Coals are the preserved remains of organic materials that have been metamorphosed over geologic time by temperature and pressure into complex organic rocks containing a mixture of gases. Organic materials have been deposited and removed continuously since the appearance of life on this planet but are only rarely preserved and buried sufficiently long to become coals. Coals first appear in the geological record as coaly layers derived from aquatic plants growing in shallow lagoons of the Lower Devonian.2 The first true coal deposits, distinct from the surrounding sediments, are associated with the appearance of land-based plants in the Middle and Upper Devonian, some 400 to 350 million years ago. The five major coal-forming periods, from oldest to youngest, are the Carboniferous (355 to 290 million years ago), the Permian (290 to 245 million years ago), the Jurassic (206 to 143 million years ago), the Cretaceous (143 to 65 million years ago), and the Cenozoic or Tertiary (beginning 65 million years ago).
As seen in table 3–1, Clayton classified almost one-third of the coals that have survived to modern times as from the Permian, about one-fifth of extant coals as Cenozoic or Jurassic in age, one-sixth from the Carboniferous, and one-eighth from the Cretaceous.3
Table 3–1. Distribution of coals by geologic age
Period Coal, % Cumulative coal, %
Cenozoic 21.1% 100.0%
Cretaceous 12.9% 78.9%
Jurassic 18.2% 66.1%
Triassic 0.7% 47.8%
Permian 29.0% 47.1%
Carboniferous 17.1% 18.1%
Devonian 1.0% 1.0%
Source: Clayton, J. L. 1993.
A slightly different age distribution was reported by Hayes, with 40% of global coal resources from the Carboniferous and Permian, 10% from the Triassic and Jurassic, and 50% from the Cretaceous and Tertiary.4 Knowledge of the geologic age of a coal deposit aids in understanding reservoir behavior but is a poor predictor of commerciality.
Estimates of world coal resources vary between 4 and 30 trillion tonnes, reflecting differences in technical definitions (coal density cutoffs, mineable seam thicknesses, and ash concentrations) and confusion between resources and reserves.5 The best estimate of global coal mass is on the order of 10 trillion tonnes, of which less than 10%, only 847,533 m (million) tonnes, are classified as proved recoverable coal reserves.6 Table 3–2 shows global coal reserves sorted by rank and continent.
Table 3–2. Distribution of coals by rank and location Continent Bituminous including
anthracite, m tonnes Subbituminous,
m tonnes Lignite,
m tonnes Total,
m tonnes Bituminous including
anthracite, % Subbituminous,
% Lignite, % Total, %
Africa 49,431 171 3 49,605 11.5% 0.1% 0.0% 5.9%
North America 116,592 101,440 32,661 250,693 27.1% 38.0% 21.8% 29.6%
South America 7,229 9,023 24 16,276 1.7% 3.4% 0.0% 1.9%
Asia 146,251 36,282 34,685 217,218 33.9% 13.6% 23.2% 25.6%
Europe 72,872 117,616 44,694 235,182 16.9% 44.1% 29.8% 27.7%
Middle East 1,386 0 0 1,386 0.3% 0.0% 0.0% 0.2%
Oceania 37,135 2,305 37,733 77,173 8.6% 0.9% 25.2% 9.1%
total world 430,896 266,837 149,800 847,533 100.0% 100.0% 100.0% 100.0%
% of total world 50.8% 31.5% 17.7% 100.0%
Source: World Energy Council. 2007.
On the basis of rank, one-half of global coal reserves are bituminous or anthracite, one-third are subbituminous, and one-sixth are lignite. Most high-rank coal reserves are concentrated in Asia and North America; most subbituminous reserves are located in Europe and North America. Lignite coal reserves are evenly distributed between North America, Asia, Europe, and Oceania.
However, coal resources have historically been assessed from a mining perspective and therefore imperfectly reflect coal gas resources. The shallow, thick, high-purity coal deposits that are considered mineable are but a small fraction of the coals that may be gas bearing. Coal resources and reserves discussed above are based on various coal depth cutoffs, all less than 1,800 m (5,900 ft) and most less than 1,000 m (3,300 ft). Minimum coal seam thickness ranges from 0.1 m to more than 1 m and, within a given country, frequently varies with coal rank.
Coal purity cutoffs are often related to Schopf’s definition of coal as a rock that is at least 50% by weight and 70%
by volume carbonaceous material.7 Such screens are at once too conservative and too liberal for estimation of coal gas resources and reserves. They are too conservative in that deep, thin, high-ash coals may have generated significant quantities of gas and are too liberal in that a shallow, thick, low-ash coal seam may have insufficient permeability for commercial gas production.
The organic precursors to coal have been deposited at all latitudes. Galloway and Hobday noted the latitude of deposition for coals depends on geologic age.8 Carboniferous coals were predominantly deposited in equatorial conditions, while Permian coals reflect cold, high-latitude, postglacial environments. Jurassic coals were deposited at mid- to high latitudes, Cretaceous coals originated in swamps at latitudes above 45°, and Tertiary coals were deposited at all latitudes. Latitude dictates climatic conditions of swamps, and plant communities of those swamps became more diverse over geologic time. Equatorial swamps are populated with luxuriant plant communities and commonly experience rapid, continuous accumulation of peat. A tropical swamp can renew itself in seven to nine years, with trees growing up to 30 m in height.9 In contrast, high-latitude swamps are often characterized by reeds, mosses, and other low plants, with episodic accumulation of organic matter, such as autumnal plant litter. Plant communities in early coal swamps were characterized by mosses, shrubs, and low ferns. As plant life evolved, vegetation in the swamps became more diverse and included large trees up to 35 m in height and a variety of grasses. Evolution of flowering plants in Cretaceous times further diversified plant communities and the resulting coals.
Peats are deposited at annual rates of 0.3 mm to 3 mm and are compressed during coalification to about 10%
of their original thickness.10 Thus, a 1 m coal seam represents 3,000 to 30,000 years of stable swamp deposition.
Alternatively, the slow depositional rate of 0.3 mm/year over the 5,000 years of recorded history would result in a coal seam thickness of 15 cm, roughly equal to the span of a human hand. The rapid depositional rate of 3 mm/y over the same time interval would lead to a coal seam thickness of 1.5 m, a bit less than the average height of modern humans.
Seven factors controlling coal properties identified by Stach et al. included type of deposition, plant communities, nutrient supply, swamp acidity, peat temperature, redox potential, and deposition setting.11 Peat, the organic precursor of coal, can be laid down by in-situ vegetation (autochthonous) or derived from plant material washed into the swamp (allochthonous) or from reworked in-situ vegetation (hypautochthonous).
Almost all mineable coal deposits are from autochthonous peats, as the other two types often have high ash.
From a reservoir perspective, autochthonous coals, with generally lower ash contents, would be expected to have higher gas contents than the other two types, but coal permeability appears to be largely independent of the source of the vegetation.
Plant communities, the second element noted by Stach et al., vary continuously in time and space across a swamp.12 For instance, a shallow lake with soft-bodied aquatic plants fills steadily from the margins as shoreline species, such as reeds and then trees, migrate inward. Or openwater swamps progress to reedy swamps and then forest swamps before fires force a return to openwater swamps. Plant communities affect a coal reservoir more on the microscopic scale, e.g., macerals, than the macroscopic scale of gas resource and producibility.
Swamps with an abundance of nutrients (eutrophic) are characterized by lush, robust growth of a diverse plant community. Swamps with minimal nutrients (oligotrophic) exhibit stunted growth from a limited number of plant species. Swamps distinct from these two end members, with mixed nutrient supply (mesotrophic), support sufficient growth for peat accumulation from a modest number of species. Most bituminous coals were laid
down in eutrophic swamps sustained by regular flooding of rivers, lakes, and seas.13 The rapid, diverse growth of a eutrophic swamp can result in high-purity, relatively permeable coals, with minimal flow barriers from shaley or sandy streaks laid down in episodic floods. The slow accumulation of organic matter in oligotrophic swamps is conducive to contamination by periodic floods and airborne dust, resulting in finely dispersed ash throughout a relatively low-permeability coal body.
Acidity of swamp water, the fourth element identified by Stach et al., affects preservation of organic material.14 Highly acidic peats promote preservation of vegetal accumulations. As pH values rise above about 5.0, a peat accumulation is subject to severe degradation.15 In general, salt marshes are more acidic than freshwater swamps.
Peat preservation is quite sensitive to acidity, and even brief episodes of elevated pH, such as seasonal flooding, can degrade peat. Highly alkaline environments support rapid degradation of plant matter and are prone to pyrite formation if sulfur is present, especially if a peat experiences a marine transgression or if iron is present in the groundwater. Pyrite, with a density of 5.0 g/cm3, can affect the density log, as discussed in the section on wireline logs below.
Peat temperature also influences preservation of plant material, as optimal bacterial degradation occurs at 35°C to 40°C (95°F to 104°F). Rapid burial and subsidence of cool swamps can preserve accumulated organic materials, whereas slow burial of a tropical swamp promotes peat degradation. While peat temperature is important for primary decomposition of organic material and the genesis of coalification, it is only a minor control on coal reservoir properties.
Redox potential, the sixth factor identified by Stach et al., addresses degradation of plant materials in the presence of oxygen.16 The low oxygen levels of anaerobic environments, often characterized by stagnant waters, promote plant preservation, whereas the high oxygen levels of aerobic swamps do not. Oxidation of peat primarily affects the resultant coal on the microscopic level through maceral formation rather than macroscopic reservoir properties.
Depositional setting, the final factor affecting coal properties identified by Stach et al., is a major control on coal reservoirs.17 Coals have been laid down in a variety of depositional environments, including alluvial fans, fluvial, delta, shore-zone, and lacustrine systems. Detailed geologic aspects of each of these environments are discussed elsewhere.18 Influences of depositional environment on coal gas reservoir engineering are discussed briefly here.
Alluvial fans are cone-shaped bodies built up by shifting networks of ephemeral streams (dry fans) or perennial streams (wet fans). Peats are developed on the distal edge of the cone or in intralobe depressions.
Stream migrations initiate and terminate deposition of peat bodies, while episodic flooding, especially in fans with high sediment loads, increases ash content of the resulting coals. The resulting coal deposits are irregularly shaped bodies generally aligned with the alluvial cone structure or along the distal edge of the cone where groundwater nourished peat swamps or the fan merged into a lacustrine environment. Examples of coals formed in alluvial fans include those of the Karoo Basin of South Africa and the Sydney Basin of Australia.19
Permian coals of the Sydney Basin formed at the intersection of large, cold climate gravel fans and margins of glacial-fed lakes. The 15 or more seams commonly present in a fan cover a gross stratigraphic interval of 400 m and are typically 1–3 m thick, with a maximum thickness of 9 m. The thin coals and a net-to-gross ratio of approximately 0.1 of this coal deposit imply multizone completions. Flow capacity of a coal zone completed in a well depends not only on permeabilities of the coal bodies proper that comprise the seam, but also on permeabilities of any preserved stream channels that defined or scoured the coal bodies. Water production from channels filled with high-permeability sandstone could prolong coal dewatering, delaying gas production.
In contrast, tight sandstones could retard water influx or perhaps even provide a flow barrier, accelerating dewatering and gas production. Individual seams in a Sydney Basin alluvial fan can cover up to 400 km2 (150 mi2). Assuming the Kokand alluvial fan of semiarid western Siberia is a modern analog of ancient alluvial fans, where a typical peat body occupies 1% of total fan area, yields a coal body area of 4 km2 (400 ha or 990 ac).20 Under the simple assumption that the peats of an alluvial fan are subjected to uniform burial and coalification with little tectonic deformation over geologic time, the resulting coal deposit is still a complex assembly of coal bodies. These coal bodies are difficult to correlate as individual seams and often provide infill opportunities during coal gas extraction.
A second coal depositional environment is fluvial systems, which primarily collect and transport sediment into basins by water flow.21 Fluvial systems can persist over geologic time, but individual flow channels are dynamic, often shifting laterally, splitting and joining, abandoning and reoccupying a given course. Peats often develop in the interchannel zones and are characterized by slow depositional rates of 20 to 80 cm per millennium.22 The resulting coals are often elongated along dip, with narrower coal bodies associated with high-energy, linear flows and wider deposits associated with meandering, lazy flows. Coals along the fluvial margin are often thin and split and show a higher ash content than those from the back swamp area, which is relatively free of sediment loading except during seasonal and major floods. Established peat swamps resist erosion, serving to stabilize flow in existing channels and preserve interchannel organic deposits.
Coals laid down in fluvial systems can be up to 20 m thick.23 Length scales of coals deposited in fluvial systems range from tens of meters to tens of kilometers.24 The bounding fluvial channels, which compact less than peats, can affect coal well performance, similar to those of alluvial fans described above, acting either as conduits for water or as permeability barriers. Less energetic, shorter-lived streams may simply wash out a channel from the surface of a peat swamp, leaving a sand lens or stringer in the resulting coal. Crevasse splays reduce coal quality on a small scale, both laterally and vertically, degrading coal quality, generated gas, and flow capacity. Fluvial coal reservoirs display considerable heterogeneity with a spectrum of permeabilities.
Similar to alluvial fans, correlation of individual seams can be difficult over more than a few kilometers. Due to the variation in thickness and areal extent of coals deposited in fluvial systems, extraction of gas from these coals requires different development schemes. Thick, areally extensive coals can sometimes be exploited with single-zone completions in widely spaced wells, whereas thin, areally limited coal bodies can require multiple- zone completions in closely spaced wells. Examples of coals deposited in fluvial systems in the United States include the deposits of the Powder River Basin of Wyoming and Montana and those of the Gulf Coast of Texas and Louisiana.25
Deltas form where rivers enter a body of water. Common deltas include wave-dominated, tide-dominated, and fluvial-dominated deltas. Deltaic coals provide a variety of coal reservoir styles. Coals in wave-dominated deltas parallel the shore face, often connecting through nominally perpendicular channels and estuaries. The laterally extensive, strike-elongate seams are separated by beach ridges sandstones, and performance of wells completed in such coals is partially controlled by the permeability contrast between coals and sandstones. If permeabilities of coals and sandstones are nearly equal, coal gas could be recovered by vertical wells on regular spacing. If permeabilities of coals were larger than those of the sandstones, reservoir permeability is anisotropic, and coal gas recovery could be accomplished by horizontal wells aligned with the major coal body axes. If the coals are less permeable than the sandstones, coal gas recovery could be complicated by strong water influx from the latter. In the United States, coals of the southeastern Piceance Basin in Colorado were deposited in both the lower and upper regions of a wave-dominated delta.26 Coals of the lower delta plain are elongate bodies, paralleling the north-south paleoshoreline, with lengths greater than the 40 km north-south dimension of the study area. Individual seams are 10 to 13 km wide in the east-west direction, with thicknesses up to 32 m.
Upper delta plain coals were reported to be thinner and less extensive than those of the lower delta, but seam dimensions were not quantified.
Tide-dominated deltas are controlled by the relative strengths of freshwater and tidal influxes and are often oblate, with a major axis as much as 200 km in length.27 Geomorphology of these deltas is discussed in detail by Galloway and Hobday.28 Tidal flats of regressive environments support a diversity of plant communities ranging from salt-tolerant species at the progradational front to freshwater trees, shrubs, and grasses on the landward margin. In a transgressive environment, estuaries often dissect the tidal flats prior to flooding by marine waters. Tidal flats frequently support luxurious plant growth, with the resulting laterally extensive peat deposits compartmentalized by estuaries and tidal inlets. Tide-dominated deltas have sourced few commercial coals, and many technical details of this environment, including reservoir engineering aspects, of this depositional environment are not as well understood as others.29
Fluvially-dominated deltas source coal deposits with different geometries. During the constructive phase of the delta life cycle, peats are laid down in the interchannel region and are frequently dip-elongated bodies of limited areal extent and thickness. Distributaries tend to be stable, extending their course basinward, leaving
the adjacent peats relatively free of washouts, but detrital splits due to sediment-laden floodwaters are common.
