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Abstract: The Greater Natural Buttes Producing Area, Southeastern Uinta Bash., Utah: A Unique Model for a Basin Centered Gas Accumulation


The Greater Natural Buttes (GNB) Producing Complex encompasses more than 400 mi{2} (1050 km{2}) in the southeastern Uinta Basin, Utah. Over 520 BCF of non-associated gas has been produced from predominately the Paleocene Wasatch Formation and, to a lesser extent, the underlying Cretaceous Mesaverde Group through 1995.

Geochemical analyses of gas produced from GNB demonstrate the source of the gas is thermally mature, type III kerogen (coats) of the Mesaverde Group. The coals are concentrated in the basal Mesaverde and disseminated throughout the remaining 2,500 ft (760 m) of the: formation. Potential reservoir rocks exist throughout the same 2,500 ft (760 m) of the Mesaverde, as well as the overlying 4,000 ft (1220 m) of the Wasatch Formation.

Sandstones deposited in a fluvial and/or braid-plain environment are the main reservoir facies. Shales and low permeability sandstones in both the Mesaverde and Wasatch and the overlying Eocene Green River Formation form impediments to gas migration.

Thermal maturity levels of the deep Mesaverde coats correlate well with the estimated ultimate recovery (EUR) of welts. Detailed cross-sections demonstrate that as vitrinite reflectance (Ro) rises above 1.0 percent, the gas column rises 750 ft (230 m) to include a zone in the Wasatch that has improved porosity and permeability as well as being overpressured.

Reservoir characterization and petrophysical modeling indicate that GNB has characteristics similar to other regional basin-centered gas accumulations. However, in the case of the Wasatch/Mesaverde petroleum system of the Uinta Basin, methane has been expelled from coats at the rank of mvB, and has displaced moveable pore-volume water over a vertical and stratigraphic thickness of 6,500 ft (1,980 m).

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado