FRACTURE CHARACTERIZATION IN CRETACEOUS MESAVERDE GROUP, UINTA BASIN, UTAH

The Mesaverde Group, Uinta Basin, Utah contains substantial gas reserves and several trillion cubic feet of undiscovered resources (USGS, 2003). Economic production requires the presence and adequate connectivity of natural fractures in order to provide sufficient drainage and permeability for wells. Scanline sampling of fractures has been conducted at five localities in the basin within different lithofacies and structural positions. The surveys evaluate the dependence of fracture set orientation and intensity on both the lithofacies type and structural position. Outcrop photographs are used in order to develop geo-referenced, oriented and scaled models using the Sirovision software. Fractures within core of the Mesaverde Group have been examined and sampled. The purpose of the fracture analysis throughout the basin is to ultimately develop a subsurface 3D fracture model of the Mesaverde Group in the Uinta Basin. Three locations have been examined on the southern gently dipping limb of the northern edge of the basin. These locations include Asphalt Ridge, Snake John Reef and localities near Rangely, Colorado. Two locations were sampled in the southern edge of the basin along the Book Cliffs at Tusher Canyon and San Arroyo Canyon. Preliminary fracture analysis reveals that all locations have a predominant northwest trending (~ 322° - 343°)fracture set. Anortheasterly trending fracture set was also identified at Asphalt Ridge (~034°), San Arroyo Canyon (~037°) and Rangely (~058°). A north – south trending fracture set (~358°) is identified at Asphalt Ridge that cuts through the other fracture sets. Fracture character is clearly a function of lithofacies. Open mode fractures and deformation bands have been observed in all lithofacies types sampled. A strong control on fracture intensity is the nature of the cement. Fracture intensity is highest in quartz-cemented sandstones (~4.25) and lowest in clay-cemented sandstones (~0.02). Therefore, fracture permeability would be higher in the quartz cemented sandstones than in the poorly cemented sandstones at depth. Calcite and quartz mineralization have been identified within some fractures however mineralization within most are rare. Results are presented from a preliminary 3D Discrete Fracture Network model based on the outcrop and core data.