas, with an estimated 3.79 trillion cubic feet (0.08 trillion cubic meters) of undiscovered, technically recoverable gas. Stone and tripoli (microcrystalline silica) are also main resources in the Devonian System. However, potential resources, such as geothermal energy and uranium minerals, and the area as a target location for carbon sequestration have scarcely been studied and may have direct implications for the economy of and commercial strategy for the north-central states.