A program was initiated to determine the extent of the coalbed methane resource and the location of the most favorable resource areas within the Illinois basin, and to obtain a better understanding of the relationships between selected variables and the gas content of coals. Preliminary gas in-place resource estimates are made based on coal desorption data and the most favorable resource areas are outlined. Limited and/or poor data precluded establishing relationships between various geological chemical/physical parameters and coal gas content. Further investigations are required to determine such relationships and to define what percentage of this resource is producible.
Coalbed gas has been of concern to the coal industry as a safety hazard since the early days of underground mining in the 19th century. Only recently, however, has methane, the major component of this gas, been considered a potential source of energy. In late 1977, the United States Department of Energy (DOE) initiated the Methane Recovery from Coalbeds Project (MRCP), to characterize and to investigate and develop means to extract the coalbed methane resources of this country. This paper discusses work performed in conjunction with paper discusses work performed in conjunction with the MRCP and research efforts conducted by the Indiana Geological Survey within the Illinois basin, or Eastern Interior Coal Region.
Adequate evaluation of the methane resource in coal of the Illinois basin involves answering two questions. First, what regions of the basin hold the greatest potential for commercialization of this resource? This question was initially addressed by an intensive literature search, communication with state geological surveys and the U.S. Bureau of Mines (USBM), and by using proposed geologic criteria. These efforts resulted proposed geologic criteria. These efforts resulted in identification of potential "gassy" target areas. Recent and current activities include coring and testing in these target areas with the hope of identifying smaller regions containing the best potential for commercialization. potential for commercialization. The second question is: What set of geologic/ chemical/physical conditions exist which cause coal seams in certain areas to be "gassy" while in other areas they are not? The approach taken to answer this question has been strictly empirical; a computer program designed to perform multiple variable statistical analyses of the data is being used at the Indiana Geological Survey. Early efforts have been inconclusive, but it is hoped that as more information is generated and added to the data base significant relationships between gas content and various parameters can be determined.
The Illinois basin contains extensive bituminous coal reserves in Pennsylvanian age rocks. The U.S. Geological Survey has estimated that the total coal resource of the Illinois basin might be 3.3 × 10(5) kg (365 billion short tons). More than 75 individual coal seams have been identified in this area, 20 of which are mined. A general stratigraphic correlation of major coal seams in the basin is provided in Figure 1. The majority of the coals are not continuous and do not maintain constant thicknesses. Individual seams range from 5 cm to 4.6 m (a few inches to 15 ft) in thickness. The coals outcrop at the basin periphery and dip gently towards the deeper portions in southeastern Illinois and western Kentucky. Lower and upper Pennsylvanian coals are thin and discontinuous Pennsylvanian coals are thin and discontinuous making them a relatively unattractive gas reservoir while the middle Pennsylvanian coals are thick, generally continuous, and provide the major reserves of the basin. The greatest cumulative thickness of coal seams presumably occurs in the southeastern portion of the basin (near the tri-state Illinois, portion of the basin (near the tri-state Illinois, Kentucky, Indiana boundary) where the thickest Pennsylvanian section occurs. All Illinois basin Pennsylvanian section occurs. All Illinois basin coal seams are covered by less than 914 m (3,000 ft) of overburden, and the major coals are within 457 m (1,500 ft) of the surface.