What is coal bed methane?
How does coalbed methane form?
What are the two types of coalbed methane?
How does coalbed methane occur in the coal?
What types of coal and coalbed methane occur in Wyoming?
Where is coalbed methane found in Wyoming?
Where are the best areas in Wyoming to look for coalbed methane?
How long have we known about coalbed methane?



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COALBED METHANE IN WYOMING
By Rodney H. De Bruin, Robert M. Lyman,
Richard W. Jones, and Lance W. Cook

What is coalbed methane?
Coalbed methane is natural gas or methane (CH4) that occurs in coal beds and has been generated during the conversion of plant material to coal (the process known as coalification). Coalbed methane produced from low rank subbituminous coal in the Powder River Coal Field, Wyoming, is composed almost entirely of methane, with a minor amount (1.5 to 2%) of carbon dioxide (CO2). Coalbed methane produced in other areas of the U.S. from higher rank bituminous coal may contain minor amounts (less than 3% each) of CO2 and nitrogen (N2), very minor to trace amounts of higher hydrocarbons (ethane, propane, butane, etc.), and sometimes a trace of hydrogen sulfide (H2S) (Rightmire, 1984).

Methane produced from a typical coalbed methane well has a heating value of about 1000±25 British Thermal Units (Btus) per standard cubic foot. One million Btus (the energy equivalent of 1000 cubic feet of methane or one MCF) approximate the energy consumed by a person in the U.S. in about 1.2 days. A million Btu's of fossil fuel can generate about 100 kilowatt-hours of electricity at an electric utility. (TOP of page)

How does coalbed methane form?
During coalification, plant material that accumulated and was preserved in ancient swamps and bogs at rates fast enough to prevent decay (oxidation) begins to compact upon burial. The material is first converted to peat as much of the water in the original material is expelled. As the temperature increases with further burial, ever-increasing ranks of coal form, starting with lignite, followed by subbituminous coal and bituminous coal. If the heat (and pressure) is great enough, anthracite (the highest rank of coal) forms. Biogenic methane (that attributed to bacterial activity) is first to form. When the temperature exceeds that in which bacteria can live, thermogenic methane (that attributed to heating) forms.

At these different stages of coalification, various hydrocarbons (called volatile matter, including methane), along with carbon dioxide, nitrogen, and water, are released. Increased temperatures throughout burial drive off volatile matter. The coalification process can stop at any time, depending on geologic conditions, leaving what we see today as varying ranks of coal. Much of the methane generated by the coalification process escapes to the surface or migrates into adjacent reservoir or other rocks, but a portion is trapped within the coal itself, primarily adsorbed on or absorbed within micropores of the coal. (TOP of page)

What are the two types of coalbed methane?
During the early stages of coalification, biogenic methane is generated as a by-product of bacterial respiration. Aerobic bacteria (those that use oxygen in respiration) first metabolize any free oxygen left in the plant remains and surrounding sediments. In fresh water environments, methane production begins immediately after the oxygen is depleted (Rice and Claypool, 1981). Species of anaerobic bacteria (those that don't use oxygen) then reduce carbon dioxide and produce methane through anaerobic respiration (Rice and Claypool, 1981).

When a coal's temperature underground reaches about 122°F (Figure 1), and after a sufficient amount of time, most of the biogenic methane has been generated, about two-thirds of the original moisture has been expelled, and the coal attains an approximate rank of subbituminous (Rightmire, 1984). As the temperature increases above 122°F through increased burial or increased geothermal gradient, thermogenic processes begin and additional water, carbon dioxide, and nitrogen are generated as coalification proceeds to approximately the rank of high volatile bituminous (Rightmire, 1984). Maximum generation of carbon dioxide, with little methane generation occurs at about 210°F. Generation of thermogenic methane begins in the higher ranks of the high volatile bituminous coals, and at about 250°F, generation of methane exceeds generation of carbon dioxide. Maximum generation of methane from coal occurs at about 300°F. With even higher temperatures and higher rank coals, methane is still generated, but at somewhat lower volumes (Rightmire, 1984). (TOP of page)

How does coalbed methane occur in the coal?
Because coal beds serve as both the source rocks and the reservoir rocks, gas storage in coal beds is more complex than in most conventional reservoirs (e.g., carbonate and sandstone). Although coalbed methane can (and does) migrate to non-coal reservoir rocks, once the gas leaves the coal beds it is no longer considered coalbed methane. Gas reservoirs composed of coal contain unique properties for gas storage that are not present in other reservoirs.

