The U.S. Geological Survey (USGS), in cooperation with
the U.S. Department of Energy, is reevaluating the resource
potential of selected domestic basin-centered gas accumulations.
Basin-centered gas accumulations are characterized by
presence of gas in extensive low-permeability (tight) reservoirs
in which conventional seals and trapping mechanisms are
absent, abnormally high or low reservoir pressures exist, and
gas-water contacts are absent.
In 1995, the USGS assessed one basin-centered gas play
and two conventional plays within the trend of Jurassic and
Cretaceous Cotton Valley Group fl uvial-deltaic and barrierisland/
strandplain sandstones across the onshore northern Gulf
of Mexico Basin. Detailed evaluation of geologic and production
data provides new insights into these Cotton Valley plays.
Two Cotton Valley sandstone trends are identifi ed based
on reservoir properties and gas-production characteristics.
Transgressive blanket sandstones across northern Louisiana
have relatively high porosity and permeability and do not
require fracture stimulation to produce gas at commercial
rates. South of this trend, and extending westward into eastern
Texas, massive sandstones of the Cotton Valley trend exhibit
low porosity and permeability and require fracture stimulation.
The high permeability of Cotton Valley blanket sandstones is
not conducive to the presence of basin-centered gas, but lowpermeability
massive sandstones provide the type of reservoir
in which basin-centered gas accumulations commonly occur.
Data on source rocks, including burial and thermal history,
are consistent with the interpretation of potential basincentered
gas within Cotton Valley sandstones. However, pressure
gradients throughout most of the blanket- and massivesandstone
trends are normal or nearly normal, which is not
characteristic of basin-centered gas accumulations.
The presence of gas-water contacts in at least seven fi elds
across the blanket-sandstone trend together with relatively
high permeabilities and high gas-production rates without fracture
stimulation indicate that fi elds in this trend are conventional.
Within the tight massive-sandstone trend, permeability
is suffi ciently low that gas-water transition zones are vertically
extensive and gas-water contacts either have not been encountered
or are poorly defi ned. With increasing depth through
these transition zones, gas saturation decreases and water saturation
increases until eventually gas saturations become suffi
ciently low that, in terms of ultimate cumulative production,
wells are noncommercial. Such progressive increase in water
saturation with depth suggests that poorly defi ned gas-water
contacts probably are present below the depth at which wells
become noncommercial. The interpreted presence of gas-water
contacts within the tight, Cotton Valley massive-sandstone
trend suggests that gas accumulations in this trend, too, are
conventional, and that a basin-centered gas accumulation does
not exist within Cotton Valley sandstones in the northern Gulf
Additional Publication Details
USGS Numbered Series
Is there a basin-centered gas accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.?