Ash contents can be high. Coals deposited during the abandonment phase of fluvially dominated deltas can be areally extensive, even blanket-type, coals covering up to 500 km2.30 Coals of the lower delta plain tend to be thinner but can cover large areas, limited by distributaries and splays. In contrast, coals deposited in the transitional area from lower to upper delta plain are generally thicker and of greater areal extent. Distributary confinement is not uncommon, leading to thick, laterally extensive, high-quality coals in the central backswamp areas. As the roof rocks deposited over these coals are among the most stable, many commercially important coals are found in this transition zone. Upper delta plain coals tend to be thick but of limited areal extent, being confined to interchannel areas. Splays from these channels can be extensive, and the resulting coal bodies are characterized by several coal “benches” interbedded with thin shales and sandstones.
Shore-zone systems support peat development in backbarrier and tidal flat settings. The shallow bays, lagoons, and marshes of the back-barrier environment reflect the interplay between freshwater fluvial and marine influences. Back-barrier coals of the Northern and Central Appalachian basins in the United States are thin, discontinuous, and high in sulfur (reflecting a strong marine influence). In contrast, those of the Gippsland Basin in Australia are individually as thick as 100 m, with up to eight mudstone partings from minor basinwide transgressions.31 Back-barrier coals are parallel to the shoreline and may be crosscut by tidal inlets.
Under favorable circumstances, these parallel peat bodies grow laterally and vertically, clogging tidal creeks and coalescing into widespread peats. Transgressive episodes favor deposition of these laterally extensive coals.
Barriers of a shore-zone system help preserve delta plain peats, with the ash fraction of the resulting coals affected by splays and washovers. Higher ash content often correlates with higher energy environments. Tidal flats are frequently high-energy environments that rework or remove organic material, resulting in slow accumulation of randomly oriented peat bodies. The resulting coals are thin, defined by estuaries cutting the tidal flat, and high in sulfur. Shore-zone coal deposits are typically thin, multilayered gas reservoirs. Each layer is composed of erratically shaped coal bodies with no clear depositional orientation. Development of shore-zone coals typically requires multizone completions and tight well spacing.
Lacustrine environments can provide conditions suitable for peat development, especially in deltas, but are often on smaller scales than in other environments discussed above. Length scales of lakes tend to be smaller than those of other depositional environments, restricting peat body development accordingly. Plant communities on a lake perimeter are controlled by shoreline gradient, with more areally extensive bodies associated with shallow, gentle slopes, and river deltas, where sediment loads and thermal contrasts control deposition, and hence, peat growth. Water levels in lacustrine settings vary more frequently and over larger intervals than in other environments. Annual lake-level fluctuations on the order of 1 m, 5 m variations on the order of a century, and 350 m over 75,000 years have been recorded.32 For perspective, coals form only where there is balance between being too low and subject to flooding with clastics and too high to prevent peat accumulation. The difference between “too low” and “too high” is on the order of a few meters.33 Lacustrine coals reflect shoreline geometry, sometime paralleling an ancient shore, sometimes showing characteristics of interchannel peats of a fluvial delta. A warm, shallow, stable lake could be filled by organic matter as plant communities on the perimeter progressively advanced across the lake, ultimately filling it with peat.
In any given basin, depositional environments often vary areally and stratigraphically, resulting in coal bodies with different reservoir characteristics. A present-day example of concurrent depositional diversity is the Barataria Basin, which is bounded by distributary channels of the Mississippi River, one abandoned, one active, and by barrier islands between the bay and the Gulf of Mexico.34 An elongate basin more than 150 km long and 60 km wide, the distal end is a saline marsh where the bay joins the Gulf of Mexico. Moving landward, the saline marsh gives way to a brackish marsh and then a freshwater marsh. The final upstream environment is forested wetland, including some hardwood stands, with characteristic lengths on the order of 5 km. Several lakes, present in the upper reaches of the basin, show characteristic lengths of 10 to 20 km. Crevasse splays are lobate or teardrop-shaped, with a maximum dimension of 15 km.
Reservoir complexity increases during coalification through several mechanisms, including compaction and fracturing. As noted above, peats compact in the ratio of 10:1 during burial. In contrast, Laubach noted sand body compaction is around 1.5:1.35 Thus, concurrent deposition of peats and sands in, for example, fluvial or