Definition: “A mixture of hydrocarbon compounds and small quantities of various nonhydrocarbons, widely used as a fuel throughout the industrialized world; it exists in the gaseous phase or in solution with crude oil in natural underground reservoirs” (Cleveland 2006). In general natural gas is the end result of the “cracking” (i.e. breaking up”) of the original long chain molecules of petroleum that had once been various biological materials into shorter and shorter pieces as a result of the application of heat and pressure from the thousands of meters of sediments overlying the organic material. The type of gas depends upon how many atoms of carbon remain linked together. Methane (CH4) for one, ethane (C2H6) for two, Propane(C3H8) and butane (C4H10) are all useful gaseous forms familiar to use in routine economic activity. Natural gas is often found along with oil and hence can be found by the same geological procedures as oil is found: surface geological features (including seeps), subsurface geology (using seismic processes etc), and geophysics. As a well is drilled the substrate removed by the hollow drilling device emerges at the surface and can be analyzed for its geological, paleontological and petrochemical properties. As more and more wells have been drilled geologists have been able to construct regional maps of the underground substrate so that we have very detailed information for many oil and gas producing regions. In some regions, such as Indiana County Pennsylvania, many thousands of wells have been drilled to extract gas from relatively low yielding but very extensive fields.


Natural gas is usually divided into “conventional” (meaning from oil and gas or gas “fields” of usually limited spatial extent and specific form, vs. “unconventional” which are from more diffuse fields as indicated below). Another categorization is as “associated” (with oil—usually conventional), and “non associated” fields. The various unconventional fields include:

Coal Bed Methane (CBM) -- “An unconventional form of natural gas formed in the coalification process and found on the internal surfaces of the coal. To commercially extract the gas, its partial pressure must be reduced by removing water from the coal bed. The large quantities of water, sometimes saline produced from coal bed methane wells pose an environmental risk if not disposed of properly” (Cleveland et al. 2006)

Marginal Wells, defined as wells that produce less than 60 Mcf per day (Interstate Oil and Gas Compact Commission, 2006). Marginal currently comprise about 9% of total U.S. gas production (Sell 2007).

Tight Gas defined as “A category of unconventional natural gas that is trapped underground in extremely hard rock, or in unusually impermeable sandstone or limestone formation; tight gas requires much greater extraction efforts for acceptable rates of gas flow” (Cleveland et al. 2006).

Off Shore defined as “A general turn for oil and gas industry operations taking place along a coastline (e.g., in Louisiana) or in open ocean water (e.g., the North Sea field). Thus, offshore drilling, offshore lease, and so on” (Cleveland et al 2006).

Methane Hydrate defined as “the most recent form of unconventional natural gas to be discovered and researched. These interesting formations are made up of a lattice of frozen water, which forms a sort of 'cage' around molecules of methane. These hydrates look like melting snow and were first discovered in permafrost regions of the Arctic” ( 2004).