Adsorption.—The process whereby gas adheres to the internal surfaces of a coal. At low to moderate pressures—typically found at normal exploration depths of less than 2 km—this layer of gas is one molecule thick. At low pressure, there is little difference between total and adsorbed gas.
Adsorption isotherm.—The gas retention capacity of a powdered coal sample at a constant temperature but at different pressures. Usually measured under equilibrium moisture conditions.
Alluvial plain.—A level or gently sloping land surface that is covered by extensive deposition of alluvium.
Ash yield.—Non-combustible inorganic residue remaining after a coal has been completely combusted. It represents the bulk of mineral matter in a coal after carbonates, sulfides, and clays are broken down during heating. Therefore, ash yield is less than the total mineral-matter content.
Biogenic gas.—Gases produced from coal by the metabolic activities of microbes.
Bituminous coal.—An intermediate rank humic coal whose rank as measured by vitrinite reflectance that exceeds 0.5% Ro.
Canister.—A gas-tight container for holding a sample as it is being desorbed.
Carbonaceous mudston (carbonaceous shale).—A dark-gray or black rock that contains an abundant amount of carbon in the form of small particles of organic mater; it is commonly associated with coal seams.
Channel sample.—A channel of consistent volume is cut across a seam and all coal within the cut is collected for analysis.
Clastic wedge.—A concave asymmetrical layer of sediments that forms from a domination of sediment input form one side of the basin, generally due to a topographic high or increased sediment input.
Cleats.—Orthogonal sets of fractures in coal caused by shrinkage related to desiccation, devolatization, and structural processes. Somewhat analogous to joints in other rocks.
Coal.—A rock dominantly composed of sedimentary organic matter. A coal contains more than 50 percent by weight and more than 70 percent by volume of sedimentary organic matter.
Coal-bed gas.—Gas produced from the desorption of coal. It is usually composed of methane, but carbon dioxide, nitrogen, and light alkane hydrocarbons are also commonly found.
Coal-bed methane (CBM).—Strictly referring to the methane produced from coalbed gas.
Coal matrix.—Solid unfractured pieces of coal that are bounded by cleats.
Coalifi cation.—The diagenetic process whereby plant debris is altered to coal during burial heating.
Coal seam.—A single bed of coal including partings within that one coaly interval.
Critical desorption pressure.—The pressure at which gas begins desorbing from coal beds.
Delta-front.—The frontal area of a delta where sediment is transported and deposited. On a prograding delta, a thick continuous sandstone is produced that displays abundant high-angle crossbedding.
Delta-plain.—The landward portion of the delta where a river channel bifurcates and forms abundant distributary channels. This area is characterized by swampy lowlands.
Desorption.—The process of removing gas from sorption sites by reducing the partial pressure in the coal.
Dewatering.—The removal of water from the subsurface. In the case of coal, the removal of water reduces the cleat pressure and causes the onset of methane production.
Dry, ash-free basis.—Coal-analysis data recalculated to mathematically remove ash yield and moisture content from the data.
Equilibrium moisture.—Moisture content after saturating a coal sample with water at 96–97 percent relative humidity at 30°C. The humidity is maintained by placing the sample in a humidor containing a saturated solution of potassium sulfate.
Fixed carbon.—The carbon remaining after the volatile matter has been expelled during combustion.
Foreland basin.—A flexural sag in the Earth’s crust that forms from loading by thrust-sheet faulting. These basins are major sediment-accumulation sites for material being transported off of the thrust sheet and being deposited in coeval depositional systems outbound from the thrust front (i.e., shallow-sea, lake, or fluvial sedimentation).
Gas reserves.—The amount or recoverable gas determined by exploration that, under current economical and technological conditions, represents a fraction of the total gas in a reservoir.
GIP.—Acronym for gas-in-place. GIP includes all gas producible by complete desorption of coal to atmospheric pressure levels. GIP overestimates the gas resource because, realistically, a coal bed can only be depressurized to the regional pressure level, not to atmospheric pressure.
Humic coal.—A coal dominantly composed of the debris from terrestrial plants.
Hydrostatic gradient.—The pressure increase with depth of a liquid in contact with the surface. The hydrostatic gradient for most petroleum basins is 10.4 kPa/m (Hunt, 1979). This varies from the lithostatic gradient (pressure with depth due to overlying rock) of 24.4 kPa/m (Hunt, 1979).
Isopach map.—A contour map showing the variation of a component across a geographical area. Generally these maps show thickness of a rock unit or interval, but they can be used for other components, e.g., facies, rock type, fossil distribution, sedimentary environment.
Macerals.—Microscopically identifiable plant debris in coal and kerogen.
Methane.—A colorless, odorless, inflammable gas which is the simplest paraffin hydrocarbon. Formula CH4. It is the principle component of natural gas.
Microporosity.—Storage areas within a rock, usually on the scale of 1/1,000th of a millimeter.
Mineral matter.—The proportion of inorganic components in a coal before combustion analysis.
Natural gas.—Any of the gaseous hydrocarbons generated below the Earth’s surface.
Net-coal thickness.—Total thickness of coal in a coal-bearing interval. The coal seams may be separated by meters of inorganic strata.
Overburden.—The layers of rock above a specific point or rock layer.
Partings.—Thin beds within a coal zone dominantly composed of mineral matter.
Peat.—A humic coal whose rank as measured by vitrinite reflectance is greater than 0.2 and less than 0.3 percent Ro.
Permeability.—The interconnectivity of pores and micropores within a rock through which fl uids or gas can be transported; measured in millidarcies (mD).
Photomicrograph.—Photograph taken on an image from a petrographic microscope of a mounted rock sample.
Production fairways.—Areas of enhanced coal-bed gas (CBG) production relative to adjacent portions of a coal bed.
Proximate analysis.—The determination of moisture, volatile matter, fixed carbon, and ash in coals.
Rank.—Stage of thermal alteration in coal and dispersed organic matter.
Reservoir rock.—Any rock with adequate porosity, fracture or joints, or sorption potential that can store liquid or gas hydrocarbons.
Reverse fault.—A fault in which the hanging wall has moved upward relative to the footwall. A reverse fault with a fault-plane angle of less than 45° is termed a thrust fault.
Sedimentary organic matter.—Plant- and microbial-derived organic matter deposited in sedimentary and diagenetic environments.
Sorbed gas.—Gas held in a coal by sorption.
Sorption.—A physical process where molecules of gas adhere to the surfaces of a microporous substance by weak intermolecular attraction due to van der Waals or electrostatic forces. After sorption, gas forms a liquid-like condensate in a coal, which is a microporous substance. See also Absorption or Adsorption.
Source rock.—Any rock that contains or originally contained signifi cant organic material that is currently or has undergone thermogenic or biogenic maturation. Rocks in which oil and (or) gas have been generated.
Swamps.—A wooded wetland that is ground-water and occasionally surface-water dominated.
Thermogenic gas.—Gases produced from heating a coal, usually occurring during burial but may also occur during contact metamorphism.
Thrust front.—The leading edge of a thrust sheet.
Thrust sheet.—One or more rock slabs that are thrust up a low-angle reverse fault. A thrust sheet generally implies the emplacement of older over younger rocks.
Transgressive-regressive cycle.—The raising and lowering of sea level through time with respect to a given geographic point. Various recognizable sedimentation and erosional events occur during these cycles.
Vitrinite.—The group of macerals composed of woody plant matter, mostly lignin and cellulose.
Vitrinite reflectance.—The proportion of light reflected back from a normally incident light beam of known intensity from a planar polished surface of the coal maceral vitrinite. Expressed as a percentage of the incident light returned. Abbreviated Ro.
Volatile matter.—The mass lost from a coal by heating to high temperatures in the absence of air after removal of moisture from the sample.
Water table.—A layer of water below the ground surface. It is generally used to infer the depth below the surface to which the sediments are saturated with ground water.