West Virginia has coal deposits that range in age from the Late Devonian through the Mississippian to the Late Pennsylvanian geologic periods. The state's oldest known coal is found in several impure beds of the Upper Devonian Hampshire Formation, along the western edge of the mountains from Pocahontas County northward into Maryland. These coal beds are generally only a few centimeters thick and have not been mined except for a short-lived operation near Harman.

Mississippian Period coal beds occur in the Price, Maccrady, and Mauch Chunk formations, but these beds rarely reach minable thickness or purity. Coal found in the Lower Mississippian Price Formation reaches minable thickness locally in the Sleepy Creek area of Hampshire and Berkeley counties. Several thin, impure coal deposits occur in the Lower Mississippian Maccrady Formation in the southeastern part of the state. Thin, laterally discontinuous coal beds occur in the Lower Mississippian Bluefield, Hinton, and Bluestone formations in southern West Virginia.

All of West Virginia's commercial coal production comes from Pennsylvanian Period coal beds, assigned to seven formations or groups. From the oldest to youngest, these are the Pocahontas, New River, Kanawha, Allegheny, Conemaugh, Monongahela, and Dunkard. They are divided into a southern, low-sulfur coalfield and a northern, high-sulfur coalfield. Coal beds of the southern coalfield are generally older (Lower and Middle Pennsylvanian), higher in rank, lower in sulfur content and ash yield, and generally of better quality than those of the younger (Middle and Upper Pennsylvanian) northern coalfield. Generally, southern West Virginia mines produce clean-burning "compliance" steam coal for power generation, and much also is used for coke production for steel making; northern coal is also used for power generation, although these mines rarely produce compliance coal. There is more minable coal in the south.

The geologic distribution of coal was mainly determined by conditions during long-ago periods of peat accumulation, including the climate and the state of evolution of terrestrial plants at the time. Plants remained small until the Late Devonian, more than 350 million years ago, when large trees evolved and widespread forests first appeared. Once plants with woody tissues capable of being preserved appeared, peat accumulation was possible.

An abundant, year-round supply of fresh water was very important to the accumulation of the peat deposits that were to become the state's coal beds. Peat forms where plant debris accumulates faster than herbivores and decay destroy it. Peat bodies are like sponges, composed mainly of the woody parts of the plants. During the Pennsylvanian Period, more than 290 million years ago, the location of the continents was different. Present West Virginia was within a few degrees of the equator, and the state's numerous coal beds formed in tropical rain forests on a low-lying coastal plain. It rained frequently, without long dry spells. Rainwater low in dissolved minerals fell onto the surface of the peat bodies, soaked in, and percolated downward and outward. The constant influx and movement through the porous peat of fresh rainwater excluded mineral-bearing ground and surface waters, keeping mineral content low and the pH of the water low. A cleaner coal resulted. The high growth rates of tropical rain forests allowed the peat to accumulate rapidly.

During the Pennsylvanian Period, present West Virginia was located in a slowly subsiding basin, a necessary condition for the preservation and burial of peat. As the basin subsided, sediments were continually deposited. Temperature and pressure rose as burial depth increased, slowly converting the peat into coal.

Coal rank is a measure of the amount of coalification, or transformation a coal bed has undergone in the progression from peat through coal. Increasing amounts of heat and pressure applied over geologic time progressively alter the original plant material through peat; lignite; subbituminous; high-, medium-, and low-volatile bituminous; anthracite; meta-anthracite; and graphite.

West Virginia's coal is bituminous in rank. Coal beds of the southern coalfields range in rank from low-volatile bituminous in the southeast and decrease progressively northwestward and strati-graphically upward to high-volatile bituminous in the area of the Kanawha Valley near Charleston. The higher rank of the older coal beds indicates increased coalification from deeper burial, and the southeastward increase in rank resulted from increased heat and pressure due to proximity to the mountain-building events known as the Appalachian Orogeny. With the exception of the Eastern Panhandle, coal beds of the northern coalfield are generally high-volatile bituminous. Coals of the Eastern Panhandle are highly variable in rank even over short distances. The lower rank of the state's northern coalfield is the result of less depth of burial, less time for coalification, and greater distance from the Appalachian Orogeny.

Not all coal beds are economically minable. Early in the 20th century, the West Virginia Geological and Economic Survey estimated that the state had an original resource base of more than 116 billion tons of coal, counting only seams greater than 12 inches thick. As of 2009, more than 23.5 billion tons of coal had been mined or lost in mining, according to records kept by the West Virginia Office of Miners' Health, Safety, and Training. Allowing for the coal that cannot be mined for various reasons, it is estimated that some 55 billion tons of recoverable resources still remain in the 2020s, theoretically enough to mine at present rates for many years.

In practice, however, many constraints operate to radically reduce the amount of coal that is actually minable. Factors affecting the minability of a coal bed include thickness, quality, roof type, distance to the transportation network, overburden thickness and type, depth below valley bottoms, and various cultural factors. As a result, the minable reserve is significantly smaller than the overall coal resource base and tends to vary over time with changes in demand, in coal production and utilization technology, and in the physical infrastructure of the state.

West Virginia's coal is distributed unequally, both geographically and strati-graphically. There are vastly more coal resources in the southern coalfield than the northern coalfield, due to a progressive drying of the climate throughout the Pennsylvanian Period. The six coal seams with the highest original resources are (in billions of tons) the Pittsburgh coal (13.3); the Lower Kittanning/No. 5 Block coal (10.3); No. 2 Gas (8.0); Sewell coal (6.3); Stockton (4.7); and Cedar Grove (4.3). The five counties with the highest original resources are (in billions of tons) Logan (8.1); Boone (8.1); Mingo (6.3); Webster (6.3); and Nicholas (6.2).