T.S. Dyman1, C.E. Bartberger2, V.F. Nuccio1, C.J. Schenk1
(1) U.S. Geological Survey, Denver, CO
(2) Consultant, Denver
ABSTRACT: Re-evaluation of Cotton Valley Gas Plays in Gulf Coast Region for Petroleum Resource Assessment
The U.S. Geological Survey (USGS) in cooperation with the U.S. Department of Energy is re-evaluating the resource potential of basin-center (BC) gas accumulations in the U.S. BC gas accumulations are characterized by abnormal reservoir pressures, presence of gas in extensive low-permeability (tight) reservoirs, and lack of conventional gas-water contacts.
In 1995 the USGS assessed three conventional plays and one BC gas play in Cretaceous/Jurassic Cotton Valley (CV) sandstones in the onshore northern Gulf of Mexico Basin. Detailed evaluation of geologic and production data provides new insights into these CV plays.
Two CV sandstone trends are identified 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 east Texas, massive sandstones of the CV exhibit low porosity and permeability and require fracture stimulation. Pressure gradients throughout most of both trends are normal, which is not characteristic of BC gas accumulations.
Presence of gas-water contacts in at least seven fields across the blanket-sandstone trend together with relatively high permeabilities and high gas-production rates without fracture stimulation indicate that fields in this trend are conventional. Within the tight, massive-sandstone trend, however, permeability is sufficiently low that gas-water transition zones are vertically extensive and gas-water contacts poorly defined. With increasing depth through these transition zones, gas saturation decreases and water saturation increases until eventually gas saturations become sufficiently low that, in terms of cumulative production, wells become non-commercial. Interpreted presence of gas-water contacts within the tight, massive CV sandstone trend suggests that accumulations in this trend are also conventional, and that a BC gas accumulation does not exist within the CV Sandstone in the northern Gulf Basin.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado