Assessment of Appalachian basin oil and gas resources: Carboniferous Coal-bed Gas Total Petroleum System

The Carboniferous Coal-bed Gas Total Petroleum System, which lies within the central and southern Appalachian basin, consists of the following five assessment units (AUs): (1) the Pocahontas Basin AU in southern West Virginia, eastern Kentucky, and southwestern Virginia; (2) the Central Appalachian Shelf AU in Tennessee, eastern Kentucky, and southern West Virginia; (3) the East Dunkard (Folded) AU in western Pennsylvania and northern West Virginia; (4) the West Dunkard (Unfolded) AU in Ohio and adjacent parts of Pennsylvania and West Virginia; and (5) the Appalachian Anthracite and Semi-Anthracite AU in Pennsylvania and Virginia. Only two of these assessment units were assessed quantitatively by the U.S. Geological Survey (USGS) in the National Oil and Gas Assessment in 2002. The USGS estimated the Pocahontas Basin AU and the East Dunkard (Folded) AU to contain a mean of about 3.6 and 4.8 trillion cubic feet (TCF) of undiscovered, technically recoverable gas, respectively.

In general, the coal beds of the Carboniferous Coal-bed Gas Total Petroleum System (which are both the source rock and the reservoir) were deposited as peat together with their associated sedimentary strata in Mississippian and Pennsylvanian (Carboniferous) time. The generation of biogenic (microbial) gas probably began as soon as the peat deposits formed. Microbial gas generation is probably occurring at present to some degree throughout the Appalachian basin, wherever the coal beds are relatively shallow and wet. As a result of significant depth of burial, compaction, and coalification during the late Paleozoic and early Mesozoic, the coal beds were heated sufficiently to generate thermogenic gas (coalbed methane) in the eastern part of the Appalachian basin.

Trap formation began with the deposition of the peat deposits during the Mississippian and continued into the Late Pennsylvanian and Permian, when strata of the Appalachian Plateaus were deformed during the Alleghanian orogeny. The seals are the connate waters that occupy fractures and larger pore spaces within the coal beds, as well as the fine-grained, siliciclastic sedimentary strata that are intercalated with the coal. The critical moment for the petroleum system occurred during the Alleghanian orogeny, when deformation resulted in the geologic structures in the eastern part of the Appalachian basin that enhanced fracture porosity within the coal beds. In places, burial by thrust sheets (thrust loading) in the Valley and Ridge physiographic province may have resulted in the additional generation of thermogenic coalbed methane in the Pennsylvania Anthracite region and in the semianthracite deposits of Virginia and West Virginia, although other explanations have been offered.