[Paper presented to the 1997 International Coalbed Methane Symposium in Tuscaloosa, AL]
Chesnut, D.R., Jr.¹, Nuttall, B.C.¹, Hower, J.C.², Greb, S.F.¹, Eble, C.F.¹, Hiett, J.K.², and Williams, D.A.¹
¹Kentucky Geological Survey, 228 Mining and Mineral Resources Bldg., University of Kentucky, Lexington, KY 40506; ²Center for Applied Energy Research, 3572 Iron Works Pike, University of Kentucky, Lexington, KY 40511
Kentucky has two bituminous coal fields, the Eastern Kentucky Coal Field in the Appalachian Basin and the Western Kentucky Coal Field in the Illinois Basin. Early studies incorrectly suggested that Kentucky had little potential for coal-bed methane because there were no deep coals, and the gas content and rank of early samples collected were too low. Unfortunately, as a result of these early studies, funding agencies provided limited support for coal-bed methane research in the State.
A new survey of information suggests that there is significant potential for coal-bed methane resources. Historic information from gas well records and State mining records provide evidence that numerous coals have contained significant quantities of methane in both of Kentuckys coal fields. New studies of the stratigraphic and structural framework of the coal-bearing rocks indicate that a large number of coals lie more than 2,000 ft. (610 m) below the surface. Vitrinite reflectance studies indicate that coal rank is sufficient for coal-bed methane production. Even more encouraging, the results of calculations using extensive coal quality data, suggest that economic quantities of coal-bed methane exist in both coal fields. Several gas exploration and coal companies have begun programs to explore for coal-bed methane in the State.
In the Eastern Kentucky Coal Field almost all of the historic production of coal has been from surface and drift mines in coal beds lying above valley bottoms. As this above-drainage coal is being depleted, mining has begun to shift to coals lying below drainage. Mining companies are increasingly confronted with problems with methane. Producing methane as a commercial resource could reduce the risk of mine explosions and provide new gas supplies for local and regional markets. This development would be facilitated if government and industry begin supporting research to examine the gas content and production potential of Kentuckys deeper coals.
The Commonwealth of Kentucky has been a leading producer of bituminous coal in the U.S. for many decades. Although Kentuckys coal resource has been considered to be vast, its coal-bed methane resource remains virtually unknown. A new survey of information suggests that there is significant potential for coal-bed methane (CBM) resources. This paper provides data that supports this potential for economic CBM in the state and is a guide to information helpful to the exploration of CBM.
Kentucky has two bituminous coal fields, the Eastern Kentucky Coal Field (EKCF) in the Appalachian Basin and the Western Kentucky Coal Field (WKCF) in the Illinois Basin (Fig. 1). Both coal fields are comprised of Pennsylvanian-age strata. Coal-bed outcrops and surface structural features of the coal fields are illustrated on 7-1/2-minute geologic quadrangle maps. Kentucky was the first state (and currently one of only two states) in the U.S. that is completely covered by these detailed (1:24,000 scale) geologic maps and they provide valuable information for the exploration of CBM. These geologic quadrangles are available at the Kentucky Geological Survey.
The depth of coals is an important consideration for CBM. Shallow coals (less than 300 feet) tend to have very low CBM content because much of the methane has leaked to the atmosphere. Therefore, a stratigraphic and structural framework that shows the depth of coal-bearing rocks is necessary when evaluating the CBM potential of a region. These frameworks are discussed below.
Subsurface information is available in the form of oil and gas well records, core, and core descriptions. The Kentucky Geological Survey (KGS) is the official repository of oil and gas well records. Drillers logs, geophysical logs, cores, and core description are on file at the Survey.
