U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3058

August 2007

National Assessment of Oil and Gas Fact Sheet

Assessment of Undiscovered Oil and Gas Resources of the 

Illinois Basin, 2007

Introduction

The U.S. Geological Survey (USGS) has 

completed an assessment of the undiscovered 

oil and gas potential of the Illinois Basin (fig. 

1). The assessed area includes parts of the States 

of Illinois, Indiana, Kentucky, Missouri, Ohio, 

and Tennessee. The assessment is based on the 

geologic elements of each total petroleum system (TPS) defined in the basin. These geologic 

elements include the hydrocarbon source rocks, 

reservoir rocks, and hydrocarbon traps. By using 

this geologic framework, the USGS defined 4 total 

petroleum systems and 19 assessment units (AUs) 

within the basin and estimated the quantity of 

undiscovered technically recoverable oil and gas 

resources within 14 of the 19 AUs (table 1).

Total Petroleum Systems

The four total petroleum systems identified in 

the Illinois Basin are the (1) Precambrian to Cambrian TPS, (2) Ordovician Ancell/Maquoketa TPS, 

(3) Devonian to Mississippian New Albany TPS, 

and (4) Pennsylvanian Coal and Shale TPS. Each 

TPS is named according to the petroleum source 

rock(s) of that system. For most of the systems, 

each TPS is associated with only one source rock. 

The Precambrian to Cambrian TPS, however, 

encompasses both source rocks in the Cambrian 

Eau Claire Formation and older, hypothetical 

source rocks in the deeper parts of the basin.

Assessment Units

Sixteen of the AUs are characterized as conventional oil and gas accumulations, and three of the 

AUs are characterized as continuous accumulations. The 16 conventional AUs are the (1) Precambrian to Cambrian Rift-Fill AU, (2) Cambrian Mount Simon to Eau Claire 

AU, (3) Cambrian to Ordovician Knox Group AU, (4) Cambrian to Ordovician Carbonates Cumberland Saddle AU, (5) Ordovician St. Peter/Everton 

AU, (6) Ordovician Dutchtown to Galena AU, (7) Lower Silurian Carbonates 

AU, (8) Upper Silurian Calcareous Siltstones AU, (9) Upper Silurian Carbonates (Reef) AU, (10) Lower Devonian Carbonates AU, (11) Middle Devonian 

Dutch Creek Sandstone AU, (12) Middle Devonian Carbonates AU, (13) 

Lower Mississippian Borden AU, (14) Lower Mississippian Carbonates AU, 

(15) Upper Mississippian Sandstones AU, and (16) Pennsylvanian Sandstones 

AU. All of these conventional AUs were assessed quantitatively, except for 

the Precambrian to Cambrian Rift-Fill AU, the Cambrian Mount Simon to 

Eau Claire AU, the Cambrian to Ordovician Knox Group AU, and the Ordovician St. Peter/Everton AU. 

The three continuous AUs are the (1) Ordovician Maquoketa Continuous AU, (2) Devonian to Mississippian New Albany Continuous AU, and (3) 

Pennsylvanian Coal Bed Gas AU. All of these continuous AUs were assessed 

quantitatively, except for the Ordovician Maquoketa Continuous AU.

ILLINOIS

IOWA

INDIANA

MICHIGAN

OHIO

KENTUCKY MISSOURI

TENNESSEE

ARKANSAS

Peoria

Memphis

Chicago

Nashville

Evansville

Louisville

Fort Wayne

Saint Louis

Springfield Indianapolis

Cedar Rapids

Lexington

Dayton

Jackson

Cincinnati

Lake 

Michigan

92°W 91°W 90°W 89°W 88°W 87°W 86°W 85°W 84°W

42°N

41°N

40°N

39°N

38°N

37°N

36°N

35°N

0 50 100 Miles

0 100 200 Kilometers

80

69

90

65

74

64

75

64

71

75

65

65

40

55

57

55

70

70

55

74

80

57

44

40

24

Figure 1. Location of the Illinois Basin (assessed area is within the red boundary). 

Using a geology-based assessment methodology, 

the U.S. Geological Survey estimated the following 

quantities of undiscovered, technically recoverable 

oil and gas resources in the Illinois Basin, USA: (1) 

a mean of 214 million barrels of oil; (2) a mean of 

4.65 trillion cubic feet of natural gas; and (3) a mean 

of 24 million barrels of natural gas liquids.
Resource Summary

For the Illinois Basin, the USGS estimated the 

quantities of undiscovered, technically recoverable 

oil and gas resources as follows (table 1): 

(1) a mean of 214 million barrels of oil, 

(2) a mean of 4.65 trillion cubic feet of natural 

gas, and 

(3) a mean of 24 million barrels of natural gas 

liquids. 

