Jonathan R. Kelafant, Scott H. Stevens, Charles M. Boyer II
Advanced Resources International
Arlington, Va.
Global coalbed gas resources are significant and provide an excellent exploration and production target. In many countries, these resources dwarf the conventional gas potential and may be the only significant hydrocarbon present.
However, the diverse geographic, geologic, and reservoir conditions of coalbeds will require an extensive exploration phase and subsequent adaptation of U.S. coalbed gas technology.
Part 1 (OGJ, Oct. 5, p. 49) of this three-part series estimated a potential world coalbed gas resource of 4,000-7,000 tcf. This significant gas resource is distributed across all continents. However, the bulk (90%) is in 12 countries.
This second part of the series will review the major coalbed gas potential in these 12 countries and, despite limited reservoir data, examine the development potential.
EXISTING DATA
Because coal has played a dominant role in the energy sector of most developed countries for over 100 years, a wealth of basic geologic data exists. These data can provide the coalbed gas explorationist with a starting point for evaluating the coal basins of the world.
The data, however, were collected primarily for coal mining purposes. Therefore, the data are not always directly applicable to coalbed gas development.
For example, the thickness of a coal seam may differ considerably depending on whether the seam is being measured for mining or for the extraction of coalbed gas.
Because of this, the development of the world's coalbed gas resource requires collecting additional data, especially related to formation reservoir properties such as permeability and pressure.
NORTH AMERICA
Apart from minor coal deposits in northeastern Mexico, most of the coal and coalbed gas resources in North America are found in the U.S. and Canada. For the U.S., extensive information on coalbed gas is available.
The potential for coalbed gas development in Canada, however, has only recently been recognized. A vast untapped gas resource, possibly comparable to Canada's conventional gas resource, is only now undergoing initial exploration.
U.S.
With a resource base of 400 tcf and current coalbed gas production of over 1 bcfd, the U.S. is the recognized leader in coalbed gas development.
The San Juan basin in Colorado and New Mexico and the Warrior (Black Warrior) basin of Alabama have had the bulk of the development activity. Production from these basins accounts for over 95% of the coalbed gas currently produced in the U. S.
While significant development has occurred in these two basins (over 5,000 wells are currently producing), the industry is now moving into the other less-developed coal basins of the eastern U.S. (primarily the Appalachian and Arkoma basins), and the western U.S. (Piceance, Uinta, Powder River, and Raton).
CANADA
The coal deposits of Canada in many ways parallel those of the U.S., with Permo-Carboniferous basins in the east and Mesozoic/Cenozoic basins in the west. They generally are similar to the U.S., but the eastern basins (primarily the Sydney basin) are small in size and, therefore, have limited coalbed gas potential.
The western basins, on the other hand, have tremendous size, and the potential for coalbed gas development is only now being realized.
The western coal basins of Alberta and British Columbia are generally considered part of the Western Canada sedimentary basin, Fig. 1. Coal is found throughout the plains, foothills, and mountains in age from Jurassic to Tertiary and in rank from lignite in the plains of eastern Alberta to semianthracite in the mountains of Alberta and British Columbia.
Structural complexity increases from the plains through the foothills.
The mountain region (where the higher rank coal is located) is structurally complex. Although this area has been extensively drilled for conventional oil and gas, the coalbed gas resource has been largely bypassed and overlooked.
Recent estimates place the coal resource at over 7 trillion tons with a potential coalbed gas resource of 200-2,700 tcf.
Unlike other regions in the world, because of the lack of an established and extensive coal mining industry, limited reservoir information (especially gas content) is available on the western Canadian basins. This, combined with the region's proven conventional gas resource, has led to a general lack of understanding of the area's coal and coalbed gas resources.
Ongoing studies will significantly add to the understanding of this potentially large gas resource.
ASIA
The coal-bearing countries of Asia (primarily China, the Asian section of Russia, Kazahkstan, and India) contain most of the world's coal deposits and, accordingly, most of the world's coalbed gas resource (Fig. 2).
Because of the resource size, the development potential of a coalbed gas industry is very high. However, while the resource base may be large, the coalbeds are often remote and inaccessible.
Substantial coal deposits with coalbed gas potential can also be found in Korea (North and South), Japan, Iran, and Turkey.
CHINA
With large reserves of gassy, high-rank coal, China has one of the world's best potentials for coalbed gas development.
The Chinese government estimates in-place coalbed gas to be 1,060-1,240 tcf. This volume is greater than the country's conventional natural gas resource.
