A Preliminary Assessment Of In Place Coalbed

International Journal of Coal Geology 38 Ž1998. 115–136

A preliminary assessment of in place coalbed methane resources in the Virginia portion of the central Appalachian Basin Jack E. Nolde ) , David Spears Virginia DiÕision of Mineral Resources, P.O. Box 3667, CharlottesÕille, VA 22903, USA Received 2 May 1997; accepted 1 July 1998

Abstract In the central Appalachian basin, Virginia leads in coalbed methane drilling and production, even though it only contains a small fraction of the total coal resources and basin area. In 1992, coalbed methane surpassed conventional and shale gas as Virginia’s largest source of natural gas. By the end of 1996 there were 814 wells producing coalbed methane in Virginia. Average daily production per well in 1996 was 114 Mcf Ž3.2 Mm3 .. Cumulative production of coalbed methane from 1988 through 1996 is 121,542,188 Mcf Ž3,445,073 Mm3 .. Factors influencing this trend include the presence of abundant coal in place including coal remaining in ground after mining, sufficient overburden, high gas-content coal beds, high gas permeability, existing pipeline infrastructure, and legislative clarification of uncertain coalbed gas ownership. Development of coalbed methane in Virginia began in late 1988. Production is from coal beds in the southwestern Virginia coalfield, a structurally distinct area along the southeastern margin of the central Appalachian basin. Ten coal beds within the Lower Pennsylvanian Pocahontas and Lee Formations and the Lower Pennsylvanian portion of the Norton Formation have been targeted for production of coalbed methane. Estimated coal in place for these coal beds is about 14.9 billion short tons. Estimated gas contents of the coal beds range from 256 ft 3 Ž7.9 m3rt. to 698 ft 3rton Ž21.5 m3rt.. These data yield an estimated in-place coalbed methane resource of 6.7 Tcf Ž0.18 Tm3 . for Virginia. The U.S. Geological Survey reported the in-place coalbed methane resource for the entire central Appalachian basin to be 5 Tcf Ž0.13 Tm3 ., with 3.07 Tcf Ž0.08 Tm3 . technically recoverable. An estimated in-place coalbed methane of 6.7 Tcf Ž0.18 Tm3 . is more compatible with 3.07 Tcf of technically recoverable coalbed methane for the central Appalachian basin. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Central Appalachian Basin; coalbed methane resources; Virginia

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Corresponding author. Tel.: q1-804-293-5121; Fax: q1-804-293-2239

0166-5162r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 Ž 9 8 . 0 0 0 3 5 - 4

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J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

1. Introduction The central Appalachian basin, as defined by Adams et al. Ž1982. covers an area of nearly 23,000 mile 2 Ž59,500 km2 . in Virginia, West Virginia, Kentucky, and Tennessee. The Virginia portion ŽFig. 1., known as the southwestern Virginia coalfield, lies at the southeastern margin of the basin, where the basin is bounded by the southern Appalachian thrust belt. The southwestern Virginia coalfield covers an area of approximately 1520 mile 2 Ž3,937 km2 ., only 7% of the total central Appalachian basin area. Nevertheless, Virginia contains about 96% of this part of the basin’s producing coalbed methane wells. In the southwestern Virginia coalfield, coal and coalbed methane are produced in Buchanan, Dickenson, Russell, and Wise Counties ŽFig. 2.. At least 57 minable coal beds occur in southwestern Virginia ŽNolde, 1994.. Of these, 10 of the deeper coal beds are currently the targets for coalbed methane development. These coal beds are, in ascending stratigraphic order ŽFig. 3., Pocahontas No. 3, Pocahontas No. 6, Pocahontas No. 9, Lower Horsepen, Fire Creek, War Creek, Middle Horsepen, Upper Horsepen, Greasy Creek, and Upper Seaboard. Estimated coal resources for these coal beds amounts to 14.9 billion short tons. The release of methane gas in coal beds is a serious safety hazard to underground miners. Since 1884, underground mine accidents related to methane gas have occasion-

Fig. 1. Index map showing location of study area with respect to part of the northern and southern Appalachian basin Žstipple. and the central Appalachian basin Žheavy stipple..

J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

117

Fig. 2. Map showing coalbed methane pools Žhachured. and major structural features in the southwestern Virginia coalfield.

ally killed or injured miners and interfered with coal mining in southwestern Virginia. In that year, an explosion in an underground mine near the Town of Pocahontas killed 114 miners ŽHarnsberger, 1919.. Modern mine safety practices, including ventilation of methane gas, have reduced the risk, but have not eliminated accidents. As recently as 1994, an explosion attributed to methane gas occurred in a mine in south-central Buchanan County. In southwestern Virginia, natural gas has been produced from Devonian and Mississippian rocks since 1948 ŽHuddle et al., 1956.. Coalbed methane production in the overlying Pennsylvanian rocks began in 1988 ŽNolde, 1995.. Cumulative coalbed methane production in Virginia for 1988 through 1996 is 121,542,188 Mcf Ž3,445,073 Mm3 . ŽTable 1.. Current calculations of coal in place and gas content data suggest that there is 6.7 Tcf Ž0.18 Tm3 . of in-place coalbed methane resources in Virginia. This paper presents a overview of our present knowledge of the coalbed methane resources of southwestern Virginia. After first outlining the history of development and production and the methodology used to determine the methane resource, the stratigraphic setting of the resource is outlined. The subsurface distribution and significance of the ten coal beds is outlined. Conclusions are drawn as to why Virginia dominates coalbed methane production in the central and northern Appalachian basin Žsee Lyons, 1997.. 1.1. Methodology Data for the present study are from published geologic maps and approximately 900 coalbed methane wells and coal-exploration holes. As part of a continuing study of the subsurface geology of the southwestern Virginia coalfield, coal distribution and thickness maps were compiled for each of the ten coal beds that are targets for coalbed

118 J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136 Fig. 3. Generalized NW–SE stratigraphic cross-section across the central portion if the southwestern Virginia coalfield. Žcoarse stipple, quartz arenite and fine stipple, lithic arenite..

J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

119

Table 1 Coalbed methane production statistics Žthousand cubic feet, Mcf. for Virginia, 1988 to 1996 Year

Number of gas wells

Annual gas production ŽMcf.

Average monthly gas production ŽMcf.

Average daily gas production ŽMcf.

Cumulative gas production ŽMcf.

1988 1989 1990 1991 1992 1993 1994 1995 1996

2 10 39 85 277 465 492 681 814

11,856 181,526 799,569 1,142,651 6,641,852 19,923,463 28,331,817 30,355,870 34,153,584

494 1513 1708 1120 1998 3570 4799 3715 3496

16 50 56 37 66 117 158 122 114

11,856 193,382 992,951 2,135,602 8,777,454 28,700,917 57,032,734 87,388,604 121,542,188

methane development. The following procedure was used to estimate coal volumes. Coal bed distribution and isopach maps were generated using Dynamic Graphics Interactive Surface Modeling ŽISM. software. The isopach interval was set at 14 in. Overburden maps with a contour interval of 500 ft Ž152 m. were overlain onto the coal distribution and isopach maps to eliminate from tonnage calculations areas under less than 500 ft Ž152 m. of cover ŽTable 2. and areas with coal less than 14 in. thickness. A minimum depth of 500 ft Ž152 m. is generally required for coal to contain any appreciable amount of gas in place ŽKelafant and Boyer, 1988; Rice, 1995.. Polygons delineating the various thickness and overburden categories were constructed. The end product was a map color-coded so that polygons for each category could be distinguished. The area, in acres, was digitized for each polygon using the US Geological Survey’s GSDIG program ŽSelner and Taylor, 1992.. Coal tonnages ŽTable 2. were then calculated by multiplying the number of acres by the median thickness of the coal bed and the factor 1800 short tons per acre-foot Ž13,238 trha m.. Only the Pocahontas No. 3 coal bed has been mined and its mined-out area was subtracted from the estimate for coal in place. Gas-content data for 65 coal samples from the central Appalachian basin were published by Diamond and Levine Ž1981.. Sixty-one of these coal samples are of low volatile bituminous rank, and the remaining four samples are of medium volatile bituminous rank. The desorption values for the 61 low volatile bituminous coal samples ŽFig. 4. were used to determine the linear regression equation representing the change in gas content with depth: gas content Ž ft 3rton . s 152 q 0.2 = depth Ž ft . This equation was used to estimate gas contents for the target coal beds, which are summarized in Table 3. Estimated gas contents range from about 256 cu ft per ton Ž7.9 m3rt. to as much as 698 ft 3rton Ž21.5 m3rt.. The following formula was used to calculate the in-place coalbed methane resource: Gas resourcess Ž coal density. = Ž median coal thickness. = Ž gas content. = Ž area .

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J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

Table 2 Estimated coal resources Žmillion short tons. for target coalbed methane beds Coal bed and Overburden category Žft. thickness 500–1000 1000–1500 category Žin.. Pocahontas No. 3 14–28 0.00 50.45 28–42 42.57 56.94 ) 42 0.00 32.28 Total 42.57 139.67

1500–2000

2000–2500

10.93 171.91 1,377.57 1,560.41

46.01 30.45 79.77 156.23

) 2500

5.52 0.52 0.00 y 6.04

Mined or lost in mining

Estimated coal resource

0.00 0.00 0.21 0.00

112.91 302.39 1,489.41 1,904.71

Pocahontas No. 6 14–28 0.00 28–42 40.93 ) 42 0.00 Total 40.93

49.15 54.73 26.65 130.53

35.67 165.27 1,137.47 1,338.41

150.10 29.27 65.87 245.24

18.02 5.00 0.00 23.02

0.00 0.00 0.00 0.00

252.94 295.20 1,229.99 1,778.13

Pocahontas No. 9 14–28 53.24 28–42 48.67 ) 42 0.00 Total 101.91

283.39 111.24 66.67 461.30

217.25 168.74 0.00 385.99

6.88 0.00 0.00 6.88

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

560.76 328.65 66.67 956.08

Lower Horsepen 14–28 88.20 28–42 59.64 ) 42 0.00 Total 147.84

318.63 135.75 29.65 484.03

367.58 249.95 118.29 735.82

70.80 206.54 92.48 369.82

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

845.21 651.88 240.42 1,737.51

Fire Creek 14–28 28–42 ) 42 Total

10.19 42.70 11.14 64.03

211.46 207.53 41.02 460.01

58.19 158.37 70.89 287.45

26.28 157.76 105.58 289.62

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

306.12 566.36 228.63 1,101.11

War Creek 14–28 28–42 ) 42 Total

17.58 56.14 79.46 153.18

364.63 331.52 150.87 847.02

100.35 216.65 260.76 577.76

45.32 183.72 388.31 617.35

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

527.88 788.03 879.40 2,195.31

Middle Horsepen 14–28 130.96 28–42 111.14 ) 42 0.00 Total 242.10

545.09 399.96 26.62 971.67

89.23 101.15 0.00 190.38

0.00 14.74 0.00 14.74

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

765.28 626.99 26.62 1,418.89

Upper Horsepen 14–28 28–42 ) 42 Total

522.09 236.40 73.01 831.50

263.25 86.13 18.86 368.24

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

785.34 382.52 91.87 1,259.73

0.00 59.99 0.00 59.99

J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

121

Table 2 Žcontinued. Coal bed and thickness category Žin.. Greasy Creek 14–28 28–42 ) 42 Total

Overburden category Žft. 500–1000 1000–1500

1500–2000

2000–2500

) 2500

0.00 0.00 0.00 0.00

640.95 0.00 0.00 640.95

398.78 143.36 0.00 542.14

40.56 61.96 0.00 102.52

0.00 0.00 0.00 0.00

Upper Seaboard 14–28 700.10 28–42 0.00 ) 42 0.00 Total 700.10 Grand Total 2193.60

398.78 130.32 0.00 529.10 5396.97

40.56 56.33 0.00 96.89 5643.87

0.00 0.00 0.00 0.00 1699.88

0.00 0.00 0.00 0.00 29.06 y

Mined or lost in mining

Estimated coal resource

0.00 0.00 0.00 0.00

1080.29 205.32 0.00 1285.61

0.00 0.00 0.00 0.00 0.21

1139.44 186.65 0.00 1,326.09 14,963.17

The gas resource estimates are in units of trillion cubic feet ŽTcf.. All calculated values are resources or in-place gas, rather than reserves or recoverable gas. In-place coal estimates and gas contents were used to determine the coalbed methane resources for the 10 target coal beds.

2. Structural setting The southwestern Virginia coalfield lies along the southeastern margin of the central Appalachian basin ŽFig. 1.. The coalfield is partly within the Cumberland overthrust block, a large thrust sheet that extends from southwestern West Virginia, through

Fig. 4. Gas content vs. depth for central Appalachian basin coal beds Ždata from Diamond and Levine, 1981..

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J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

Table 3 Estimated coalbed methane resources Žtrillion cubic feet, Tcf. for target coal beds by overburden category, 0.12 Tcf gas has been produced from 1988 to 1996 Coal bed and thickness category Žinches.

Overburden category Žft. 500–1000 1000–1500

1500–2000

2000–2500

) 2500

Estimated gas resource ŽTcf.

Pocahontas No. 3 14–28 28–42 ) 42 total

0.00 0.01 0.00 0.01

0.02 0.02 0.01 0.05

0.01 0.09 0.69 0.79

0.03 0.02 0.05 0.10

0.01 0.00 0.00 0.01

0.07 0.14 0.75 0.96

Pocahontas No. 6 14–28 28–42 ) 42 total

0.00 0.01 0.00 0.01

0.02 0.02 0.01 0.05

0.02 0.08 0.57 0.67

0.09 0.02 0.04 0.15

0.01 0.01 0.00 0.02

0.14 0.14 0.62 0.90

Pocahontas No. 9 14–28 28–42 ) 42 total

0.02 0.02 0.00 0.04

0.11 0.04 0.03 0.18

0.11 0.09 0.00 0.20

0.01 0.00 0.00 0.01

0.00 0.00 0.00 0.00

0.25 0.15 0.03 0.43

Lower Horsepen 14–28 28–42 ) 42 total

0.03 0.02 0.00 0.05

0.13 0.05 0.01 0.19

0.18 0.13 0.06 0.37

0.04 0.12 0.06 0.22

0.00 0.00 0.00 0.00

0.38 0.32 0.13 0.83

Fire Creek 14–28 28–42 ) 42 total

0.01 0.01 0.01 0.03

0.09 0.08 0.02 0.19

0.03 0.08 0.04 0.15

0.02 0.10 0.10 0.22

0.00 0.00 0.00 0.00

0.15 0.27 0.17 0.59

War Creek 14–28 28–42 ) 42 total

0.01 0.02 0.02 0.05

0.15 0.13 0.06 0.34

0.05 0.11 0.13 0.29

0.03 0.11 0.23 0.37

0.00 0.00 0.00 0.00

0.24 0.37 0.44 1.05

Middle Horsepen 14–28 28–42 ) 42 total

0.04 0.03 0.00 0.07

0.22 0.16 0.01 0.39

0.04 0.05 0.00 0.09

0.00 0.01 0.00 0.01

0.00 0.00 0.00 0.00

0.30 0.25 0.01 0.56

Upper Horsepen 14–28 28–42 ) 42 total

0.00 0.02 0.00 0.02

0.21 0.09 0.03 0.33

0.13 0.04 0.01 0.18

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.34 0.15 0.04 0.53

J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

123

Table 3 Žcontinued. Coal bed and thickness category Žinches.

Overburden category Žft. 500–1000 1000–1500

1500–2000

2000–2500

) 2500

Estimated gas resource ŽTcf.

Greasy Creek 14–28 28–42 ) 42 total

0.19 0.00 0.00 0.19

0.16 0.06 0.00 0.22

0.02 0.03 0.00 0.05

0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00

0.37 0.09 0.00 0.46

Upper Seaboard 14–28 28–42 ) 42 total Grand Total

0.21 0.00 0.00 0.21 0.68

0.16 0.05 0.00 0.21 2.15

0.02 0.03 0.00 0.05 2.84

0.00 0.00 0.00 0.00 1.08

0.00 0.00 0.00 0.00 0.03

0.39 0.08 0.00 0.47 6.78

southwestern Virginia, and into Kentucky and Tennessee ŽHenika, 1994.. The coalfield is bounded on the southeast by closely spaced faults of the southern Appalachian thrust belt. This belt consists of the Hunter Valley, St. Paul, and Richlands thrust faults ŽFig. 2.. To the northwest, the Cumberland overthrust block is bounded by the Pine Mountain fault, a low-angle thrust fault that extends beneath part of the coalfield. The Cumberland thrust sheet is complicated by a number of internal structures, which include transverse faults and gentle folds ŽHenika, 1994.. All of these structures are interpreted to have been formed during the late Paleozoic Alleghany orogeny. Local structure does not appear to influence coalbed methane production, except to the extent that syndepositional structures may have influenced the accumulation of coal.

3. Stratigraphic and tectonic overview Paleozoic strata of the central Appalachian basin are related to the tectonic evolution of the Paleozoic continental margin. The first phase started during late Precambrian time, when rifting of Grenville-age crust resulted in the formation of a marginal ocean. Near the edge of the continent, a thick sequence of siliciclastic sediments was deposited. Overlaying these basal siliciclastic rocks is a thick carbonate shelf deposit of Cambrian and Ordovician age. This passive margin became active at the start of the Middle Ordovician creating a foreland basin. This basin was filled with black muds and turbidites. The lowering of source areas slowed siliciclastic input and a carbonate shelf and bank was re-established. Carbonate deposition, with some quartzose sand and chert formation, lasted until Late Devonian time. These sediments were covered by black muds and turbidites of the Chattanooga Shale, during the Acadian orogeny. The upper part of the sequence is the Price delta of Late Mississippian age, comprising the Price and Maccrady Formations. This clastic sequence became inundated and a carbonate shelf developed creating the muds forming the Late Mississippian Greenbrier Limestone. As carbonate deposition slowed, red, green, gray muds, and sand of latest Mississippian

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age were deposited, composing the Bluefield, Hinton, and Bluestone Formations. In southwest Virginia, deep gas production comes from the Devonian and Mississippian formations. These formations range in thickness from about 3400 ft Ž1036 m. along the Kentucky and Virginia Boundary to as much as 5000 ft Ž1524 m. in the northeastern part of the coalfield. The Mississippian strata were in turn covered by a coal-bearing clastic wedge during Early Pennsylvanian time, that resulted from Alleghany continental collision. These Lower Pennsylvanian rocks in the southwestern Virginia coalfield ŽFig. 3., range in thickness from 800 ft Ž244 m. in the northeastern part of the coalfield to as much as 5150 ft Ž1570 m. in the southwestern part ŽNolde, 1994.. They have been mapped as the Pocahontas and Lee Formations, and lower part of the Norton Formation. The Pocahontas Formation contains coal beds in sequences of sandstone, siltstone, and shale. The sandstones are medium to light gray, fine grained, micaceous and feldspathic. The siltstones and shales are medium to dark gray, carbonaceous, and contain abundant plant material. Major coal beds are the Pocahontas Nos. 3 and 6. The Pocahontas No. 3 coal bed is the thickest and most economically valuable coal in Virginia. The strata between the base of the formation and the Pocahontas No. 3 coal bed are as much as 225 ft Ž69 m. thick and thin northwestward to where they were eroded during deposition of the lower quartz arenite member of the Lee Formation. Pocahontas-equivalent strata above the No. 3 coal bed intertongue with the lower quartz arenite. These strata range from 300 to 500 ft Ž91 to 152 m. in thick. Strata between the base of the Pocahontas No. 8 coal bed and the Upper Seaboard coal bed consist of laterally equivalent rocks assigned to the Lee Formation and the lower part of the Norton Formation. The Lee Formation contains quartz arenites which grade laterally to the southeast into lithic arenites in the Norton Formation ŽFig. 3.. Principal coal beds in the Lee Formation are, in ascending order, Pocahontas No. 9, Lower Horsepen, Fire Creek, War Creek, Middle Horsepen, Upper Horsepen, Greasy Creek, and Upper Seaboard. This interval of strata averages 700 ft Ž213 m. in thickness.

4. Target coal beds The stratigraphy and distribution of the ten target coal beds are discussed briefly in the following sections. Figs. 5–14 show the approximate area of distribution of these coal beds, and Table 2 summarizes the estimated tonnage of coal in place. A preliminary estimate of total gas in place in southwestern Virginia for the ten target coal beds is 6.7 Tcf Ž0.18 Tm3 .. The top three coal beds with coalbed methane resources are Pocahontas No. 3, Lower Horsepen, and War Creek ŽTable 3.. Coalbed methane distribution within each of the various overburden intervals ŽTable 4. indicates that 91% of the total gas in the coal bed occurs at depths from 1000 to 2500 ft Ž304 to 762 m.. This interval also contains approximately 85% of the total coal in place ŽTable 2.. As part of the 1995 National Assessment of United States Oil and Gas Resources performed by the US Geological Survey, Rice Ž1995. reported the total in place coalbed methane resource for the central Appalachian basin to be 5 Tcf Ž0.13 Tm3 . with 3.07 Tcf Ž0.08 Tm3 .

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Fig. 5. Map showing approximate extent of the Pocahontas No. 3 coal bed, beneath more than 500 ft of overburden.

technically recoverable. Our data suggest that the U.S. Geological Survey in-place coalbed methane resources for the central Appalachian basin should be revised upward. 4.1. Pocahontas No. 3 coal bed The Pocahontas No. 3 coal bed underlies about 367,500 acres Ž148,700 ha. in the southwestern Virginia coalfield ŽFig. 5.. The coal occurs primarily in the southeastern portion of the coalfield. The No. 3 coal bed lies from 1385 to 2587 ft Ž422 to 790 m.

Fig. 6. Map showing approximate extent of the Pocahontas No. 6 coal bed, beneath more than 500 ft of overburden.

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Fig. 7. Map showing approximate extent of the Pocahontas No. 9 coal bed, beneath more than 500 ft of overburden.

below drainage throughout most of the area. The Pocahontas No. 3 is about 225 ft Ž69 m. above the Bluestone Formation in the southeastern part of the coalfield. The coal bed, including partings, ranges from less than 1.0 ft Ž0.3 m. to as much as 7.6 ft Ž2.3 m. thick. The average thickness of the No. 3 coal bed is 3.4 ft Ž1 m.. The No. 3 coal bed thins to the northwest ŽFig. 5., and is eventually truncated by an unconformity below the lower quartz arenite member of the Lee Formation.

Fig. 8. Map showing approximate extent of the Lower Horsepen coal bed, beneath more than 500 ft of overburden.

J.E. Nolde, D. Spearsr International Journal of Coal Geology 38 (1998) 115–136

127

Fig. 9. Map showing approximate extent of the Fire Creek coal bed, beneath more than 500 ft of overburden.

The total estimated original volume of the Pocahontas No. 3 is 1.90 billion short tons. Of this amount, 0.21 billion short tons has been mined and lost in mining. This yields an estimate for the volume of coal of 1.69 billion short tons. 4.2. Pocahontas No. 6 coal bed The Pocahontas No. 6 coal bed covers an area of about 355,800 acres Ž143,900 ha. in the southwestern Virginia coalfield ŽFig. 6.. The No. 6 coal bed lies about 200 ft Ž61 m. above the Pocahontas No. 3 coal bed and is limited to the southeastern parts of the coalfield area. The interval between the Pocahontas No. 3 to No. 6 coal beds gradually

Fig. 10. Map showing approximate extent of the War Creek coal bed, beneath more than 500 ft of overburden.

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Fig. 11. Map showing approximate extent of the Middle Horsepen coal bed, beneath more than 500 ft of overburden.

decreases westward to about 130 ft Ž40 m.. Average thickness of the No. 6 coal bed is 1.4 ft Ž0.4 m.. Northwestward, the No. 6 coal is replaced by the lower quartz arenite of the Lee Formation. This yields an estimate for coal in place of 1.78 billion short tons. 4.3. Pocahontas No. 9 coal bed In southwestern Virginia, the Pocahontas No. 9 coal bed ŽFig. 7. covers an area of about 383,400 acres Ž155,000 ha.. In this area, the Pocahontas No. 9 coal bed lies 445 ft Ž135 m. above the Pocahontas No. 3 coal bed. Where the Pocahontas No. 6 coal is

Fig. 12. Map showing approximate extent of the Upper Horsepen coal bed, beneath more than 500 ft of overburden.

