Abstract The Illinois Basin is a broad, spoon-shaped structural depression encompassing approximately 53,000 sq mi in Illinois, southwestern Indiana, and western Kentucky. The U.S. Geological Survey has estimated the bituminous coal reserves in Pennsylvanian rocks to be as much as 365 billion tons. More than 75 individual coal seams have been identified in this area, 20 of which are mined. The majority of the coals are not continuous and do not maintain constant thicknesses. Individual seams range from a few inches to 15 ft in thickness over large areas. The coals outcrop at the basin’s periphery and dip gently towards its deeper portions in southeastern Illinois and western Kentucky. Lower and Upper Pennsylvanian coals are thin and discontinuous while the Middle Pennsylvanian coals are thick, generally continuous, and provide the major reserves of the basin. Thin Lower and Upper Pennsylvanian coals have not been studied in much detail and are not as well correlated as the thicker coalbeds of the Middle Pennsylvanian. The greatest cumulative thickness of coal seams presumably occurs in the southeastern portion of the basin (near the tristate boundary) where the thickest Pennsylvanian section occurs. All Illinois basin coal seams are covered by less than 3,000 ft of overburden, and the major coals are within 1,500 ft of the surface. The Springfield'Harrisburg (No. 5) coals in Illinois and their correlatives, Springfield V in Indiana and No. 9 coals in Kentucky, are the most extensive and uniformly thick coals in the Illinois basin; estimated coal reserves are over 67 billion short tons. The Herrin (No. 6) coal is also thick and extensive in Illinois and contains estimated coal reserves of over 77 billion short tons. Some deeper coals, the Colchester (No. 2), which is uniformly present over the entire basin, and the Davis (No. 6) and Mannington (No. 4) occurring primarily in Kentucky, contain combined reserves estimated at over 39 billion short tons. The potential methane resource from selected coalbeds in the Illinois basin can be estimated from desorption data generated by the U.S. Bureau of Mines (USBM), the Illinois and Indiana State Surveys, and field tests performed under the MRCK The Herrin (No. 6) and the Springfield-Harrisburg (No. 5) coals and their correlatives in Indiana have been sampled the most often. The estimated gas content of the Herrin (No. 6) ranges from 32 to 125 cubic feet per ton (cf/ton). A similar range of values, 32 to 147, has been calculated for the Springfield-Harrisburg (No. 5) coals. This variability cannot be directly related to sample depth, since some of the gassier coals were from relatively shallow horizons—the opposite of what might have been expected—but is related to other geologic controls on the gas content, to analytical errors, or to the method used to determine the “remaining” gas. MRCP data on the methane gas content of Illinois basin coals is presently limited to coring and well testing in Clay and Marion counties of Illinois Posey County, Indiana, and Webster County, Kentucky. In Clay County, the Danville (No. 7), Herrin (No. 6), Briar Hill (No. 5a), Harrisburg (No, 5), and Seelyville coals were cored between depths of 994 and 1,352 ft and samples were desorbed for approximately 5 months using the USBM’s direct method. Total gas contents ranged between 32 and 48 cf/ton. In Marion County, the same coal seams, excluding the Seelyville, were cored and sampled at depths between 664 and 736 ft. Gas contents ranged from 22 to 35 cf/ton. All major coals in Posey County, Indiana, were sampled and showed a gas content from 26 to 144 cf/ton. Results from Webster County, Kentucky, showed a gas content of 45 cf/ton for the Nos. 13 and 9 seams. Investigations of data have shown that the gas content of the coals in the Illinois basin is generally low, ranging from less than 40 to 150 cf/ton. Based on the limited data available, ranges for the maximum and minimum expected in-place gas resource have been made for the Danville, Herrin, Springfield-Harrisburg, and their equivalent coals. The Danville coals are anticipated to have a minimum of approximately 500 billion and maximum of nearly 1.7 trillion cubic feet (Tcf) of in-place gas. The Herrin, likewise, is estimated to contain 2.5 and 9.6 Tcf of gas and the Springfield-Harrisburg, 2.2 and 9.9 Tcf of gas. The minimum in-place gas resource for these three seams totals over 5 Tcf. It is reasonable to assume that the methane contained in major deeper coals (Colchester, Davist etc.) may add significantly to this figure. It should be noted that although the specific gas content of coals in the Illinois basin is quite low, the simple magnitude of the coal resource produces large hvplace gas resource estimates. The gas content of coals in the Illinois basin is generally higher towards the southeastern portion of the basin and initial target area defined by the MRCP. On the basis of information gathered in this report, two target areas (totaling approximately 4,300 sq mi) hold the greatest probability for early commercial gas production. Target Area A, located in western Kentucky, contains a thick section of deep coals in a highly disturbed structural belt. Target Area B, in southeastern Illinois and southwestern Indiana, contains previously reported gassy coals and thick coal sections at considerable depths.

Abstract: While coalbed methane (CBM) is a significant source of natural gas production globally, uncertainties regarding the proportions of biogenic and thermogenic natural gas in CBM reservoirs still remain. We integrate major gases, hydrocarbon composition, hydrocarbon stable isotopes and noble gases in fluids from 20 producing CBM wells to more accurately constrain the genetic source of natural gases in the eastern Illinois Basin, USA. Previous studies have indicated primarily biogenic production of methane (>99.6%) with negligible contributions from thermogenic natural gases. However, by integrating noble gases, we identify quantifiable (up to 19.2%) contributions of exogenous thermogenic gas in produced gases from the Seelyville and Springfield coal seams. Thermogenic gases are distinguished by a positive relationship between methane, ethane and helium-4, lower C 1 /C 2 +, heavier δ 13 C-CH 4 , more radiogenic noble gases ( 4 He, 21 Ne*, 40 Ar*), and lower abundances of atmospherically derived gases ( 20 Ne, 36 Ar). Biogenic gases displayed lighter δ 13 C-CH 4 , higher C 1 /C 2 +, higher levels of atmospheric gases and lower abundances of radiogenic noble gases. Our data suggest that natural gases from a deeper, exogenous thermogenic source likely migrated to the Pennsylvanian-aged coals at an unknown time and later mixed with biogenic methane diluting the geochemical signature of the thermogenic methane within the Springfield and Seelyville coal seams.