Geologic Definition and Resource Assessment of Continuous (Unconventional) Gas Accumulations- the U.S. Experience

By

Christopher J. Schenk¹

(1) U.S. Geological Survey, Denver, CO

 The U.S. Geological Survey (USGS) is currently assessing continuous gas resources of the U.S. (including basin-centered gas, shale gas, tight reservoir gas, and coal-bed gas) as these resources are becoming increasingly important to the U.S. energy mix. Based on geologic criteria, a continuous gas accumulation (1) is regional in extent, (2) can have diffuse boundaries, (3) has existing "fields" that commonly merge into a regional accumulation, (4) does not have an obvious seal or trap, (5) does not have a well-defined gas-water contact, (6) has hydrocarbons that are not held in place by hydrodynamics, (7) commonly is abnormally pressured, (8) has a large in-place resource number, but a very low recovery factor, (9) has geologic "sweet spots" of production, (10) typically has reservoirs with very low matrix permeabilities, (11) commonly has natural reservoir fracturing, (12) has reservoirs generally in close proximity to source rocks, (13) has little water production (except for coal-bed gas), (14) has water commonly found updip from gas, (15) has few truly dry holes, and (16) has Estimated Ultimate Recovery (EUR) of wells that are generally lower than EUR's from conventional gas accumulations. The USGS has developed a cell-based methodology for assessment of continuous gas accumulations, in which a probability distribution of potential untested geologic cells (a cell is related to the drainage area of a well) is paired to a probability distribution of EUR's of untested cells to arrive at a probability distribution for undiscovered resources in a continuous accumulation.