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Natural fractures and their relationships to structure, stress, and permeability in the Raton Basin
Fractures in Cretaceous and early Tertiary strata record several deformation events that were imposed on the formations that fill the Raton Basin in Colorado and New Mexico. A regional, generally WNW-ESE striking extension-fracture set is present across much of the basin, but fracturing also includes both dip-slip and strike-slip conjugate shear-fracture sets as well as irregular deformation-band shear fractures. In some areas of the basin, the extension fractures are dynamically-compatible with associated conjugate shear fractures, both recording a maximum compressive stress that was horizontal, trending predominantly WNW-ESE. Fracture strikes vary from NW-SE to ENE-WSW, approximately normal to the arcuate front of the Laramide thrust-fault system that forms the western edge of the basin, implying that fracturing was the result of a horizontal compressive stress anisotropy created by indentation of the thrust system into the basin margin. Fracture anomalies occur over local structures including a N-S basement wrench-fault system that connects two large anticlines within the basin, the Tercio and Vermejo Park anticlines, where N-S strike-slip offset along the basement wrench faults caused folding and fracturing in the overlying strata.
The Laramide stress system in the basin changed from thrust-related WNW-ESE horizontal compression to the present-day N-S maximum horizontal compressive stress in mid-Tertiary time as the thrust system became inactive and was replaced by regional E-W extension. No new fracture sets were formed by the re-oriented stress system, although stress-release fractures normal to the regional set formed in outcrops as overlying strata were eroded.
Fracture datasets were derived from three sources during this study: outcrops, image logs, and cores. Each source provides a somewhat different perspective on the fractures that enhance permeability in Raton Basin reservoirs. Taken together, the three datasets provide the basis for a relatively complete conceptual model of the Raton fracture system. Fracture-controlled permeability anisotropy will be greatest in the WNW-ESE direction, parallel to the strike of the dominant set of Laramide-age natural fractures, but hydraulic stimulation fractures will propagate N-S, across the strike of those fractures under the influence of the present-day stress system. The apertures of the WNW-ESE fractures will be susceptible to closure under that stress system since the maximum horizontal compressive stress is approximately normal to fracture strike.
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