Scientific Theory
This theory states that natural gas was formed millions of years ago when plants and tiny sea animals were buried by sand and rock. Layers of mud, sand, rock, plant, and animal matter continued to build up until the pressure and heat from the earth turned them into a tar-like substance called kerogen.
As temperatures continued to increase and the kerogen continued "cooking," more complex compounds of carbon and hydrogen known as oil were formed. Natural gas was generated at the same time as oil. However, peak generation occurs when oil begins to break down because of high geothermal temperatures (greater than 400 degrees Fahrenheit).
As natural gas molecules form, they migrate from the shale "source rock" into more porous areas such as sandstone. They continue moving to either the earth's surface where they escape into the atmosphere or are trapped when their path is blocked by nonporous rock. In the latter case, the impermeable rock layers cause natural gas accumulation to occur.
The Deep Gas Theory
In 1979, Thomas Gold of Cornell University developed the "deep gas theory" in contrast to the biological explanation of natural gas's origin. He contended that "on Earth, as in other planets, most hydrocarbons were formed from non-biological sources."
His theory proposes that the earth is made up of primordial materials that combined in space billions of years ago when the basic structure of the earth evolved. The materials are believed still buried far below the earth's crust where they have been trapped for 4.5 billion years.
Cracks and fissures in the earth's crust allow the gases to migrate into reservoirs and to the surface. In this manner, it is believed the supply of hydrocarbons produced from the primordial material was instrumental in the creation of the earth's atmosphere.
The deep gas theory further proposes that oil molecules are capable of surviving greater temperatures and pressures beneath the earth's surface and that many of the hydrocarbons that migrate up to the two- to three-mile depths do break up into methane gas. This would explain the presence of both oil and gas found at two- to three-mile depths. It also supports the theory that a much greater supply of oil is present in "deep wells" that range in depth from 50,000 to 60,000 feet below the earth's surface.
Unconventional Sources of Natural Gas
In the U.S. alone, there are estimates of trillions of cubic feet of potentially recoverable natural gas resources to be found in unconventional sources. These reserves have been largely excluded from the estimates of potential natural gas supplies in U.S. since their recovery has not been reasonably demonstrated.
Some of these unconventional sources are:
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Devonian Shales
Includes two belts of thick black shale that are separated by an area of thinner shale in the Appalachian basin of the U.S. They occupy 90,000 square miles. Devonian shale contains five to 65 percent organic matter and has for centuries been known as a natural gas rock. Of the 1,000 trillion cubic feet of natural gas that these shales contain, about 10 percent is expected to be recoverable.
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Geopressurized Zones
These zones are areas that have abnormally high pressures for their particular depth. Such zones are formed by the rapid deposition and compaction of clays. Large volumes of compacted clay are deposited on top of more porous debris such as sand and silt. Water and gas squeezed from the clays during compaction enter the sands and silts under high pressures, hence geopressure. Experts estimate between 5,735 and 49,000 trillion cubic feet of natural gas are located within geopressurized zones in the Gulf Coast region, between 10,000 and 25,000 feet below the earth's surface.
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Tight Sands
These concrete-hard geological formations contain considerable volumes of natural gas of marine or continental origin. Most tight sands consist of relatively thin sandstone or siltstone beds interbedded with shale. An estimated 800 trillion cubic feet of natural gas is located in tight sands throughout the U.S.
