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.