The Ordovician Dutchtown to Galena AU has 

the greatest potential for undiscovered oil, with an 

estimated mean of 72 million barrels of undiscovered, technically recoverable oil. The Upper 

Silurian Carbonates (Reef) AU and the Middle 

Devonian Carbonates AU also have substantial 

potential for undiscovered, technically recoverable oil. The Devonian to Mississippian New Albany Continuous AU has 

the greatest potential for undiscovered gas, with an estimated mean of 3.79 

trillion cubic feet of undiscovered, technically recoverable gas. 

Additional Information

Supporting geologic studies of the Illinois Basin total petroleum systems 

and assessment units are in progress. Assessment results, as well as information on the assessment methodology, are posted on the USGS web site 

http://energy.cr.usgs.gov/oilgas/noga as they become available.

Illinois Basin Assessment Team

Christopher S. Swezey (Task Leader; cswezey@usgs.gov), Joseph R. 

Hatch, Sean T. Brennan, Joseph A. East, Elisabeth L. Rowan, John E. 

Repetski, Ronald R. Charpentier, Troy A. Cook, Timothy R. Klett, Richard 

M. Pollastro, and Christopher J. Schenk.

Table 1. Illinois Basin oil and gas assessment results.

[All tabulated results are for technically recoverable resources. MMBO is million barrels of oil. BCFG is billion cubic feet of gas. MMBNGL is million barrels of natural gas liquids. Results shown are fully risked estimates. For gas accumulations, all liquids are included under the NGL (natural gas liquids) category. F95 represents 

a 95-percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. 

TPS is Total Petroleum System. AU is Assessment Unit. CBG is Coal Bed Gas. Gray shade indicates not applicable or not assessed quantitatively] 

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Oil 0 1 5 2 0 2 8 3 0 0 1 0

Gas 0 0 0 0 0 0 0 0

Oil 2 6 11 6 2 7 15 8 0 0 1 1

Gas 0 0 0 0 0 0 0 0

Oil 5 13 23 13 4 14 28 15 0 1 2 1

Gas 0 0 0 0 0 0 0 0

Oil 0 1 5 1 0 0 6 2 0 0 0 0

Gas 0 0 0 0 0 0 0 0

Oil 6 26 64 29 6 27 75 32 0 1 5 2

Gas 0 0 0 0 0 0 0 0

Oil 0 7 21 8 0 7 25 9 0 0 1 0

Gas 0 0 0 0 0 0 0 0

Oil 0 5 16 6 0 5 18 7 0 0 1 0

Gas 0 0 0 0 0 0 0 0

Oil 13 46 102 51 13 49 122 56 1 3 7 3

Gas 0 0 0 0 0 0 0 0

Oil 0 0 61 10 0 0 45 8 0 0 2 0

Gas 0 0 0 0 0 0 0 0

Oil 2 10 25 11 2 8 25 10 0 0 1 1

Gas 0 0 0 0 0 0 0 0

Oil 2 4 10 5 2 5 12 6 0 0 1 0

Gas 0 0 0 0 0 0 0 0

Oil 11 63 162 72 13 78 205 86 1 5 15 6

Gas 25 150 419 177 0 3 9 4

Ordovician Ancell/Maquoketa TPS

Precambrian to Cambrian

TPS

Total Continuous Resources

Pennsylvanian Coal Bed Gas AU CBG

67 352 419

0 0 0 0 1,300 3,248

Upper Silurian Calcareous

Siltstones AU

Total Conventional Resources

Ordovician Maquoketa

Continuous AU

Pennsylvanian Coal and Shale

TPS

Gas Not assessed quantitatively

Devonian to Mississippian New

Albany TPS

Devonian to Mississippian

New Albany Continuous AU Gas

Upper Mississippian

Sandstones AU

Field

Type

Devonian to Mississippian

New Albany TPS

Lower Devonian Carbonates AU

Middle Devonian Dutch Creek

Sandstone AU

Pennsylvanian Sandstones AU

Lower Mississippian Borden AU

Lower Mississippian

Carbonates AU

Middle Devonian Carbonates AU

Ordovician Dutchtown

to Galena AU

Total Petroleum Systems

and Assessment Units

Not assessed quantitatively

Not assessed quantitatively

Cambrian to Ordovician

Knox Group AU

Ordovician Ancell/Maquoketa

TPS

Total Undiscovered Resources

Precambrian to Cambrian

Rift-Fill AU

Ordovician St. Peter/Everton AU

Cambrian Mount Simon

to Eau Claire AU

Lower Silurian Carbonates AU

Upper Silurian Carbonates

(Reef) AU

Cambrian to Ordovician Carbonates

Cumberland Saddle AU

1,003

Not assessed quantitatively

Not assessed quantitatively

41 182 505 214

13 5

2 13 46 18

0 0 1 1

8,116 3,792 1 4

0 0 0 0 182 396 861 443

0 0 0 0 1,482 3,644 8,977 4,235 1 4 14 6

41 182 505 214 1,549 3,996 9,980 4,654 3 17 60 24 Total Undiscovered Oil and Gas

Resources

Continuous Oil and Gas Resources Conventional Oil and Gas Resources