Western Kentucky Coal Field.--Detailed information about the stratigraphic and structural framework of this coal field can be found in Greb and others (1992). The deepest part of the Illinois Basin (also known as the Eastern Interior Basin) is in Kentucky. Coal-bearing rocks as deep as 2,500 feet occur south of the Rough Creek fault system (Figs. 2, 3). Coals in this field range in age from Early Pennsylvanian to Late Pennsylvanian (Westphalian A through Stephanian), although most production has been from the Carbondale Formation in the late Middle Pennsylvanian part (Westpahlian D) of the stratigraphic section (Fig. 4). The coal-bearing formations consist of shale, siltstone, sandstone, coal and some limestone. Massive sandstones within the Caseyville Formation (Early Pennsylvanian) occur at the expense of typical coal-bearing rocks, although some thin coal beds are associated with the sandstones.
Eastern Kentucky Coal Field.--Chesnut (1992) reported detailed information about the stratigraphic and structural framework of the Eastern Kentucky Coal Field. Unlike the Western Kentucky Coal Field, where topography is gently rolling, the topography of the Eastern Kentucky Coal Field is heavily dissected with topographic relief more than 2,000 feet in some places. Because of the topographic relief, the terms below drainage and above drainage are especially useful when describing the position of certain coal-bearing rocks. Figure 5 is one of six cross sections from a report (Chesnut, 1994) that describes coal-bearing rocks below drainage in eastern Kentucky. Coal-bearing rocks are as deep as 2700 feet below drainage in parts of southeastern Kentucky.
The coal-bearing rocks in eastern Kentucky range from Early Pennsylvanian to Late Pennsylvanian in age. However, most of the past coal mining has been in the Breathitt Group (Fig. 6) of Early and Middle Pennsylvanian age. The coal-bearing formations consist largely of shale, siltstone, sandstone, and coal. Massive sandstones such as the Sewanee, Bee Rock, and Corbin occur in the lower part of the Breathitt Group at the expense of typical coal-bearing rocks of the Breathitt, although some coal beds are associated with the sandstones. Accurate coal correlations across this coal field are found in Rice and Hiett (1994).
The history of mining is valuable information when evaluating a region for CBM resources. Abandoned deep mines may be reservoirs for methane, especially in the gob (roof breakdown). In addition, methane encountered during the mining of coal is a good indication of economic gas contents.
More than 84 ignitions have been recorded in Kentuckys two coal fields since 1887 (Fig. 7)(Humphrey, 1959;1960; Richmond and others, 1982 ; Kentucky Department of Mines and Minerals Annual Reports). Many of the ignitions occurred in mine shafts and old works where methane was allowed to accumulate, but there have also been numerous ignitions at the face.
Western Kentucky Coal Field.--Because of the gently rolling topography of this coal field, most of the deep mining has been by slope or shaft methods. The majority of underground mines are room and pillar mines, although longwall operations also occur. Coal has been shaft mined to depths of 1250 feet below the surface.
By far the most CBM ignitions (30) in Kentucky have been recorded in the Springfield (Western Kentucky No. 9) coal of western Kentucky (Fig. 8). The large number of ignitions may partly be a function of the large amount of mining in the Springfield coal, the number one producer in Kentucky (27.9 MT annual production), and the fact that the Springfield coal was one of the first coals to be extensively deep mined below drainage in Kentucky. Most of the explosions in the Western Kentucky Coal Field have occurred in Hopkins (14), Union (12), and Webster (9) counties, which are structurally in the deepest part of the basin (Fig. 2). It is not uncommon for underground mines in the Springfield coal to have repeated ignitions at depth. Multiple ignitions in underground mines may be a function of ventilation and duration of mining, but also illustrate potential for CBM accumulation in certain coal beds.
The most concentrated area of methane ignitions in western Kentucky was around the town of Madisonville, Ky. where seven ignitions have been reported from mines in the Central fault zone (Fig. 9). In at least one instance, ignitions in both the Herrin (Western Kentucky No. 11) and Springfield coals occurred in the same vicinity along one of the faults. Other areas have similar density of mining, without the same concentration of ignitions suggesting that the faults may have enhanced the gas content of the coal in this area.
As in other coal fields, structural traps may prove to be important in CBM exploration strategies. In Figure 9 it is also interesting to note that many of the ignitions occurred within 2 kilometers of the surface outcrop of the coal, less than 400 under cover, which may indicate that CBM is available at shallow depths. This suggests that some seams may have large amounts of methane even near the surface.
