AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Tight Gas Sands and Natural Fractures in the Cretaceous Mesaverde Group, Greater Natural Buttes Field, Uinta Basin, Utah
(1) Utah Geological Survey, Salt Lake City, UT.
Greater Natural Buttes field (GNB) in the Uinta Basin is the largest gas field in Utah. GNB lies in an area of gentle northwest dip on the southern flank of the basin. The field produces natural gas primarily from low-permeability sandstone in the Mesaverde Group and the Tertiary Wasatch Formation. GNB has produced over 2.1 TCF of gas from about 3950 wells and continues to be the most active drilling area in Utah.
Most of the gas production is from highly compartmentalized, lenticular, fluvial channel sandstones in the Price River Formation in the upper Mesaverde Group. The Sego and Castlegate Sandstones in the lower Mesaverde typically have higher water saturation and are not generally productive, but have lithologic and fracture characteristics similar to productive sandstones up section. Production in these tight-gas-sand reservoirs is achieved through massive hydraulic fracture treatments, so understanding the natural fracture systems and reservoir heterogeneity created by various depositional environments can aid in hydraulic fracturing optimization and lead to better drilling and completion strategies.
Cores in the Castlegate, Sego, and Price River Formations display classic low-permeability depositional, petrophysical, and geomechanical characteristics that are being used to create reservoir models and simulations for hydraulic fracturing. These cores show that the Mesaverde is mostly sandstone interbedded with black, bioturbated, and carbonaceous shale and silty mudstone. Individual sandstones are very fine to fine grained and have low-angle cross-bedding, flaser bedding, small ripples, sparse burrows, and moderate bioturbation. Depositional environments range from marginal marine to lower coastal plain. Porosity varies from 2 to 8% and permeability varies from 0.002 to 0.07 mD.
Over 90 fractures were identified in core. Most are natural, open fractures, but some are closed and filled or partially filled with calcite. Some fractures appear to propagate from thin coal seams, while others terminate at shale or clay partings. Most fractures are inclined, but horizontal and vertical orientations are also present. Portions of GNB cores illustrating various depositional environments and fracture types will be available for viewing and discussion.