Continuous Hydrocarbon Reservoirs

"The U.S. Geological Survey has defined a new type of hydrocarbon accumulation which is not trapped in the conventional sense because the accumulation is not significantly affected by the water column. These unconventional accumulations are areally large and are termed continuous because the reservoir rock is charged with oil or gas throughout. Many examples of continuous accumulations can be found in the United States. In conducting the 1995 National Assessment of Oil and Gas Resources, U.S. Geological Survey scientists included this new category of accumulation because of the large quantities of technically recoverable hydrocarbons, mostly gas."

Dr. James Schmoker, U.S. Geological Survey

The U.S. Geological Survey (USGS) is attempting to estimate, through the use of new methods, the amount of hydrocarbon resources in continuous-type reservoirs that will be available to humans over the next few decades.

The USGS now includes unconventional oil and gas resources in its National Assessment of Oil and Gas Resources and investigates complex properties of continuous hydrocarbon reservoirs. Standard methods for quantitative resource assessment are inappropriate because continuous-type accumulations are large single fields having areal dimensions of miles and are not significantly affected by hydrodynamic influences. USGS geologists not only quantify the amount of technically recoverable resource, but more importantly, they estimate the amount of resource that might become available for human activity over a reasonable period of time. For example, as much as 5,000 trillion cubic feet of gas is locked up in continuous accumulations of the Green River Basin alone, yet only a fraction of it will ever be recovered. By comparison, the total proved reserves of gas in the entire United States are approximately 160 trillion cubic feet.



Normal methods for defining a conventional reservoir do not apply to continuous reservoirs.

Continuous reservoirs differ from conventional reservoirs in that a downdip water-hydrocarbon contact is absent. A useful analogy for continuous hydrocarbon accumulations is a water reservoir. In a water reservoir, the fluid is not contained in a single rock type and is not confined to a localized trap, but is distributed widely; the water forms a continuous accumulation. The key geologic variables of a continuous accumulation relate to the ability of reservoir rocks to release gas or oil. Productivity of wells in a continuous play varies greatly from inconsequential to that resembling a conventional well. However, truly dry holes are technically not possible because oil or gas is present everywhere in the reservoir; plugging a well is an economic decision based on poor flow rates.



USGS studies indicate that much of the hydrocarbon resource in continuous reservoirs is on Federal lands.

USGS studies focus on two issues: First, the volume of hydrocarbons in continuous deposits that is recoverable; and second, how much of that resource is located on Federal lands. In the lower 48 States, estimates of continuous-gas resources exceed estimates of undiscovered conventional gas resources, whether on Federal lands or not. Although large volumes of technically recoverable, unconventional oil and gas resources might exist in Alaska, the economics of their recovery put these resources out of reach at present. Even large conventional gas accumulations in Alaska, such as the one at Prudhoe Bay, are not currently economically recoverable.



USGS scientists have defined the geological environment of continuous hydrocarbon accumulations.

On the basis of equivalent energy, more than 90 percent of the technically recoverable hydrocarbon resources occurring in continuous deposits are in the form of gas not associated with oil. Primary occurrences of such gas are in low-permeability sandstones, fractured shales, and chalks. A number of continuous accumulations are already being commercially produced, but their regional geologic nature might not be recognized; thus, portions of a single, continuous accumulation often are viewed as separate fields. Deliverability of gas hydrocarbons to the wellbore commonly depends on networks of vertical fractures that form as a result of burial of the rock. Fracture systems can extend many miles but relatively little gas is contained in this network of fractures; most of the gas is contained in the rock matrix and escapes to the fractures as surrounding gas pressure is lowered by the influence of the well.



Estimates of the amount of recoverable hydrocarbons under various economic conditions are needed to guide long-term energy policy.

Estimates of potential additions to reserves are based on existing technology and practices, but development of these reserves will vary depending on price. Using a statistically representative suite of wells, USGS scientists calculate ultimate recoveries. These data form the basis for a productivity distribution representing the remaining wells expected to be drilled. A limiting long-term consideration is that the price of gas produced from domestic continuous reservoirs must be competitive with gas from conventional fields and with imported liquefied natural gas. Economic analysis by USGS scientists will allow planners to compare the cost of extracting gas from low-permeability sandstone reservoirs, for example, in the Green River Basin of southwest Wyoming, with the cost of producing and delivering gas from conventional deposits in Alaska or from the coalbeds in the San Juan Basin.

For More Information:

Email: schmoker@usgs.gov

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This USGS Fact Sheet can be found at <URL:http://energy.usgs.gov/factsheets/Hydrocarbon/hydro.html>
Maintained by L. Friedrich Last updated 23-Jan-1997