Measurements of methane liberation at the fans of deep mines in western Kentucky, provided by Mine Safety and Health Administration (MSHA), indicate that the Baker (Western Kentucky No. 13) and Springfield (Western Kentucky No. 9) coals have the largest amount of methane. One mine in the Baker had a maximum of 1.69 million cubic feet per day, and a mine in the Springfield coal had a maximum of 1.75 million cubic feet per day. Limited data from mines in the Springfield suggest an average value of 333 thousand cubic feet per day.
Eastern Kentucky Coal Field.--Because of the mountainous topography and abundant near-surface coal resource in eastern Kentucky, most deep mining has been by drift methods; mostly in room and pillar operations above drainage.
The low number of methane ignitions in eastern Kentucky (39) can undoubtedly be attributed to the fact that most of the regions mining was from drift mines in coals that outcropped at the surface. An analysis of known ignitions in the region shows that most have been concentrated in the deeper part of the basin (Fig. 7) in Harlan (9 ignitions), and Pike (8) counties. Six ignitions have also been recorded in Martin County, all from the same mine in the Pond Creek (Lower Elkhorn) coal. This coal is part of the Elkhorn coal zone, which has had the largest number of recorded ignitions in eastern Kentucky (Fig. 10). The concentration of ignitions in this coal zone is probably a function of the typical occurrence of this coal at or just below drainage. The Scotia mine disaster, which killed 26 miners and inspectors in two separate explosions, 60 hours apart, was in the Imboden coal, a Lower Elkhorn equivalent. The coal had been producing 250,000 cubic feet of methane a day, which was not considered excessively gassy (Richmond and others, 1982). Discussions with MSHA ventilation experts in eastern Kentucky indicate that where the Pond Creek coal and its equivalent coals are mined below drainage it generally liberates 500,000 cubic feet of methane a day, with as much as one million cubic feet liberated daily from one mine. These preliminary measurements suggest the potential for increasing methane concentrations in the stratigraphically lower coals, below drainage, in eastern Kentucky.
For purposes of discussion coal quality here includes ultimate and proximate analyses, rank, and petrographic descriptions. Certain trends in coal quality reflect CBM content and certain coal-quality parameters can be used to predict this content For example, increasing rank (metamorphic grade) of coal usually parallels increasing gas content. Bright coals, those containing abundant vitrinite macerals (organic mineral analogues), also tend to have higher gas contents. For these reasons, coal quality data can be very important in CBM exploration. Several thousands of coal quality analyses are available in the KGS Kentucky Coal Resource Information System (KCRIS) and Kentuckys Center for Applied Energy Research (CAER) databases. Some of these data were used to illustrate the following trends.
Rank.--Coal rank is the metamorphic grade of a coal and reflects the level of changes that a coal has undergone since its deposition as a peat. Rank is often measured by either moisture content, calorific value (e.g., BTU) (measured by a laboratory calorimeter), volatile matter content, or by vitrinite reflectance Rmax (determined microscopically). In general, metamorphism results in the formation of large polyaromatic molecules, with functional groups (e.g. methoxy, carboxyl, and hydroxyl) being liberated with increasing ring saturation. This causes the release of, among other things, methane gas.
To highlight areas with good CBM potential, we have constructed a highly generalized rank map based on vitrinite maximum reflectance (more detailed rank maps are available from KGS and CAER). According to Dr. Jack Pashin (Alabama Geological Survey, person. commun., 1996) economic contents of CBM in the Black Warrior Basin of Alabama are restricted to areas where vitrinite maximum reflectance Rmax is greater than 0.9.
