The U.S. Geological Survey (USGS) assessed the potential for undiscovered conventional and continuous oil and gas resources in several priority basins beginning in 2001. Conventional resources were those that were generally trapped by hydrodynamic processes, and commonly there was a well-defined, oil- or gas-water contact at the base of each accumulation, leading to discrete, countable accumulations. Continuous resources (including accumulations known as basin-centered gas, shale gas, tight gas, and coalbed gas) were defined as those accumulations generally not trapped by hydrodynamic processes. Transition zones were recognized between areas of conventional and continuous resources. A continuous oil or gas accumulation may have some or all of the following characteristics: (1) regional in extent, (2) diffuse boundaries, (3) existing “fields” commonly merge into a single regional accumulation, (4) no obvious seal and trap, (5) no well-defined, oil- or gas-water contact, (6) hydrocarbons apparently not held in place by hydrodynamics, (7) commonly abnormally pressured, (8) large in-place resource volume, but very low recovery factor, (9) geologically controlled “sweet spots”, (10) little free water production (except from coal-bed gas accumulations), (11) water commonly found up dip from hydrocarbons, (12) few truly “dry” holes, (13) reservoirs generally in close proximity to source rocks, (14) Estimated Ultimate Recovery (EUR) of oil or gas from wells are generally lower than EURs from wells in a conventional accumulation, 15) reservoirs with very low matrix permeabilities, and (16) natural reservoir fracturing common. The key to the assessment of continuous resources was the geologic definition of sweet spots that represent the concurrence of several favorable geologic parameters such as fracturing, thermal history, gas content, reservoir thickness, or matrix rock properties. Sweet spots were areas that were most likely to be the economically developed parts of a continuous accumulation.

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