However, China faces a number of obstacles in developing its coalbed gas. Because China's natural gas production is less than 1 tcf/year, the transport and distribution infrastructure is limited, and most gas pipelines are concentrated in Sichuan Province, far from the country's major coalbed gas basins.
Nevertheless, many underground coal mines in China are deep and extremely gas-prone. The Chinese government, therefore, is vigorously promoting coalbed gas development for both mine degasification and as a new source of natural gas.
The coal basins of eastern China are the nearest to populated and industrialized centers. Thus, this area has the best potential natural gas market.
However, these eastern coal basins are small and heavily deformed. Some basins have experienced considerable igneous intrusion and recent subsidence. Coals are often high in rank, but other reservoir properties may be less favorable.
The first two coalbed gas projects in China are located in the northeast. The first stimulated, vertical-well projects utilizing U.S. technology are under way in northern China, one at Shenyang and another at Tangshan.
The small coal basins in eastern China many hold promise for coalbed gas development because of coal mine degasification requirements and close proximity to markets.
Shanxi Province has a number of gassy, bituminous coal mining areas, notably Datong and Yangquan. The Huainan coal basin in Anhui Province, and the Jixi and Hegang coal mining areas in Manchuria are also prospective. These have some of the highest gas content measured for bituminous coal in China.
The key to a successful project in these geologically complex areas will be the identification of favorable reservoir properties (i.e., permeability and gas saturation), coupled with favorable proximity to gas markets.
The interior basins of central and western China have a much larger coal resource and are generally less deformed. This may result in more favorable reservoir conditions. However, these basins are poorly surveyed and far from gas markets.
Development of coalbed gas in these western basins requires construction of major regional pipelines to transport the gas to market. The development of this coalbed gas in tandem with conventional natural gas may provide sufficient economies of scale to make these interior coalbed gas basins commercially attractive.
The Junggar basin, in remote northwest China, covers about 50,000 sq miles. Multiple coal seams are present with thicknesses totaling up to 300 ft in the Jurassic Xishanyao formation and 60 ft in the Jurassic Badaowan formation.
The thickest coal deposits are concentrated in the narrow east-to-west trending Tian Shan foredeep which passes through the city of Urumqi. This portion of the basin has a high resource concentration and a nearby, though limited, market for coalbed gas.
Unfortunately, the deposits lie in the most highly deformed part of the basin although in the interior of the Junggar basin, the strata dip more gently.
Basin uplift has exposed, high-volatile bituminous coal around the basin edge. This implies high-rank coal toward the basin interior.
Another basin, the Ordos, extends over about 100,000 sq miles, some 12 times larger than the San Juan basin. This vast basin lies within the stable continental interior of north-central China.
The Ordos basin is much less deformed than coal basins in most other parts of China. The strata generally dip less than a few degrees toward the axis of the basin, and faulting is rare.
The two principal coal sequences in the Ordos are the Shanxi (Permian) and Yanan (Jurassic) formations. Individual coal seams in the Yanan formation are particularly thick and laterally continuous. For example, 60-ft thick seam groups extending over more than 1,000 sq miles have been reported.
Coal rank increases with present burial depth from sub-bituminous along the outcrop of the basin to high-volatile bituminous or higher rank in the deeper portion of the basin.
The Tarim basin, located in the far west of China, is the country's largest sedimentary basin, extending over 220,000 sq miles. While the coal resources are large, they are still poorly defined.
Coal-bearing strata are 5,000-7,000 ft thick, with coal seams buried at depths of 5,000-30,000 ft. The geothermal gradient in the basin is low and favorable coal rank generally occurs only at great depth.
For example, in the eastern portion of the basin, the rank of the Carboniferous coal is only high-volatile bituminous at a depth of 15,500 ft.
RUSSIA
The Asian section of Russia (east of the Ural mountains) is likely to have some of the largest coalbed gas resources in the world. Three basins contain most of the coal resource:
- Pechora basin, cut by the Arctic Circle and adjacent to the Barents Sea (sits on the flanks of the Urals and could be considered in either Europe or Asia)
- Kuznetsk basin along the southern slopes of the west-central Siberian plain
- Tungusk basin near the center of Siberian Russia.
Although extensive coal deposits (and coalbed gas resource) can be found in Russia, the significant conventional gas resource of west-central Siberia raises issues as to gas markets and prices.
