Natural gas occurrence
| Energy: A Historical Perspective and 21st Century Forecast | |
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| Series | Studies in Geology |
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| Chapter | Sources of Energy |
| Author | Dr. Amos Salvador |
| Link | Web page |
| PDF file (requires access) | |
Natural gas is known to occur under many different conditions. Gas accumulations have been assigned to two general groups depending on the feasibility of the recovery of their contained natural gas: Conventional natural-gas accumulations are those in reservoirs with a permeability high enough to allow gas to flow at rates that make its production profitable at current development and marketing costs. They are generally supported by water. Unconventional accumulations, on the other hand, are those from which gas is not profitably recovered for one reason or another at a certain time. Schmoker[1] describes unconventional gas accumulations as "continuous accumulations . . . that exist more or less independently of the water column," and discusses several methods for their assessment.
Some of the sources of natural gas labeled unconventional in the past have produced gas profitably in the United States for at least a decade or two and will undoubtedly become major sources of energy in the decades to come. Not only in the United States but in many other countries of the world, such unconventional sources of natural gas are known to occur, and although they are not given serious recognition now, they will certainly be developed sometime in the future. Foremost in this category of former unconventional sources of natural gas in the United States are gas in coal beds (the coalbed methane), gas in low-permeability sandstones (tight sands), basin-centered gas accumulations, and gas in organic black shales (also known as fractured shales). As of 2002, they contributed 26% of the total United States gas production.[2]
Coalbed methane, which not too long ago was considered an unconventional type of gas accumulation (and the cause of deadly accidents in underground coal mines), is now being produced profitably in several basins in the United States (about 7% of the total United States natural-gas production) and in Australia and is being investigated in other parts of the world. Natural gas from tight-sand reservoirs is also becoming a substantial part of the total natural gas produced in the United States. Lesser production has been obtained for many years from organic black shales and from basin-centered reservoirs. Production of the natural gas in methane hydrates and in solution in geopressured formation waters will probably not be economically viable for many years, if ever. These last two occurrences of natural gas may be better called speculative.
See also
- Historical sources of energy
- Natural-gas liquids
- Natural gas: history and forecast
- Gas hydrates
- Coalbed methane
References
- ↑ Schmoker, J. W., 2002, Resource-assessment perspectives for unconventional gas systems: AAPG Bulletin, v. 86, no. 11, p. 1993-1999.
- ↑ Law, B. E., and J. B. Curtis, 2002, Introduction to unconventional petroleum systems: AAPG Bulletin, v. 86, no. 11, p. 1851-1852.
External links
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