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
Coalbed methane fields
Central Appalachian Basin
CBM production in the central Appalachian basin is virtually all from CBM fields of Virginia (Fig. 1), where it comes mainly from the Nora (Dickenson and Russell Counties) and Oakwood (Buchanan County) fields; four smaller CBM fields of more limited CBM production occur in Wise and Buchanan Counties (Nolde, 1995). The Nora field contains a relatively larger number of conventional gas wells (R.C. Milici, U.S. Geological Survey, written commun., 1996) The Valley coal fields and the Richmond and Taylorsville Basins of Virginia do not produce commercial CBM.
Northern Appalachian Basin
Historically, CBM from the Pittsburgh coal bed has been produced in commercial quantities since 1932 and 1956 from the Big Run and Pine Grove fields, respectively, of Wetzel County, West Virginia (Repine, 1990; Patchen et al., 1991). Wells in these historic fields have been shut in. There was also historic CBM production from the Freeport coal zone in Carroll County, Ohio.
As shown in Figure 2, there are six CBM fields in southwestern Pennsylvania and two in the northern West Virginia (West Virginia Geol. Survey and Pennsylvania Topographic and Geologic Survey, 1993; Bruner et al., 1995). These are the Oakford, Gump, New Freeport, Lagonda, Waynesburg and Blairville fields in Pennsylvania, and the Big Run and Pine Grove fields in West Virginia. The multipurpose borehole in Monongalia County, West Virginia, as shown in Figure 2, was used for horizontal degasification from the Pittsburgh coal bed from 1972 to1980.
Coalbed methane stratigraphy
The most important coal beds with CBM production and(or) potential for production in the central and northern Appalachian basin are shown in Figure 3. The coal stratigraphy of the Southwest Virginia coalfield, where most of the 1995 CBM production in the central Appalachian basin exists, can be found in Englund and Thomas (1990) and Nolde (1994). In northern West Virginia and southwestern Pennsylvania, the coal stratigraphy is summarized in Arkle et al. (1979), and the coal beds of importance for CBM exploration and development are given in Bruner et al. (1995). For Ohio, the coal-bed stratigraphy is summarized in Collins (1979). For Tennessee, the coal stratigraphy is summarized in Glenn (1925) and Wilson et al. (1956), and for Maryland in Swartz and Baker (1920) and Lyons and Jacobsen (1981).
Depths to coal beds and coalification
In most CBM studies, coal beds less than 500 ft and more than 6,000 ft below the surface are excluded in resource calculations (Kelafant and Boyer, 1988; Patchen et al., 1991; Rice, 1995), although there are rare cases of CBM production at shallower depths. In Virginia, the principal known CBM reservoirs are the Lower Pennsylvanian Pocahontas and Lee coal beds at depths of 500-3000 ft (Fig. 3; Stevens et al., 1996, p. 43). A summary of depths to individual CBM target beds in the central Appalachian basin is in Rogers (1994). In the Big Run and Pine Grove fields of northern West Virginia, CBM was being produced from the Pittsburgh coal bed at depths from 475 to 997 ft (Patchen et al., 1991). Target coal beds in three coal tests in Greene County by Equitrans Inc.(a subsidiary of Equitable Resources Exploration) were at depths of 2,100 to 2,350 ft (PRI, 1991).
The CBM fields in northern West Virginia and southwestern Pennsylvania are in areas where the cumulative coal thickness varies from 10 to 30+ ft (generally 10-19 ft) and where single coal beds of mainly high volatile B/A bituminous rank are as much as 12 ft thick. The Pittsburgh coal bed, which was the principal CBM producer in West Virginia in 1994, is a thick and laterally extensive Appalachian coal bed (Cross, 1952).
Stach et al. (1982, p. 242) distinguished four coalification jumps in bituminous and anthracitic coals. The first and second coalification jumps correspond to the start and end of oil generation--vitrinite reflectance of 0.6% and 1.3% Rm, respectively. The third and fourth coalification jumps, which correspond to the release of large amounts of methane and aromitization of vitrinite, are at 2.3% and 3.7% Rm (Stach et al., 1982) respectively. Important economic gas deposits first appear where the vitrinite refelectance is 1.0% Rm (high volatile A bituminous coal) and peak at about 2.0% Rm, which corresponds to semianthracite, according to Stach et al. (1982, p. 45, 402-403). The gas Ždeath lineŪ is unknown according to these authors. However, it is clear that much of the economic CBM is generated between the first and fourth coalification jumps, which correspond mainly to high volatile bituminous coal to semianthracite.
It is generally assumed that most of the thermogenic methane comes from liptinite macerals when they reach a maturation of high volatile A bituminous coal (e.g., see Rogers, 1994). Although liptinite macerals are certainly an important source of CBM, they cannot account for the comparatively larger amounts of CBM in low volatile bituminous coal and anthracite that must have produced substantial amounts of CBM from non-liptinite macerals, probably from the cleaving of aliphatic chains from vitrinite during aromitization. Rogers (1994) has shown that 80-95% of the CBM thermally generated in coals of low volatile bituminous and anthracitic ranks escaped when CBM exceeded the adorptive capacity of the micropores. This author suggested that CBM retention is about an order of magnitude less in Appalachian coals than methane generated at bituminous ranks and that as much as 30,000 cf/ton of CBM could be generated through the anthracite rank. If the gas content of coals in the Anthracite region of eastern Pennsylvania is at a maximum of 687 cf/ton (see section on desorption data), then these anthracites are retaining only a few percent of their original thermogenic CBM.
The target coal beds for CBM in the central Appalachian basin are dominantly low volatile bituminous coal and a smaller amount of medium volatile bituminous coal (Nolde, 1995). The shallower coal beds such as the War Creek, L. Seaboard, and Jawbone (Fig. 3) are mainly of low and medium volatile bituminous rank, but high volatile A bituminous rank is also known (Kelafant and Boyer, 1988).
In the bituminous coal fields of the northern Appalachian basin, the rank of the coal ranges from high volatile B bituminous coal to low volatile bituminous coal, generally increasing in rank in an eastward direction towards the Allegheny Front. Lyons (1988) has suggested that the rank of the coal in Maryland follows structure, the highest ranks following the axial trends. This may be an important consideration in CBM development just west of the Allegheny Front in Maryland and Pennsylvania.
In Virginia, the Valley coal fields contain low volatile bituminous coal and semianthracite (Merrimac and Langhorne coal beds, Price Formation, Lower Mississippian) (Englund et al., 1983; Simon and Englund, 1983). The total gas from these coals from two test wells averages about 220 cf/ton at depths from 1,110-1,462 ft; total coal thickness for the Merrimac and Langhorne coal bed intervals varied from 0.45-6.70 ft) (Stanley and Schultz, 1983). The Merrimac and Langhorne coal beds average 5 ft and 3 ft thick, respectively, where they have been historically mined (see data in Campbell et al., 1925). At the time of their report, these beds reportedly did not have any economic potential for CBM development . However, these gas data indicate that there is a CBM economic potential for these two coal beds if thick and continuous coal beds can be located in these coal fields.
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