Coalbed methane potential in the Appalachian states of Pennsylvania,West Virginia, Maryland, Ohio, Virginia, Kentucky, and Tennessee--An overview
Paul C. Lyons
Open-File Report 96-735
Potential for undiscovered CBM
The CBM potential of coal beds for undiscovered CBM is related to thickness, rank, permeability, depth below the surface, and other factors. Within the Black Warrior and Appalachian basins, the gas content of coals increases with depth for coals of the same rank and also increases from high volatile A and B to low volatile bituminous coal. However, the CBM content of low volatile bituminous coals from various basins shows great differences in gas contents (McFall et al., 1986; Kelafant and Boyer, 1988), which suggests factors other than just rank are involved in CBM potential.
Central Appalachian Basin
Curiously, the central Appalachian basin--in contrast with the Black Warrior, northern Appalachian, San Juan, and Piceance basins--has the highest CBM content at depths between 1,500 and 3,000 ft (Kuuskraa and Boyer, 1993). This may be related to the greater permeability of central Appalachian basin coal beds due to structural or other regional factors.
A substantial part of Appalachian CBM technically recoverable CBM resources are in the central part of the Appalachian basin (Gautier et al., 1995; Rice, 1995; Attanasi and Rice, 1995). These resources using present-day technology were estimated at 14.84 Tcf (trillion cubic feet), including 4.43 Tcf confirmed and 10.41 Tcf hypothetical resources.
In the central Appalachian basin, six target seams of medium and low volatile bituminous rank are targeted for CBM production (Kelafant and Boyer, 1988). In stratigraphic order (see Fig. 3), with corresponding estimated gas in place (>500 ft depth, >1 ft coal), these are:
|Coal bed (Wv./Va. names)||Gas in place (Tcf)|
|Fire Creek/ L. Horsepen||0.7|
|Pocahontas No. 4||1.1|
|Pocahontas No. 3||1.6|
Rice (1995) determined a mean estimated ultimate recovery per well of 521 MMcfg and 3.068 Tcf of technically recoverable CBM in the central Appalachian basin, which is at odds with the in place CBM resources of 5.0 Tcf Kelafant and Boyer, 1988), which should be a much higher value is Riceís (1995) estimate is reasonable. Earlier DOE estimates, as referred to in Kelafant and Boyer (1988), indicate 10-48 Tcf of CBM in place in the central Appalachian basin, and Riceís (1995) estimate of 3.068 Tcf is more compatible with the earlier estimates.
Kelafant and Boyer (1988) estimated an additional 0.6 Tcf in minor CBM coal beds in the Pocahontas and New River Formations. The great potential for CBM development in Virginia is shown by the growth in annual production (Fig. 5) which in 1994 is 28,331,817 Mcf, corresponding to a value of about $62,747,013 at $2.15/Mcf (Jack Nolde, Virginia Division of Mineral Resources, personal commun., April, 1996)
In the Valley Coal fields of southwestern Virginia (Fig. 8) in the Valley and Ridge Province, there is probably some CBM potential for recoverable CBM (Nolde, 1995; see also Stanley and Schultz, 1983). The chemical analysis of 20 samples from test drilling in 1982-83 (Englund et al., 1983) indicates the rank varies from medium volatile bituminous coal to semianthracite (Simon and Englund, 1983). These are among the optimum ranks for thermogenic generation of CBM (Das et al., 1991).
Nolde (1995) has estimated at least 0.3 Tcf of in-place CBM in the Richmond basin of Virginia (Fig. 8). This work was done by Virginia Polytechnic Institute. This Triassic basin is virtually unexplored as a basin for CBM development.
In southeastern West Virginia, there is a substantial potential for CBM development. The average gas content for deep coal beds in Wyoming and Rayleigh Counties, West Virginia (Kelafant and Boyer, 1988) is 385 and 322 cf/ton, respectively. There were no CBM gas-content data reported for nearby McDowell County (Diamond et al., 1986). These data suggest a CBM potential for these three counties similar to that in Buchanan and Dickenson Counties, Virginia, which have average gas contents of 514 and 200 cf/ton, respectively (Diamond et al., 1986; Kelafant and Boyer, 1988). These two Virginia counties have most of the current CBM production in the central Appalachian basin. Webster County to the north in central West Virginia has little or no potential for CBM development judging from the average gas content of 22 cf/ton (Kelafant and Boyer, 1988).
There is an unknown CBM potential in southeastern Kentucky. There is little published information on the CBM potential of that area of Kentucky (B.C. Nuttall, Kentucky Geological Survey, personal commun., April, 1996). However, judging from the average gas contents of 52-90 cf/ton (Kelafant and Boyer, 1988), the potential of this area for undiscovered recoverable CBM is limited.
