Abstract

Cretaceous strata in the east-central Piceance basin of northwestern Colorado were subjected to several phases of stress due to horizontal tectonic compression, burial, and uplift. These rocks contain a regional set of west-northwest extension fractures, observed in abundant vertical and deviated core from the U.S. Department of Energy's Multiwell Experiment and Slant Hole Completion Test wells. Time-depth relationships, fracture orientation, and fluid inclusion analyses indicate that the fractures formed at about 36-40 Ma, during a phase of increased Laramide west-northwest compression, and in a pressure-temperature regime compatible with the geologically reconstructed maximum burial depths. This regional fracturing is an example of load-parallel extension fracturing and basinwide dilatancy at depth, under conditions of high pore pressure and anisotropic, tectonically created, horizontal stress.

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