According to Yee and others (1993), coalbed methane is stored in four ways: 1) as free gas within the micropores (pores with a diameter of less than .0025 inches) and cleats (sets of natural fractures in the coal); 2) as dissolved gas in water within the coal; 3) as adsorbed gas held by molecular attraction on surfaces of macerals (organic constituents that comprise the coal mass), micropores, and cleats in the coal; and 4) as absorbed gas within the molecular structure of the coal molecules. The amount of methane present within a particular volume of coal is very large. Coals at shallower depths with good cleat development contain significant amounts of free and dissolved gas while the percentage of adsorbed methane generally increases with increasing pressure (depth) and coal rank.(TOP of page)

What types of coal and coalbed methane occur in Wyoming?
Bituminous and subbituminous coal beds occur in all 10 coal fields of the state (Figure 2); these coal beds contain both biogenic and thermogenic coalbed methane. [For clarification, this pamphlet uses the term "coal field" instead of "basin," as the former term refers specifically to that part of a structural or sedimentary basin that contains coal-bearing rocks. Since coalbed methane is found only where coals occur, we prefer to use this term instead of the more general term "basin."] Wyoming's coal beds are Cretaceous and Tertiary in age and occur in a variety of structural and stratigraphic settings (Jones, 1990, 1991). [Much of the following discussion on Wyoming coal fields is extracted from Glass (1997). That report contains an excellent list of references for Wyoming coal quality, reserves, and coal bed descriptions.]

Cretaceous coals may attain the rank of high volatile A bituminous, but many Cretaceous coals are lower in rank and have not attained enough thermal maturity to have generated large amounts of thermogenic coalbed methane. However, some of these lower rank Cretaceous coals may contain biogenic coalbed methane. Deeply buried Cretaceous coals in the Bighorn, Wind River, and Green River coal fields have probably reached ranks that correspond to significant thermogenic methane generation.

Tertiary coal beds in Wyoming are generally lignite to subbituminous in rank. Some coals may be high volatile bituminous in rank where they have been deeply buried and have reached sufficient maturity for thermal generation of methane. These coal beds are located in the deeper parts of the Wind River, Bighorn, Hanna, and Green River coal fields. Less thermally mature Tertiary coal beds in the Wasatch and Fort Union formations of the Powder River Coal Field contain biogenic coalbed methane. Several individual Tertiary coal beds are 100 feet (or more) thick and contain large amounts of coalbed methane, even though the gas yield per ton of coal is relatively low. (TOP of page)

Where is coalbed methane found in Wyoming?
Methane associated with coal beds has been observed in nearly all the coal-bearing areas in Wyoming (Figure 3). Evidence for its occurrence includes direct measurements in wells and coal cores, surface venting of gas, gas-related explosions and fires in underground coal mines, inferences from the rank of coal, and thermal histories of coal fields (Table 1).

Although coalbed methane exists in all coal regions of Wyoming, some of the state's coal deposits are shallow and too thermally immature to have generated substantial amounts of thermogenic gas. However, many of these shallow coals do have biogenic methane entrapped in them. The most significant quantity of biogenic methane in Wyoming exists in the relatively shallow, thick coal beds in the Powder River Coal Field. While methane content is relatively low (estimates range from 30 to 40 standard cubic feet of methane per ton of coal) in this coal field, a number of thick, shallow Tertiary coal beds (with large coal tonnages) account for the accumulation of large quantities of biogenic coalbed methane. In contrast, Cretaceous and some Tertiary coal beds deeply buried in many Wyoming basins are much more thermally mature and no doubt have generated and do contain large volumes of thermogenic gas. (TOP of page)

Where are the best areas in Wyoming to look for coalbed methane?
Exploration targets for coalbed methane can be defined by, but not limited to, the following criteria:

• known, thick, abundant, and laterally continuous coal beds;
  coal-bearing areas with coals of appropriate rank;
• adequate conditions for accumulation and preservation of coalbed methane (i.e., a favorable reservoir);
• depth to the coal bed, which influences economic and mechanical limits on development; and
• other evidence such as degree and location of fracturing (cleats) and faulting, geothermal gradient, high pressure or overpressured areas in the subsurface, and the presence of gas fields producing from known coal-bearing rocks

Jones and De Bruin's (1990) map of exploration targets in Wyoming (Figure 3) is still valid. These targes were based on the first four criteria (above) plus identified occurrences of coalbed methane, data available from coalbed methane tests, and production of coalbed methane.

McCord (1980) considered coal beds less than 5000 feet deep to be the primary targets for initial exploration in the Green River Coal Field; this depth limit has been applied to other Wyoming coal fields as well. Visco-elastic properties of coal beds at depth may inhibit effective production of methane directly from coal beds. The shallow coal beds in the Powder River Coal Field are well above this depth limit.

Exploration targets in the Hams Fork Coal Field and the extreme western Green River Coal Field are those described by McCord (1980) for coal beds in the Frontier, Adaville, and Mesaverde formations under less than 5000 feet of overburden. An additional target area is the Almy area near Evanston, Wyoming, where coal beds in the Evanston Formation may contain coalbed methane. Data are inadequate to define any additional exploration targets and therefore the potential for coalbed methane in the remainder of the coal field is unknown.

Cretaceous and Tertiary coal beds that crop out around the Rock Springs uplift are considered to be exploration targets to a depth of 5000 feet (McCord, 1980). Several areas that contain thick, shallow, subbituminous Tertiary coal beds in the eastern and southeastern parts of the Green River Coal Field may be exploration targets for biogenic coalbed methane. Coalbed methane targets are also located in the Almond Formation to depths of 5000 feet in the eastern Green River Coal Field and along the Cherokee Ridge in the southeastern part of that coal field. The remainder of the Green River Coal Field may contain deep coal beds that have high enough rank to generate significant amounts of coalbed methane. A large part of the gas in conventional Cretaceous reservoirs in this coal field may be from coal beds.

Exploration targets for coalbed methane in the Wind River and Bighorn coal fields are defined primarily by coal beds in the Mesaverde Formation under less than 5000 feet of cover. Steeply-dipping Lance and Meeteetse coal beds in the Waltman area of the Wind River Coal Field may present additional targets for coalbed methane development.

In the Hanna Coal Field, coalbed methane targets occur in the upper part of the Mesaverde Group and in the lower part of the Medicine Bow Formation to depths of 5000 feet. This target area is restricted to the western half of the coal field and is controlled primarily by the structure of the basin. In the interior of this coal field, coalbed methane targets occur in numerous coal beds of the Ferris and Hanna formations. Both biogenic and thermogenic methane probably exist in this coal field.

The Powder River Coal Field contains a large resource of biogenic coalbed methane associated with numerous thick, laterally continuous, relatively shallow (less than 3000 feet deep) Tertiary coal beds (Figure 4). The primary targets are confined to coal beds of the Tongue River Member of the Fort Union Formation and the Wasatch Formation. The eastern edge of the target area is defined as the outcrop or subcrop (behind the oxidized or "burned" coal) of the Wyodak and equivalent coal beds (Figure 5). The western edge of the target area is defined by the inferred subsurface extent of the "Big George" coal bed and/or the inferred subsurface extent of the Wyodak coal bed and its equivalents (Figures 4 and 5). Additional biogenic coalbed methane may be present in some of the thicker Wasatch Formation coal beds in the Buffalo and Sheridan areas (Figures 4 and 5), northwestern Powder River Coal Field, Wyoming. (TOP of page)

How long have we known about coalbed methane?
Occurrences of coalbed methane, either biogenically derived from shallow, lower rank coal beds or thermogenically derived from more deeply buried, higher rank coal beds, have been documented in many Wyoming coal areas (Jones and De Bruin, 1990). Coalbed methane has been observed in water wells in the Powder River Coal Field since the 1950's (Olive, 1957) and some of the historical ranches that have used coal beds for water sources have encountered coalbed methane since 1916 (Jones and De Bruin, 1990). (TOP of page)

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