In the Cumberland Plateau of Tennessee, there is an unknown potential for undiscovered recoverable CBM. Coal beds are up to 14 ft thick and occur at maximum depths from about 600 to 1,900 ft below the surface (Wilson et al., 1956; Luther, 1960). Some of the thicker coal beds are the Big Mary, Windrock, Joyner, Poplar Creek, Wilder, and Sewanee coal beds. The thicker beds generally average 3.5 to 4.5 ft thick, except for the Big Mary coal bed that averages 6 to 8 ft thick (Glenn, 1925). Chemical data in Glenn (1925) indicate that most of the coals are of high volatile B and A bituminous ranks. There is little known about the gas content of these coals. The Sewanee coal bed has a total gas content ranging from 32 to 83 cf/ton)at depths of 821-825 ft (Diamond et al., 1986), which are low gas contents. However, more gas tests need to be made in beds at greater depths in order to determine the CBM potential of these coal beds.
Northern Appalachian Basin
In the northern Appalachian basin, the in place CBM resources have been estimated by Adams et al., (1984). These are shown in stratigraphic order (see Fig. 3):
|Coal bed or group (gp.)||Area (sq. mi)||Gas in place (Tcf)|
|Waynesburg coal bed||7,000||2.0|
|Pittsburgh coal bed||12,600||7.1|
Rice (1995) accepted this estimate of in-place CBM resources for his national assessment and reported 11.48 Tcf (10.41 Tcf for syncline play and 1.07 Tcf for anticline play) as technically recoverable gas. He used a mean estimated ultimate recovery per well of 121 and 216 MMcfg for the anticline and syncline plays, respectively. More work is necessary to refine these estimates. The greater CBM potential of the lower coal beds in the northern Appalachian basin is due to their higher rank and greater total gas content (Kelafant and Boyer, 1988; Hunt and Steele, 1991a; Markowski, 1993; Bruner et al., 1995).
The CBM potential of the Anthracite region of eastern Pennsylvania (Fig. 8) has not been determined. Two core holes were drilled in the mid-1970s (J. R. Levine, Consulting geologist, Tuscaloosa, Alabama, personal commun., April, 1996) and desorption data were reported in Diamond and Levine (1981) and Diamond et al. (1986). High gas contents were measured for the Peach Mountain coal bed in Schuylkill County at 685 ft. There are also some data in Diamond et al. (1986) showing considerably lower CBM contents in the same county. These data suggest a possibility for CBM development in some parts of this region where permeability and structural factors are not a problem.
A potential for recoverable CBM may exist in the coal fields of western Maryland and adjacent parts of Pennsylvania. The most promising areas in Maryland are the Georges Creek (Fig. 8) and Upper Potomac (northern part) coal fields where the rank is highest and the total coal and overburden is greatest (Swartz and Baker, 1920; Lyons and Jacobsen, 1981). In these fields, the rank varies from medium volatile to low volatile bituminous coal. The most promising targets are Allegheny coals--Mount Savage, Kittanning and Freeport coals--which occur up to about 1500 ft below the surface along the axis of the synclines. These coals are commonly 2-5 ft thick in these fields; the Upper Freeport is as much as 11 ft thick in the southern part of the Upper Potomac coal field. The Pottsville coals (Sharon, Quakertown, and Mercer; see Fig. 3) are thin (usually about 1-2 ft thick) and discontinuous, and, in spite of their greater depth, probably would not be good targets for CBM development in Maryland, except as part of mutiple-bed CBM production.
Ohio has a fair potential for CBM development from Allegheny coal beds underneath Monongahela and Dunkard strata (see Couchot et al., 1980, fig. 3) immediately west of the Ohio River. Data on deep coal resources of Ohio are in Struble et al. (1971), Collins and Smith (1977), and Couchot et al., (1980). In eastern Ohio, the counties with the greatest CBM potential are Belmont, Monroe, Washington, and Meigs Counties where there is the thickest and most areally extensive cover of Dunkard and Monongahela strata (see Couchot et al.,1980, fig. 3) above Allegheny coal beds at depths greater than 500 ft. The most promising coal beds for CBM recovery are the Bedford (in Upper Mercer coal zone) and Allegheny coals--Brookville, Lower and Upper Kittanning, and Lower and Upper Freeport--which collectively are as much as 18 ft thick or more in certain areas. These coals beds lie as much as 1,500 feet below the surface and are of high volatile A/B bituminous rank (Berryhill, 1963). There is a very limited CBM potential for the Meigs Creek coal bed (=Sewickley coal bed; Berryhill, 1963) and Pittsburgh coal bed in local areas where there is a thick Dunkard cover and where these coal beds are thickest, such as in Belmont and Washington Counties (Berryhill, 1963; Collins and Smith,1977; Couchot et al., 1980). In these counties these two beds occur in mineable thicknesses as much as 5.7 and 9.6 feet thick, respectively.
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