SEDIMENTOLOGIC CONTROLS ON THE FRACTURE DISTRIBUTION AND NETWORK DEVELOPMENT IN MESAVERDE GROUP SANDSTONE LITHOFACIES, UINTA BASIN, UTAH

The Mesaverde Group, Uinta Basin, Utah contains several Tcf of undiscovered gas resources and is the source of current significant production. Economic gas production requires the presence and adequate connectivity of natural fractures to hydraulically induced fractures in order to provide sufficient drainage and permeability for wells. We examine the nature of the natural fracture network with scanline sampling, ground photogrammetric mapping, core analyses, and microstructural analyses. We examine five localities in the basin within different lithofacies and structural positions to test hypotheses regarding lithofacies, diagenetic, and structural controls on fracture distribution. Mesaverde Group core has been examined and sampled to evaluate subsurface fracture characteristics and to correlate lithofacies observed in outcrop to the subsurface. The purpose of the fracture analysis is to develop a subsurface 3D Discrete Fracture Network (DFN) model of the Mesaverde Group in the Uinta Basin. Regional fracture sets identified at each locality include a northwest (~352°), northeasterly (~029°) and a west-northwest (~283°) trending fracture set. Scanline surveys were conducted within 8 major lithofacies that have been identified and described using color, bed thickness, grain size, bed continuity, and sedimentary structures. The compositions of the sandstones sampled primarily consist of very fine to medium grained quartz arenites, and medium - grained lithic and feldspathic arenites. Fracture character is observed to be largely dependent on the sedimentologic characteristics of the sandstone lithofacies. A primary control on fracture distribution observed in the field is the nature of the cement. Fracture intensity is highest in silica-cemented sandstones (4.25) and lowest in clay-cemented sandstones (0.07). Calcite and quartz mineralization within fractures have been identified, however, mineralization is rare. Results are presented from a preliminary 3D DFN model based on the outcrop and core data. Subsequent DFN models developed will be input into a series of mechanical and flow models that will simulate far – field in – situ stresses, modeling the interaction between hydraulic fractures and the natural fracture network and how fluid may flow through the system.