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Abstract: Characterization of Natural Fractures in the Frontier Formation, Green River Basin, Wyoming; Influence of Bed Thickness on Fracture Intensity and Interconnectivity

Hugo Harstad, Lawrence W. Teufel, John C. Lorenz

Significant gas reserves are present in the low-permeability sandstones of the Frontier Formation in the Green River Basin, Wyoming. Successful exploitation of these reservoirs through horizontal drilling requires understanding the orientations and distributions of natural fractures. Detailed maps of fracture systems have been constructed from outcrop pavements of the Frontier sandstones at the edge of the basin. They show relatively uniform distributions of vertical regional extension fractures and that bed thickness is the dominant geologic parameter controlling fracture behavior. Spatial frequency distribution of fractures has been derived from statistical analysis of fracture orientation, spacings, lengths, and interconnections and correlated to bed thickness. Fractur spacing and length decrease and fracture interconnectivity increases with decreasing bed thickness. A natural fracture reservoir model has been developed using this fracture relationship. The model shows that the relationship between bed thickness and fracture frequency distribution and the contrast between matrix and fracture permeability determines the reservoir's effective transmissibility. Wellbore and welltest data provide validations for the geologic predictions of fracture characteristics and for simulation predictions of reservoir response.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada