Thermal Maturity, Porosity, and Facies Relationships Applied to Gas Generation and Production in Tertiary and Cretaceous Low-Permeability Sandstones, Uinta Basin, Utah
NUCCIO, VITO F., JAMES W. SCHMOKER, and THOMAS D. FOUCH, U.S. Geological Survey, Denver, CO
Uinta basin rocks include a thick sequence of low-permeability (tight) sandstones that contain a large volume of gas. Most known gas accumulations occur in the eastern part of the basin in the Upper Cretaceous Mesaverde Group; uppermost Cretaceous to early Eocene North Horn Formation; and the Paleocene and Eocene Wasatch, Colton, and Green River formations. Most successful tight-gas completions are in Tertiary strata, but rocks in the underlying, largely undrilled Mesaverde Group in the north-central part of the basin are potential targets as well. To characterize the gas-bearing sandstones, we have constructed a series of interrelated maps and cross sections showing thermal maturity, porosity, fluid pressures, and lithofacies relations for the Uinta basin.
Vitrinite reflectance (Rm) for each of these low-permeability sequence increases to the north and levels of thermal maturation are sufficient for gas generation over large areas of the basin. Measured Rm values at the base of the Mesaverde increase from 0.65% at shallow depths along the southern edge of the basin to 1.5% in the central basin at depths of about 11,000 ft (3350 m). Measured Rm values at the top of the Mesaverde increase from 0.50% at outcrops along the southern
edge of the basin to 2.2% near the town of Altamont, at depths of approximately 18,000 ft (5500 m).
Projection of maturity values and fluid-pressure data into undrilled parts of the basin reveals the probability of a regional, overpressured, basin-entered gas accumulation, where gas is likely to be generating at present. Published estimates of amounts of erosion in the region vary from 1000 ft (300 m) to almost 11,000 ft (3350 m). Our interpretation favors the lesser erosional estimates because significant cooling of strata due to uplift and erosion would slow or stop hydrocarbon generation.
Plots of porosity vs. Rm allow prediction of porosity for Mesaverde sandstones in unexplored areas of the basin. Porosities of nonmarine Mesaverde sandstones with thermal maturity less than about 0.7% or greater than roughly 2.0% decrease as thermal maturity increases and follow "normal" sandstone trends. However, between 0.7 and 2.0%, in the window of hydrocarbon generation, porosities do not decrease as thermal maturity increases. Overpressured, gas-saturated Mesaverde sandstones are likely to have porosities in the 5 to 9% range.
Reservoir quality is related to depositional facies. In Tertiary formations, the best gas reservoirs are in diagenetically altered, fluvial-channel, deltaic, and open-lacustrine sandstones. In the underlying Cretaceous strata, gas source rocks and reservoirs occur in a sequence of fluvial lenticular sandstones and in marine, blanketlike sandstones.
Our data indicate a large area of organic-rich, overpressured strata probably underlies the northern part of the Uinta basin. Wells drilled in the Mesaverde and lower part of the Tertiary, in the areas where Rm at the base of the Mesaverde is greater than 1.1%, should have the best potential for gas production. Overpressured gas reservoirs (Rm >1.1%) are likely to have no free water and be enveloped by successive zones of mixed water and gas (Rm 1.1 to 0.75%), and of water only (Rm <0.75%).
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)