The lack of local markets and the poorly established infrastructure may require that an integrated, wellhead-to-burner tip development strategy be pursued.
The Pechora basin contains an estimated 175 billion tons of coal ranging in rank from high to low-volatile bituminous. Located along the flanks of the Ural mountains, the coal bearing Permo-Carboniferous sediments of the basin have undergone substantial deformation.
While extensive geologic information exists for the basin, because of the active mining industry, harsh climatic conditions may limit the exploitation of the coalbed gas resource estimated at 80-120 tcf.
Located in southwestern Siberia, 2,500 miles east of Moscow, the Permo-Carboniferous and Jurassic coal seams of the Kuznetsk cover an area of over 10,000 sq miles. Coal at a depth in excess of 6,000 ft has been identified. Estimated coalbed gas resource is 350-500 tcf.
Structural deformation along the western and northern basin edges has resulted in significant faulting and folding, with the dips of coal horizons often approaching vertical (90).
As with the other Russian basins, extensive coal mining has locally defined the structural and stratigraphic sequence. However, because of the large size of the basin, significant areas remain relatively unexplored.
The massive Tungusk basin stretches from the Arctic Circle in the north to near the Mongolian border in central Siberia in the south. Largely unexplored, this basin may be the largest coal basin in the world.
Because the basin is far from population centers, the coal mining activity has been limited primarily to the basin edge. No reliable estimates of coal or coalbed gas exist for this remote basin.
KAZAKHSTAN
Kazakhstan, by way of its recent independence, has become the eighth largest coal-bearing country in the world. While small when compared to its northern neighbor Russia, Kazakhstan still boasts 170 billion tons of coal resource and an estimated 40 tcf of coalbed gas resource.
The three primary coal basins, Ekibastus, Maikyubensk, and Karaganda, are in western Kazakhstan and are extensively mined. Coal rank varies from low-rank sub-bituminous in the Maikyubensk basin to anthracite in the southern portion of the Karaganda basin.
INDIA
India's coalbed gas resource is favorably positioned to markets. Its currently limited pipeline infrastructure should not present a major obstacle to development.
India has 160 billion tons of coal resource in seams thicker than 1.5 ft and shallower than 4,000 ft.
Permian Gondwana coals are extensive in southeast India, while a small amount of Tertiary coal occurs in the northeast and northwest sections of the country.
Coal-bearing Gondwana basins in India extend over 14,000 sq miles. Half of this area lacks significant well control.
Of India's total coal resources, most (over 60%) are shallower than 1,000 ft while the remainder lie at depths from 1,000 to 4,000 ft.
Extensive coal may also occur buried beneath the Rajmahal traps (flood basalts).
One of the most promising locations for coalbed gas may be the Damodar Valley basin, 100 miles northwest of Calcutta. The 2,500 sq mile area contains bituminous coals ranging in thickness from 10 to 60 ft.
AUSTRALIA
Australia will likely be among the first countries to follow the U.S. in developing a coalbed gas industry.
Four major Permian hard-coal basins are in eastern Australia. These are the Sydney-Gunnedah, Bowen, Galilee, and Cooper basins (Fig. 3). Portions of these basins contain thick, gassy bituminous coal seams at depths suitable for coal seam gas development.
Coal is also present in Mesozoic sediments which overlie the Permian sequence. However, with the possible exception of the Ipswitch and Clarence-Moreton basins in northern New South Wales and southern Queensland, the Mesozoic coal seams are generally too discontinuous and too low in rank to be prospective for coalbed gas.
Unlike most of Australia's conventional natural gas deposits, the country's coalbed gas resource is generally situated within reach of major urban centers along the east coast of Australia.
However, existing transport links are generally not well placed to move this gas to market. Therefore, pipelines will constitute a significant investment for most Australian coalbed gas prospects.
SYDNEY-GUNNEDAH BASIN
The Sydney-Gunnedah basin is the best explored of the Australian coal basins. The basin has numerous coal exploration boreholes and petroleum tests.
Triangular in shape, the basin extends westward from the edge of the continental shelf off New South Wales and merges inland with the Gunnedah basin near the Liverpool range.
The lightly explored Gunnedah basin is a deep, 100-mile-wide Permian trough which connects the Sydney and Bowen basins. Onshore the Sydney-Gunnedah extends over an area of nearly 25,000 sq miles. Coal seams in the Sydney basin are concentrated in the Illawarra, Singleton, and Newcastle/Tomago coal measures, of late Permian.
The coal resource of the Sydney basin is vast, estimated at 380 billion tons. Two-thirds lie at depths of 650-2,000 ft and may be suitable for coalbed gas exploration.
Coal rank of the top of the Permian coal increases from high-volatile bituminous in the west to medium/low-volatile bituminous in the east near Sydney.
Because of the regional coal rank and depth trends, coalbed gas content is generally higher in the southern half of the Sydney-Gunnedah basin. High horizontal stress anisotropy, however, has been measured in the basin. This may adversely affect coalbed reservoir conditions.
The less explored Gunnedah portion of the basin is considered less prospective than the Sydney because coal seams are shallower than 3,000 ft and generally thin.
BOWEN BASIN
The Bowen basin lies at the northern end of the Sidney-Bowen trough in east-central Queensland. Major Permian coal groups include the Reids Dome beds, Collinsville, Moranbah, and Rangal.
The Bowen basin consists of an outcropping northern half and a deep southern extension (Taroom trough) that connects to the Gunnedah basin. Coal rank in the basin generally increases towards the ease, from bituminous to semianthracite.
The Bowen basin has a significant amount of coalbed gas, estimated at 136 tcf for only the northern third of the basin.
GALILEE BASIN
In central Queensland is the Galilee basin, the largest Permian coal basin in Australia. The basin extends over 55,000 sq miles. The Permian coalbeds are buried beneath and are generally obscured by the Mesozoic Eromanga basin.
Because the Galilee basin is lightly drilled and has few outcrops, its coal resource is not well defined, particularly in the western portion of the basin. Thin, discontinuous coal seams occur in the partially eroded early Permian Aramac coal measures.
The late Permian Bandanna formation and equivalents contain thicker, more continuous coal seams, ranging from 40 ft up to 150 ft in thickness in the Koburra trough.
COOPER BASIN
This relatively unexplored basin (in terms of coal and coalbed gas) in southwestern Queensland and northeastern South Australia, covers an area of 50,000 sq miles. The Cooper basin is entirely covered by the Mesozoic Eromanga basin, resulting in no exposed outcrops of the coal formations and deep coal depths that generally range from 6,000 to 8,000 ft.
Thick coal seams have been identified in the Permian Toolachee and Patchawarra coal measures and the coal resource may be vast, currently estimated at more than 1,000 billion tons of mostly bituminous coal.
AFRICA
The bulk of Africa's coalbed gas resource is contained within the extensive Permian (Gondwana) Karoo basins of southern Africa (Fig. 4). South Africa, Zimbabwe, and Botswana are the three principal coal producing countries in the region. These three countries have a combined coal resource in excess of 150 billion tons and a coalbed gas resource estimated at 40 tcf.
The producibility of southern Africa's coalbed gas resource is untested. One of the main points favoring producibility is that the coal basins are located in areas with extensional tectonics. In these settings, the resulting normal to low horizontal stresses may allow coal cleats and larger scale fracture systems to remain open, thereby enhancing permeability.
Reported gas seeps and gas blowouts in coal exploration core holes also indicate potential coal gas producibility.
SOUTH AFRICA
The world's eighth largest coal producer, South Africa dominates the African coal industry. Coal deposits are generally shallow, less than 1,000 ft, and are found in 18 coal fields covering about 115,000 sq miles.
The great bulk of the coal reserves are concentrated in the Transvaal region of the country.
A recent study by the South African National Energy Council identified the Waterberg basin in the northwest Transvaal and the southeast portion of the Transvaal coalfield near Paardekop-Amersfoort as having the highest potential for coalbed gas development.
The coal seams in these two areas are deeper at 1,500 ft. Reported gas desorption measurements near Amersfoort, South Africa, yielded values of 150-350 cu ft/ton for seams at a depth of 1,000 ft.
ZIMBABWE
The second largest coal producing country on the African continent, Zimbabwe is producing about 5 million tons/year from a single mine, the Wankie Colliery. The country's coal reserves are concentrated in two areas. In the northern part of the country the coals are within and parallel to the Zambezi River valley (the Wankie and Lusulu Lower Karoo intrabasins). In the south and southeastern portions, the coals border South Africa and Mozambique.
Wankie and Lusulu intrabasin coal seams are generally high-volatile bituminous, relatively shallow, and gently dipping. Coalbeds of southern Zimbabwe are markedly different because they are extensively faulted and heavily intruded by dolerite dikes and sills.
The normal faults have displaced the coal to depths of over 2,000 ft and the intrusions have locally raised the rank of the coal to anthracite.
Gas content measurements in Zimbabwe from two core holes in the Sabi valley averaged nearly 400 cu ft/ton. Resource in-place calculations for the Sabi valley coreholes yield values of over 35 bcf/sq mile, comparable to areas in the San Juan basin.
BOTSWANA
Coal exploration in Botswana is confined to the subcrop of the Karoo coal measures along the eastern and southern rim of a large basin which is thought to underlie almost half the country. Exploration into other portions of the basin is limited because most of the basin is overlain by the Kalahari desert.
The only coal mine in the country is the Morupule mine that exploits the nearly 30-ft thick Morupule main seam. In addition to the main seam, two unmined seams ranging from 10 to 20 ft thick are also present.
Botswanan coal seams are high-volatile bituminous in rank and relatively flat-lying.
EUROPE
European countries have a long history of producing and using gas associated with coal mining operations. However, interest in developing coalbed gas as a stand-alone venture is only recent.
Successful coalbed gas development in Europe could have a tremendous impact on its energy sector given that nearly every European country possesses coal resources.
A review of all the countries is beyond the scope of this article and therefore this section will focus only on three of the major European coal producers: Germany, Poland, and the U.K. (Fig. 5).
However, significant coal and coalbed gas resources have been identified and are the target of exploration efforts in Ukraine, Spain, Czechoslovakia, Hungary, France, Belgium, and The Netherlands.
The Carboniferous coal seams of Europe share many similarities. Most of the seams are only moderately gassy. Gas contents are mainly in the 150-350 cu ft/ton range. Locally, however, values of up to 700 cu ft/ton have been reported for high-rank bituminous and anthracitic coal.
The coal resource is also contained in numerous, relatively thin seams distributed over a large stratigraphic interval. Many European basins are structurally complex and indications are that some coal seams are undersaturated in gas because of previous uplift, erosion, and natural degassing.
GERMANY
Germany's bituminous coal resource is concentrated in the western part of the country, primarily in the Ruhr and Saar basins. In these basins, numerous (50-100) coal seams may occur within a thick Carboniferous section. Total net coal thickness of the section can exceed 600 ft. Individual seams are generally thin, averaging less than 3 ft.
Seams outcrop along the basin edge and reach depths of over 6,000 ft in the basin interior. Germany also has considerable lignite that is located mainly in the eastern portion of the country. These Tertiary lignite seams are generally shallow, low in gas content, and range from 10 to 300 ft in thickness.
POLAND
The coalbed gas potential of Poland has received considerable attention during the past 2 years. Ranked fifth in the world in terms of coal production, Poland has three major Carboniferous coal basins: the lower Silesian, upper Silesian, and Lublin basins.
Lower Silesian basin coalbeds are medium to low-volatile bituminous and generally thin, 3-6 ft thick. Net coal thickness can be considerable, up to 120 ft, because 40-50 seams may be present in a section.
Southeast of the lower Silesian basin lies the upper Silesian basin which appears to have the highest development potential. Upper Silesian coal seams are thicker, generally ranging from 3 to 20 ft, and more numerous with 100-400 seams present within the Carboniferous section.
The Lublin basin was discovered just over 20 years ago and as a result is less defined than Silesian basins. While the Lublin basin may represent a substantial coalbed gas resource, the reported gas content measured in mines is low.
U.K.
The major coal fields of the U.K. are all Carboniferous in age and can be grouped into four geographic regions: southern, central, northern, and Scottish. In the southern region, the South Wales coal field is the major coal basin and contains numerous seams (up to 70); each is 3-12 ft in thickness. Coal rank ranges from high-volatile bituminous to anthracite, and the basin is heavily disturbed by faulting and folding.
The Yorkshire-Nottinghamshire basin is the principal coal basin in the Central coal region. Coal rank is bituminous and numerous seams, ranging from 1 to 10 ft thick, are present. In general, the basin is structurally uncomplicated although some faulting is present.
The Northumberland-Durham basin dominates the Northern coal region. Coal seams and basinal structure are similar to the Yorkshire-Northumberland basin with the exception that most of the resource is located under the North Sea.
Coal seams in the Scottish region are predominantly high-volatile bituminous, although igneous intrusions have locally raised the rank of the coal to anthracite. Seams range from 2 to 20 ft thick and are numerous. Faulting is common in the basin.
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