Figure 11 are vitrinite reflectance maps of Kentucky coal fields that emphasize this reflectance cutoff. Data for these maps were from coals situated above drainage. As most CBM exploration targets would be below the mapped coals, reflectance of the target coals would generally be higher than the mapped coal reflectance. Coal rank in eastern Kentucky generally increases to the southeast. The highest rank measured in Kentucky was 1.1 Rmax from a coal in southeastern Kentucky next to the Virginia border. Hower and Rimmer (1991) identified several anomalous rank highs, superimposed upon the prevailing rank trend. They proposed that these anomalies were caused by differing geothermal gradients associated with basement discontinuities identified by Ammerman and Keller (1979). High reflectance trends in western Kentucky appear to be fault controlled; higher reflectance areas are situated in graben settings. In both coalfields, areas with higher rank (Figure 11) were similar to areas with reported methane emmissions in underground mines (Figure 7). Higher rank areas are also found in the Webster syncline portion of the Moorman syncline.
Kim Formula.--Because of the near absence of directly-measured gas contents of coals in Kentucky, other methods must be used to estimate gas contents. The Kim relationship (Kim, 1977; Gas Research Institute, 1995) is an empirical formula developed specifically for use in estimating gas content of coals utilizing proximate analysis and depth information. A proximate analysis includes: as determined moisture, volatile matter, fixed carbon and ash yield. The Kim formula uses these readily available data to calculate an estimated gas storage capacity.
With due consideration of its limitations, the Kim relationship was used for qualitative evaluation of the coal data in this reconnaissance study of CBM in Kentucky. Proximate analyses data were obtained from KCRIS; a total of 704 records were used for the calculation. The calculated gas content (Fig. 12) ranged from a minimum of 15 cubic feet of gas per ton of coal (cf/ton) to a maximum of 600 cf/ton, with a median value of 324 cf/ton. In the Black Warrior Basin of Alabama, a coal-bed gas conent of at least 250 cf/ton is considered economic and the best production comes from coals containing at least 400 cf/ton (Jack Pashin, 1996, personal commun.) Nearly 75 percent of the Kentucky coal samples studied are in these categories shown as "Good" and "Best" on Figures 11 and 12.
Number of Samples
Median Gas Content
Smath and others, 1985
Diamond and others, 1986
Diamond and others, 1986
Direct methods of estimating gas content of coals require periodically measuring the volume of gas desorbed from a coal sample that has been sealed in a canister. Some desorption data for Kentucky coals has been published. In their review of coal-bed gas in the central Appalachians, Kelafant and Boyer (1988, fig. 8) show several county average CBM contents for Kentucky, but concentrate their study mainly on the Pocahontas coals of Virginia and West Virginia. Data from Smath and others (1985) and Diamond and others (1986) is summarized in Table 1. Samples from Diamond and others may have been collected from outcrop and above-drainage mine faces in eastern Kentucky. Gas contents of coals in above-drainage mines are predicted to be very low, so the low gas contents are not surprising. Coal samples used by Smath and others were from boreholes in western Kentucky drilled specifically to determine the gas contents. For some reason, this borehole program mainly encountered thin coals although western Kentucky is known to have extensive reserves of thick coal. Unfortunately, the poor gas shows from these early studies served to block funding for subsequent research on CBM in Kentucky.
The U.S. Geological Survey considers CBM an "undiscovered" resource in their latest assessment of the national petroleum resource (Beman and others, 1996; Charpentier and others, 1996; Gautier and others, 1996). From data supplied in Gautier and others (1996), CBM resources are estimated to be 130.2 billion cubic feet of gas in place for the Western Kentucky Coal Field and 153.4 billion cubic feet of gas in place for the Eastern Kentucky Coal Field.
Eastern Kentucky Coal Field.--In 1957, E. M. Howard drilled three wells on the Nolan Heirs lease in Carter Coordinate section 9-E-74, Harlan County, eastern Kentucky (Fig. 13). [Carter Coordinates are a longitude and latitude-based grid used to officially locate oil and gas wells in Kentucky.] According to records on file in the Geologic Data Center of the KGS, two of the wells were completed in coals with initial open flows of 75 and 80 thousand cubic feet of gas per day. The producing coal has been tentatively identified as the Lower Elkhorn. The Lower Elkhorn is the coal from which the most coal mine methane ignitions have been reported in eastern Kentucky (Fig. 8) These wells apparently were being used as a domestic gas supply as late as 1980. The production history and cumulative production volume of these wells is unknown.
