Coal deposits
Formation
Coalification
In geologic terms, coal is a sedimentary rock containing a mixture of constituents, mostly of vegetal origin. Vegetal matter is composed mainly of carbon, hydrogen, oxygen, nitrogen, sulfur, and some inorganic mineral elements. When this material decays under water, in the absence of oxygen, the carbon content increases. The initial product of this decomposition process is known as peat. Peat can be formed in bogs, marshes, or freshwater swamps, and in fact huge freshwater swamps of the geologic past provided favourable conditions for the formation of thick peat deposits that over time became coal deposits. The transformation of peat to lignite is the result of pressure exerted by sedimentary materials that accumulate over the peat deposits. Even greater pressures and heat from movements of Earth’s crust (as occurs during mountain building), and occasionally from igneous intrusion, cause the transformation of lignite to bituminous and anthracite coal.
Major coal eras
Coal deposits are known to have formed more than 400 million years ago. Most anthracite and bituminous coals occur within the 299- to 359.2-million-year-old strata of the Carboniferous Period, the so-called first coal age. The formation of coal deposits continued through the Permian, Triassic, and Jurassic periods into the “second coal age,” which includes the Cretaceous, Paleogene, and Neogene periods. Coals of the Cretaceous Period (145.5 million to 65.5 million years ago) are generally in the high-volatile to medium-volatile bituminous ranks. Cenozoic coals, formed less than 65.5 million years ago, are predominantly of the subbituminous and lignitic ranks.
Rank and grade
The rank of a coal indicates the progressive changes in carbon, volatile matter, and probably ash and sulfur that take place as coalification progresses from the lower-rank lignite through the higher ranks of subbituminous, high-volatile bituminous, low-volatile bituminous, and anthracite. The rank of a coal should not be confused with its grade. A high rank (e.g., anthracite) represents coal from a deposit that has undergone the greatest degree of devolatilization and contains very little mineral matter, ash, and moisture. On the other hand, any rank of coal, when cleaned of impurities through coal preparation, will be of a higher grade.
Resources and reserves
Distribution worldwide
Coal deposits are found in sedimentary rock basins, where they appear as successive layers, or seams, sandwiched between strata of sandstone and shale. There are more than 2,000 coal-bearing sedimentary basins distributed around the world. World coal resources—that is, the total amount of coal available in the world—are approximately 11 trillion tons. The distribution of the estimated coal resources of the world is approximately as follows: Europe (including Russia and the former Soviet republics) 49 percent; North America 29 percent; Asia 14 percent; Australia 6 percent; and Africa and South America 1 percent each. Distinct from coal resources are coal reserves, which are only those resources that are technically and economically minable at a particular time. The current recoverable coal reserves of the world are estimated at 760 billion tons. Their distribution by continent is: Europe 44 percent; North America 28 percent; Asia 17 percent; Australia 5 percent; Africa 5 percent; and South America 1 percent.
Economic factors
Among the most important factors that influence the movement of a coal deposit from a resource to a reserve or vice versa are the price of coal in the energy market and the costs of producing the coal for that market. Currently, seams less than 30 centimetres (1 foot) in thickness are not considered economically recoverable. Furthermore, extraction from seams at great depth—i.e., over 1,000 metres (3,300 feet)—presents great difficulties. Other geologic features, such as excessively steep seams, extensive faulting and folding, washouts created by erosion and sedimentation, and burnout of the coal seams by igneous intrusion, all affect the amount and quality of coal that can be recovered from a seam.
Prospecting and exploration
The fundamental objective of coal prospecting is to discover coal resources through a search. In areas where coal mining has not been previously practiced, the search process should result in obtaining coal samples that give reasonable evidence of the existence of a coal seam. Once a seam has been discovered, considerable further work is necessary in order to advance knowledge of the particular geologic aspects and the extent of the coal deposit. The term coal exploration is used to describe these activities. Coal exploration includes activities and evaluations necessary to gather data for making decisions on such issues as the desirability of further exploration, the technical feasibility of mining (including favourable and unfavourable factors), and economic feasibility (including size of mine, coal quality assessment, marketability, and preparation of mined coal for market requirements).
Mapping
Geologic mapping is an important task in exploration. Mapping involves compiling detailed field notes on coal seams, strata above and below the seam, rock types, geologic structures, stream data, and man-made structures. Good maps and mapping techniques provide a means for planning and accomplishing exploration, development, reclamation, day-to-day operations, and equipment moves. Calculation of material volumes, location of physical elements, and determination of mining conditions are expedited by the use of maps. Maps also provide a method for recording data so that they can be organized and analyzed for ready reference.
Aerial photography and mapping methods (photogrammetry) are increasing in usefulness, particularly in the exploration and mining of surface deposits. Photogrammetric methods are relatively easy and inexpensive, can be adjusted to any scale, and are highly accurate in any terrain. Aerial photography can be conducted at an altitude designed to produce maps that show drainage configuration, roads, buildings, lakes, streams, timber, power lines, railroads, and fences or other features that may be missed by a ground survey.
Drilling
Drilling is the most reliable method of gathering information about a coal deposit and the mining conditions. It provides physical samples of the coal and overlying strata for chemical and physical analysis.
Spatial patterns
Numerous factors are associated with a drilling program. One is the spatial pattern of the holes in an exploration area. When very large areas are being studied, hole spacings vary greatly and generally are not in any set pattern. When the program is narrowed to a specific target area, a grid pattern is most common. In areas where coal is known to exist, closely spaced drill-hole patterns are required.
Core drilling and rotary drilling
A second factor associated with a drilling program is the choice between core drilling and rotary drilling. In core drilling, a hollow drill bit is attached to a core barrel so that cylindrical samples of the strata can be obtained. (Since the drill bit is faceted with diamonds for cutting the strata, this method is also called diamond core drilling.) Photographing the cores as they come out of the hole can provide data of great reliability. In rotary drilling, the samples obtained are the chips and pulverized rock produced by the abrasive and chipping action of the drill bit. Rotary drilling is faster and comparatively less expensive than core drilling. In fact, it is not uncommon to drill down to the top of the coal seam by rotary drilling and then replace the drill tools for core drilling. In most programs, only 10 to 25 percent of the holes are actually cored for detailed information on overlying strata and coal. Coring of the coal seam itself, however, should closely approach 100 percent; if it does not, the analytical information obtained should be considered suspect.
Dozer cutting
Exploration of coal outcrops may be accomplished with dozer cuts at regular intervals. Dozer cutting provides information on the attitude of the coal and on the nature of the overburden—important factors with regard to machine operation.
Geophysical exploration
In geophysical exploration, the seismic, electric, magnetic, radiometric, and gravitational properties of earth materials are measured in order to detect anomalies that may be caused by the presence of mineral deposits. Their form of exploration may begin with airborne methods in regional and target-area investigations and continue with on-ground methods during detailed investigations. The most widely utilized airborne methods are, in increasing order of use, magnetic, magnetic plus radiometric, magnetic plus electromagnetic, and electromagnetic. These methods are almost always accompanied by aerial photography.
Ground geophysical methods have a major advantage over the airborne methods in that they are in direct contact with the earth. The principal methods are electrical, magnetic, electromagnetic, radiometric, gravimetric, and refraction-seismic. The drill-hole geophysical survey, called logging, is an important method of extending data acquisition beyond the drill hole. A combination of logging methods is advantageous: gamma-ray and density logging for identifying the type of coal present; gamma-ray (radiometric), resistivity (electric), and calliper logs for determining the thickness of the seam; and sonic and density logs for determining the condition of the roof and floor strata.
