Natural Gas

Natural gas, a highly flammable fossil fuel, is an important nonrenewable energy resource. Less polluting than other fossil fuels like coal and oil, natural gas is deemed relatively environmentallyfriendly, helping to increase demand for the fuel in [Page 1216]recent years. As a hydrocarbon, natural gas is primarily composed of methane, a colorless, odorless, lighter-than-air gas.

Methane is the simplest of all the hydrocarbons in molecular makeup, consisting of one atom of carbon and four atoms of hydrogen. While natural gas is relatively clean-burning, methane itself is a greenhouse gas, more efficient at trapping heat than even carbon dioxide (the major offending gas in the enhanced greenhouse effect). Burning natural gas, as with all hydrocarbons, releases carbon dioxide into the atmosphere. Increased levels of methane and carbon dioxide in the atmosphere are thought to contribute to global warming and such related potential environmental hazards as climate change, polar ice melting, glacial retreat, and the rise in sea levels.

As is the case with other fossil fuels, natural gas is thought to have been formed hundreds of millions of years ago, with dead organic matter sinking to the bottom of ancient seas. Plankton and algae, thought to be the source material for natural gas, sank to the bottom of the seas, and slowly began to be covered in silt and other sedimentary materials. Over many millions of years, the weight of the accumulating sediment, combined with the weight of ocean water, exerted tremendous pressure on the organic material. With the pressure, heat also acted on the organic source materials and over millions of years transformed these materials into hydrocarbons in the form of natural gas. This process is similar to the formation of crude oil, and natural gas and oil are often found at similar locations today. Over time and under extreme pressure, oil and gas were forced into relatively porous sandstone or limestone, referred to as reservoir rock. Subsequent deformation of the earth's crust acted to trap oil and gas into pockets under impermeable cap rock like marble or granite.

The three major geologic forces trapping oil and gas into pockets within the earth's crust are folding, faulting, and pinching out. Folding results from horizontal pressure being exerted on the cap rock, forming a fold (or anticline). Faulting represents a fissure in the cap rock, with a large section of cap rock slipping down, forming a hydrocarbon-trapping cavity. In the pinching out process, impermeable rock is forced upward into the reservoir rock, resulting in pockets trapping oil and gas. In each case, as gas is lighter than oil, natural gas migrates to the top of these deposits. Geologists are able to locate oil and natural gas deposits, often deep within the earth's crust, by looking for evidence of these geologic processes.