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Fig. 13. Map showing approximate extent of the Greasy Creek coal bed, beneath more than 500 ft of overburden.

absent, the Pocahontas No. 9 occurs about 70 ft Ž21 m. above the lower quartz arenite member of the Lee Formation. The average thickness of the Pocahontas No. 9 is 1.3 ft Ž0.4 m.. This yields an estimate for coal in place as 0.96 billion short tons ŽTable 2.. 4.4. Lower Horsepen coal bed In southwestern Virginia, the Lower Horsepen coal bed ŽFig. 8. covers an area of about 567,900 acres Ž230,000 ha.. The coal bed is about 300 ft Ž91 m. above the top of the lower quartz arenite member of the Lee Formation and about 50 ft Ž15 m. above the

Fig. 14. Map showing approximate extent of the Upper Seaboard coal bed, beneath more than 500 ft of overburden.

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Table 4 Estimated coalbed methane resources Žtrillion cubic feet, Tcf. by overburden category, 0.12 Tcf of gas has been produced from 1988 to 1996 Overburden category Žft.

Coal density Žlbrft 3 .

Median coal thickness Žft.

Gas content Žft 3 rton.

Area Žacres.

Estimated gas resource ŽTcf.

500–1000 1000–1500 1500–2000 2000–2500 ) 2500 Total

2000 2000 2000 2000 2000

2.01 1.91 2.81 2.65 1.77

302 402 502 602 652

559,569 1,399,958 1,007,422 338,289 12,964

0.68 2.15 2.84 1.08 0.03 6.78

Pocahontas No. 9 coal bed. The average thickness of the Lower Horsepen is 1.8 ft Ž0.55 m.. This yields an estimate for coal in place of 1.74 billion short tons. 4.5. Fire Creek coal bed In southwestern Virginia, the Fire Creek coal bed ŽFig. 9. covers an area of about 367,900 acres Ž148,900 ha.. The Fire Creek coal bed is limited to the southern part of the coalfield, where it lies 30 to 60 ft Ž9 to 18 m. above the Lower Horsepen. In the northeast part of the coalfield a sandstone occupies the Fire Creek coal interval. The Fire Creek coal commonly consists of two splits, separated by as much as 15 ft Ž4.5 m. of siltstone. The coal bed is well developed in the central part of the area where thicknesses of more than 3.5 ft Ž1.1 m. are present. The coal has an average thickness of 2.3 ft Ž0.7 m.. This yields an estimate for coal in place of 1.10 billion short tons. 4.6. War Creek coal bed In southwestern Virginia, the War Creek coal bed ŽFig. 10. covers an area of about 588,300 acres Ž238,000 ha.. In the coalfield area the War Creek is 600 to 700 ft Ž183 to 213 m. above the Pocahontas No. 3 coal bed. The interval between the lower and middle quartz arenite members thins to the northwest from about 400 ft Ž122 m. to 100 ft Ž30 m. and the War Creek lies near the middle of that sequence. The War Creek coal bed is eventually replaced by the northwestward-thickening and coalescing lower and middle quartz arenite members of the Lee Formation. Although the bed is rather extensive in areal distribution, the coal is not particularly thick in the central and southern part of the coalfield. The War Creek coal bed has an average thickness of 2.2 ft Ž0.7 m.. This yields an estimate for coal in place of 2.19 billion short tons. 4.7. Middle Horsepen coal bed In southwestern Virginia, the Middle Horsepen coal bed ŽFig. 11. covers an area of about 475,200 acres Ž207,500 ha.. In the area, the Middle Horsepen is 110 to 120 ft Ž33

