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# Describing Petroleum Reservoirs of the Future

*"Future energy resources of the United States, particularly gas, will be found in what we consider today to be unconventional reservoirs, especially low-permeability reservoirs in shales, siltstones, fine-grained sands, and carbonates.  These are not, in fact, undiscovered resources, since their occurrences are fairly well-known.  However, we do not have adequate geologic data to evaluate the contribution such reservoirs will make to the National energy endowment in the future.  The U.S. Geological Survey has a significant role in conducting basic and applied research on the characteristics of these important resources."
Charles W. Spencer, U.S. Geological Survey

**Unconventional resources are probably very large, but their character and distribution are not yet well understood.**
U.S. Geological Survey (USGS) research on reservoir characterization involves modeling the geometry and distribution of porosity and permeability, as well as determining the chemical and physical sensitivity of hydrocarbon reservoirs.  An important focus of this research is on very low-permeability (tight) gas reservoirs, tar and heavy oil deposits, coalbed gas (especially methane), and gas hydrates.**The USGS has conducted geologic research in reservoir characterization for nearly two decades.**
Cooperative programs with the Department of Energy (DOE) and Sandia National Laboratories since 1977 have led to a greatly improved understanding of the character of very low-permeability  (tight) gas reservoirs in the western U.S.  These studies are critical  in development of new economic recovery technology, such as horizontal drilling.  In addition, significant advances in understanding deep (>15,000 feet) gas reservoirs have resulted from cooperative research with the Gas Research Institute.  Estimates of hydrocarbon resources have been prepared in cooperation with the National Petroleum Council.**Current USGS studies suggest that enormous quantities of gas and oil may be tied up in unconventional reservoirs.**
For example, potentially producible gas from low-permeability horizons in the Northern Great Plains of Montana and the Dakotas could exceed 100 trillion cubic feet  Coalbed methane could yield hundreds of trillions of cubic feet, and tar sands (heavy oils) could add billions of barrels of oil.  Gas hydrates (a form of methane plus water ice) in the permafrost regions of Alaska and in the U.S. Exclusive Economic Zone could add enormous quantities of gas.  USGS scientists prepare estimates of how much of these resources can be economically recovered using today's technology, and what kinds of technological advances might be needed to economically recover additional amounts of hydrocarbon from these sources.  These estimates require a good understanding of how large the resource is, where it is located, and under what physical and chemical conditions it exists.**USGS investigations have led to larger gas-resource estimates for some western basins.**
The approach taken by the USGS to arrive at resource estimates includes the near-term and the very long-term development of the resource.  Although future technology and economics cannot be predicted very well, USGS analyses assume that technology will improve significantly and gas prices will be competitive with other energy sources.  Estimates of gas contained in five Rocky Mountain basins indicate that over 100 trillion cubic feet of gas is recoverable under conditions of higher gas prices using current technology.  Studies in the Piceance Creek and Greater Green River basins indicate that estimates of gas recoverable with advanced technology exceed previous estimates by as much as six times.  Advanced technology assumes exotic drilling and well-completion methods, some of which are currently being tested with reasonable success.  The recognition of geologic criteria, such as abnormally high or low pore pressures, level of thermal maturity, subsurface temperature distribution, and well-log analysis, has enabled USGS scientists to define the vertical and lateral extent of gas-bearing reservoirs more accurately than was previously possible.**USGS reservoir studies are a key to exotic drilling techniques.**
Conventional methods of producing gas from tight reservoirs usually requires some form of artificial stimulation, such as hydraulic fracturing.  Scientists do know that natural rock fractures are an important factor in the economic recovery of gas from tight reservoirs.  Advanced methods of gas production in these environments are taking advantage of gas flow from natural fractures in the reservoir rock.  The distribution, orientation, and density of these fractures is being studied by the USGS and other DOE-supported research groups.  In addition to these physical attributes, reservoir engineers also need detailed analyses of the effects of interstitial clays and fluids.  USGS models provide a scientific basis for providing this information.  In a recent experiment conducted by DOE, a horizontal hole was drilled that intersected many open, natural fractures in a tight sandstone reservoir.  The nature of the natural fractures and other characteristics of the reservoir were sufficiently well-determined that drilling could be accurately directed. **The size of unconventional energy resources will be an important factor in the next USGS National Assessment.**
Detailed estimates of the amount of oil and gas recoverable under varying scenarios of economics and technology are needed to guide long-range energy policy for the United States.  Today's unconventional resources will play a critical role in the Nation's energy base in the next century.  Gas in particular may command a premium price over other fossil fuels because it is less damaging to the environment.  Heavy oils and tar sands, coalbed gas, and gas hydrates will also receive increasing attention.For More Information:Charles W. SpencerU.S. Geological SurveyBox 25046MS 940Denver Federal CenterDenver, CO  80225

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This USGS Fact Sheet can be found at <URL:http://energy.usgs.gov/factsheets/Petroleum/reservoir.html>Maintained by L. Friedrich  Last updated 31-Jan-1997*