In 1990, Equitable Resources Exploration completed their No. KF1300 Fee well in section 14-I-83, Letcher County (Fig. 13). Eleven unnamed coal seams at drilling depths between 1,113 feet and 2,504 feet were stimulated with a multiple-stage foam and nitrogen fracture treatment including sand as a proppant. After stimulation, the reported open flow was 5,000 cubic feet of gas daily with a shut-in pressure of 395 pounds after 48 hours. No production history is available to evaluate the success of this well.
Western Kentucky Coal Field.--The Energy Information Administration of the U.S. Department of Energy reports the Sebree Consolidated field, Carter Coordinate N-24, of western Kentucky (Fig. 13) as the only coal-bed gas field in Kentucky (Energy Information Administration, 1995). While there are 32 gas wells in the area with production reported from the coal-bearing Carbondale and Tradewater Formations, no wells are specifically identified as coal-bed gas wells. However, CBM has been produced from gas wells in old works of the Springfield coal in Illinois, and similar situations may exist for production in western Kentucky.
While this paper presents compelling evidence of a viable resource, future research is essential for evaluating and locating CBM resources in Kentucky. Very little is known about the deep below-drainage coals in either coalfield because boreholes were rarely drilled into these coal-bearing rocks. A reconnaissance drilling program to investigate the occurrence and gas content of deep coals is desirable and a consortium of industry and government geological surveys would be an ideal partnership to undertake such a program.
Other research and data collection must focus on coal rank and quality data, and their relationship to depth. The Kim relationship must be validated with desorption data from Kentucky coals. Finally, the gas content between coals of differing composition must be investigated.
CBM exploration by industry should consider both coal fields as a potentially economic source of coal gas.
Ammerman, M.L., and Keller, G.R., 1979, Delineation of Rome Trough in eastern Kentucky by gravity and deep drilling data. American Association of Petroleum Geologists Bulletin, v. 63, p. 341-353.
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Figure 1. The Western and the Eastern Kentucky Coal Fields are identified by the outcrop of Pennsylvanian rocks.
Figure 2. Outcrop and structural features of the Western Kentucky Coal Field.
Figure 3. Cross section through the Western Kentucky Coal Field.
Figure 4. Generalized stratigraphic column of the Western Kentucky Coal Field.
Figure 5. Strike-oriented cross section of the Eastern Kentucky Coal Field.
Figure 6. Generalized stratigraphic column of the Eastern Kentucky Coal Field.
Figure 7. Areas of reported methane ignitions in deep mines. The ignition in western Kentucky may have been caused by a mine cutting through a gas well.
Figure 8. Stratigraphic occurrence of ignitions in western Kentucky. All of the ignitions in the Davis coal were from the same mine.
Figure 9. Ignitions associated with fault zones in western Kentucky. The crosshatched pattern shows the location of an underground mine in the Herrin coal, which also had an ignition.
Figure 10. Stratigraphic occurrence of ignitions in eastern Kentucky. Abbreviations for stratigraphic formations shown are AC, Alvy Creek Formation; GR, Grundy Formation; Pl, Pikeville Formation, HY, Hyden Formation; FC, Four Corners Formation, and PR, Princess Formation. Lower Elkhorn coal includes the following local coal names: Pond Creek, Blue Gem, Straight Creek, Imboden, and Pathfork coals. Upper Elkhorn 2 coal includes the following equivalents: Alma and Warfield coals. Upper Elkhorn 3 coal includes the Kellioka, Darby, Thacker, and Millers Creek coals.
Figure 11. Map showing areas of high (> 0.9) vitrinite maximum reflectance and significant gas contents (calculated by Kim formula).
Figure 12. Calculated gas contents using the Kim method.
Figure 13. Map of existing CBM production.
Table 1. Median coal-bed gas content in cubic feet of gas per ton of coal for eastern and western Kentucky.