Resource beginning to be tapped globally

Coalbed Methane’s Role Growing

Unconventional resources such as coalbed methane will become progressively more important worldwide as population continues to grow at an unprecedented – and possibly unsustainable – rate.

As a result, coalbed methane and other unconventional resources undoubtedly will play an important role in supplying energy needs to both economically developed and emerging nations worldwide in the foreseeable future.

Preliminary worldwide coalbed methane resources are estimated to range between 5,800 and 24,215 Tcf (164 to 686 Tm3).

The largest potential resources, which also have the largest degree of uncertainty, are in the Former Soviet Union with 4,000 to 16,116 Tcf (113 to 456Tm3). North America ranges between 951 to 4,383 Tcf (27 to 124 Tm3), whereas South America and Europe range from 15 to 32 Tcf (42 to 91 Bm3) and 161 to 269 Tcf (4.6 to 7.6 Tm3), respectively. Africa ranges between 27 to 55 Tcf (760 to 1,557 Bm3), and the Middle East has no coalbed methane resources. The Asia Pacific region, which includes China, ranges from 646 to 3,360 Tcf (18 to 95 Tm3).

The rate of coalbed methane resource development within individual countries will be highly variable due to local economic factors and government energy priorities and policies.

The United States remains the world leader in coalbed gas exploration, booked reserves and production, although coalbed methane production is expanding internationally – particularly in Canada and Australia, and commercial production from India will undoubtedly accelerate in the near future.

In North America, there is commercial coalbed gas production or exploration in approximately 12 U.S. and several Canadian basins. Coalbed methane now represents 9 percent of 2006 dry-gas production and 9 percent of proved dry-gas reserves in the United States, with the major producing areas located in the San Juan, Powder River, Black Warrior, Raton, Central Appalachian and Uinta (Ferron and Book Cliffs) basins.

Other U.S. areas with significant exploration or production are the Cherokee, Arkoma, Illinois, Hanna, Gulf Coast and Greater Green River basins.

Annual coalbed methane production in the United States continues to increase, but not as rapidly as in previous years. The annual 2006 coalbed methane production was up slightly (1,758 Bcf; 49.8 Bm3) from 2005 coalbed methane production, which was 1,732 Bcf (49.0 Bm3) from an estimated 54,000 wells (figure 1).

Coalbed methane reserves decreased slightly from 19.892 Tcf (563.3 Bm3) in 2004 to 19.620 Tcf (555.6 Bm3) in 2006, representing a decrease of only 72 Bcf (7.7 Bm3) (figure 2).

There are two important facts to remember:

  • Demand for natural gas in the United States is expected to increase 50 percent over the next 20 years as additional co-generation power plants and natural gas electric power generation facilities are constructed.
  • The global demand for energy will continue to increase regardless of the current economic downturn as the middle classes of emerging economies such as China and India continue to expand.

Therefore, development of unconventional resources such as coalbed methane and gas shales worldwide is critical for economic stability and continued growth.

Enhanced coalbed methane recovery using sequestered greenhouse gases (carbon dioxide) and microbial conversion of the coal and sorbed gases into methane may represent a solution to solving energy and environmental objectives simultaneously and development of these technologies is accelerating to meet global demand.  

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The Energy Minerals Division (EMD), a division of AAPG, is dedicated to addressing the special concerns of energy resource geologists working with energy resources other than conventional oil and gas, providing a vehicle to keep abreast of the latest developments in the geosciences and associated technology. EMD works in concert with the Division of Environmental Geosciences to serve energy resource and environmental geologists.

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This article describes a 250-m (820-ft)-thick upper Eocene deep-water clastic succession. This succession is divided into two reservoir zones: the lower sandstone zone (LSZ) and the upper sandstone zone, separated by a package of pelitic rocks with variable thickness on the order of tens of meters. The application of sequence-stratigraphic methodology allowed the subdivision of this stratigraphic section into third-order systems tracts.

The LSZ is characterized by blocky and fining-upward beds on well logs, and includes interbedded shale layers of as much as 10 m (33 ft) thick. This zone reaches a maximum thickness of 150 m (492 ft) and fills a trough at least 4 km (2 mi) wide, underlain by an erosional surface. The lower part of this zone consists of coarse- to medium-grained sandstones with good vertical pressure communication. We interpret this unit as vertically and laterally amalgamated channel-fill deposits of high-density turbidity flows accumulated during late forced regression. The sandstones in the upper part of this trough are dominantly medium to fine grained and display an overall fining-upward trend. We interpret them as laterally amalgamated channel-fill deposits of lower density turbidity flows, relative to the ones in the lower part of the LSZ, accumulated during lowstand to early transgression.

The pelitic rocks that separate the two sandstone zones display variable thickness, from 35 to more than 100 m (115–>328 ft), indistinct seismic facies, and no internal markers on well logs, and consist of muddy diamictites with contorted shale rip-up clasts. This section is interpreted as cohesive debris flows and/or mass-transported slumps accumulated during late transgression.

The upper sandstone zone displays a weakly defined blocky well-log signature, where the proportion of sand is higher than 80%, and a jagged well-log signature, where the sand proportion is lower than 60%. The high proportions of sand are associated with a channelized geometry that is well delineated on seismic amplitude maps. Several depositional elements are identified within this zone, including leveed channels, crevasse channels, and splays associated with turbidity flows. This package is interpreted as the product of increased terrigenous sediment supply during highstand normal regression.

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Unconventional Resources is an online course that enables participants to learn about shale gas, shale oil and coalbed methane.

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