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to 37 m. above the War Creek coal. The coal is generally over 100 ft Ž30 m. below the base of the middle quartz arenite and 300 to 400 ft Ž91 to 122 m. above the top of the lower quartz arenite of the Lee Formation. To the northwest, the Middle Horsepen is replaced by the middle quartz arenite of the Lee Formation. In the southeastern part of the coalfield the Middle Horsepen is commonly less than 2 ft Ž0.6 m. in thickness. From this area the coal becomes thinner to the north, northwest, and west to less than 1 ft Ž0.3 m.. The coal has an average thickness of 1.7 ft Ž0.5 m.. This yields an estimate for coal in place of 1.42 billion short tons. 4.8. Upper Horsepen coal bed In southwestern Virginia, the Upper Horsepen coal bed ŽFig. 12. covers an area of 512,900 acres Ž207,500 ha.. In the northeastern part of the coalfield the Upper Horsepen is the lower of two coal beds in a coal zone. The lower coal is 800 to 820 ft Ž244–250 m. above the Pocahontas No. 3 coal bed. The upper coal is about 30 ft above the lower. In the northern and central coalfield area the Upper Horsepen coal bed is about 50 ft Ž15 m. above the Middle Horsepen coal bed. In northwestern part of the area ŽFig. 12. the Upper Horsepen is replaced by the middle quartz arenite member of the Lee Formation. The coal thickness averages 1.6 ft Ž0.5 m.. This yields an estimate for coal in place of 1.26 billion short tons. 4.9. Greasy Creek coal bed In southwestern Virginia, the Greasy Creek coal bed ŽFig. 13. covers an area of 468,500 acres Ž189,500 ha.. In northeastern part of the area, the Greasy Creek coal bed lays 300 ft Ž91 m. above the Upper Horsepen coal. The coal is persistent throughout the northeast where it is 3 ft Ž0.9 m. in thickness. To the northwest in the north-central part of the area the Greasy Creek intertongues with or is replaced by the middle quartz arenite member of the Lee Formation. The coal has an average thickness of 1.8 ft Ž0.5 m.. This yields an estimate for coal in place of 1.28 billion short tons. 4.10. Upper Seaboard coal bed In southwestern Virginia, the Upper Seaboard coal bed ŽFig. 14. covers an area of about 442,273 acres Ž179,000 ha.. In the northeastern part of the coalfield the Upper Seaboard coal bed is 40 to 50 ft Ž12 to 15 m. above the Greasy Creek coal bed. To the northwest the interval increases to 100 ft Ž31 m. above the Greasy Creek coal bed. Further to the northwest, the Upper Seaboard terminates laterally into the upper part of the middle quartz arenite member. The Upper Seaboard coal zone in the central and southern part of the coalfield generally contains two coal beds. They are consistently 25 to 30 ft Ž6-9 m. apart and 150 ft Ž46 m. above the Greasy Creek coal. These two coals are the Upper Seaboard ‘A’ Župper bed. and the Upper Seaboard Žlower bed.. The coals are relatively continuous and lie close above the middle quartz arenite member of the Lee Formation. However, they are generally thin and range between 1 ft Ž0.3 m. and 2.2 Ž0.7 m. ft in thickness. The

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coal has an average thickness of 1.6 ft Ž0.5 m.. This yields an estimate for coal in place of 1.33 billion short tons.

5. History of coalbed methane development In 1973, TWM Petroleum entered into an agreement for the lease of oil and gas rights on acreage owned by Island Creek Coal, a subsidiary of Occidental Petroleum. The agreement was for drilling vertical ventilation holes into the deep mines in the Pocahontas No. 3 coal bed in Buchanan County. The agreement expired in 1975. In 1978, Occidental Petroleum, working with the US Department of Energy, developed a method for recovering methane from horizontal boreholes in the Virginia Pocahontas No. 5 mine. Also, in 1978, the Clinchfield Coal began working with the US Department of Energy on a project at the McClure No. 1 mine in Dickenson County. They drilled five vertical boreholes to test the possibility of producing methane gas from the Jawbone coal bed. Desorption tests on two cores indicated a gas content of 280 cu ft per ton Ž8.7 m3rt.. They also studied the effect of borehole spacing on gas drainage rates. The project ended in 1980 and Clinchfield Coal ŽPittston Coal Group. was convinced that there was potential for coalbed methane production in the Nora gas field. The Pittston Coal Group began working with Equitable Resources Energy ŽEREX. to develop the resource. EREX began drilling for coalbed methane in 1988 ŽFig. 15; Table 5.. They completed two wells during 1988. EREX completed the 1 Squire Smith on September 9, 1988 and the 101 Jessee Wampler on October 11, 1988. In 1989, EREX drilled 11 wells. In 1990, EREX drilled 42 coalbed methane wells. Also during 1990, Consolidation Coal drilled 7 wells, OXY, USA drilled 18 wells, and Island Creek Coal and Pocahontas Gas Partnership ŽPGP. drilled one well each. With the extension of the federal Tax Credit during 1990 to December 31, 1992, coalbed methane drilling increased to 118 wells in

Fig. 15. Number of coalbed methane wells and vertical ventilation holes drilled per year.

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Table 5 Number of coalbed methane wells and vertical ventilation holes drilled per year by operator, 1988 to 1996 Operator

Pre-1988

Coalbed methane wells Consol 0 EREX 0 Island Creek 0 OXY USA 0 Pocahontas 0 Ratliff 0 Virginia Gas 0 Total 0

1988

1989

1990

1991

1992

1993

1994

1995

1996

Total

0 2 0 0 0 0 0 2

0 11 0 0 0 0 0 11

7 42 1 18 1 0 0 69

42 44 3 24 0 0 6 119

0 53 17 9 64 0 0 143

1 46 12 0 30 0 3 92

21 52 8 0 30 1 0 112

49 8 0 1 34 0 0 92

77 50 0 0 31 0 0 158

197 308 41 52 190 1 9 798

0 4 0 4

0 0 0 0

0 0 0 0

2 0 0 2

2 24 98 127

Vertical Õentilation holes conÕerted to coalbed methane wells Consol 0 0 0 0 0 Island Creek 1 0 1 1 1 Pocahontas 10 9 14 32 8 Total 11 9 15 33 9

0 19 25 44

1991. Pocahontas Gas Partnership drilled 73 vertical ventilation holes near the Consolidation Coal, Buchanan No. 1 mine in south-central Buchanan County. Also, PGP applied for permits to convert these 73 vertical ventilation holes to coalbed methane wells. Some of the holes were drilled in the late 1980s. Island Creek Coal, during 1991, applied for a permit to convert one vertical ventilation hole to a coalbed methane well. OXY, USA drilled the Ball A-1 well for disposal of water produced during coalbed gas production. They are using the Mississippian Price Formation as the disposal unit. Ratliff Gas drilled one well near the Beatrice Mine. Virginia Gas, an active operator with 9 wells, drilled a discovery well in north-central Buchanan County in 1991. Subsequently they drilled and completed five additional wells in the area and three wells about four miles Ž6.4 km. to the southwest of the discovery well. The Oakwood Gathering Company began construction of a gathering system to collect gas from the OXYrBuchanan Production operations in 1991. In addition, PGP worked on its gathering system that began in 1991. This trend carried over into 1992, with the drilling of 142 wells. With the end of the federal Tax Credit for new coalbed methane wells on December 31, 1992, drilling declined to 92 coalbed methane wells during 1993 ŽTable 5.. In 1993, Consol acquired the assets of OXY, USA and the Oakwood Gathering Žpipeline. System. Also in 1993, Island Creek Coal converted 4 vertical ventilation holes to coalbed methane wells. Since applying for permits, PGP has converted 98 vertical ventilation holes near the Virginia Pocahontas Beatrice mine and Consolidation Coal Buchanan No. 1 mine to produce coalbed methane. These mines are in the Pocahontas No. 3 coal bed. Subsequently over the period of 1993 to 1996, PGP drilled 125 additional coalbed methane wells. Since development began in 1988, 814 coalbed methane wells have been drilled as of December 1996. Also during that time, 125 vertical ventilation holes for deep mines were drilled or converted to coalbed methane wells. Production decline curves for two

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Fig. 16. Production curves of monthly gas production from 1 Squire Smith and 101 Jessee Wampler coalbed methane wells.

wells completed in 1988, the 1 Squire Smith and 101 Jessee Wampler are shown in Fig. 16. Data are for the first 99 months of production. Cumulative production from the two wells, for October through December, was 11,856 Mcf Ž336 Mm3 .. Development

Fig. 17. Annual and cumulative coalbed methane production for Virginia, 1988 to 1996.

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rapidly extended to the northeast, west, and southeast during 1991 and 1992 ŽFig. 2.. Annual production and cumulative production curves are compiled in Table 1 and shown in Fig. 17. Table 1 also shows average production per month and average daily production per well. Total coalbed methane production during 1996 was 34,153,584 Mcf Ž968,072 Mm3 .. Cumulative coalbed methane production for 1988 through 1996 is 121,542,188 Mcf Ž3,445,073 Mm3 .. The major operators in Virginia are Equitable Resources Energy Company Ž288 wells., Pocahontas Gas Partnership Ž258 wells., and Consolidation Coal Company Ž202 wells. ŽTable 4.. Minor coalbed methane operators are Island Creek Coal Company, Ratliff Gas Company, and Virginia Gas Company.

6. Effect of regulations on coalbed methane development The federal Windfall Profit Act of 1980 ŽNonconventional Fuels Tax Credit under Section 29. spurred exploration and development of coalbed methane for wells drilled between December 31, 1979, and December 31, 1992. Several legal issues in the past have influenced coalbed methane development. The principal issue was concerning coalbed gas ownership. If a land owner holds the surface and mineral rights, there is no question that he also owns the coalbed methane resource. However, in Virginia it is common that mineral rights are severed from the surface owner, and the coal owner severed from the oil and gas rights. Potential claimants are the coal owner, coal lessee, oil and gas owner, the oil and gas lessee, and the surface owner. The ownership issue was complicated in Virginia by the 1977 Migratory Gas Act, which gave the surface owner certain rights to migratory gases. If coalbed methane is considered a migratory gas, the law gives ownership to the surface owner. It has been argued that coalbed methane is a non-migratory gas, provided the gas stays in the coal bed and produced by a well in that coal bed. It also has been argued that the law was unconstitutional because it can take or transfer property rights from the coal owner or oil and gas lessee to the surface owner without compensation. Part of the ownership question was resolved when the 1990 Virginia General Assembly repealed the 1977 Act. Besides repealing the Migratory Gas Act, the General Assembly passed legislation allowing development and production, and forced pooling of interests with proceeds held in escrow while ownership rights are being resolved.

7. Conclusions Virginia leads in the drilling and production of coalbed methane in the central and northern Appalachian basin. Geologic and economic factors for this lead include the presence of abundant coal resources, sufficient overburden, high gas-content coal beds, existing pipeline infrastructure, and legislative clarification of uncertain gas ownership. Ten coal beds are targets for methane production. Estimated coal in place for these coal beds is about 14.9 billion short tons. Estimated in place coalbed methane resource in Virginia is 6.7 Tcf Ž0.18 Tm3 ..

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Acknowledgements We would like to thank Gene Rader and Ian Duncan of the Virginia Division of Mineral Resources and Ken Englund, Robert Milici, and Paul Lyons of the US Geological Survey for comments and critical reviews. The authors accept sole responsibility for information and interpretations in this paper.

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