Characterization of Fracture Damage Zones in the Abiquiu Formation, Plaza Blanca, New Mexico

The methods used for characterizing fracture damage-zones in the Plaza Blanca, New Mexico region yielded a data-rich study. New concepts such as a “splay zone” are introduced to help explain a complex accommodation zone formed by hard linked SE-dipping normal faults called the Plaza Blanca Fault (PBF) splay. These faults are part of a broader connected fault system within the Rio Grande Rift. The total extension in the Plaza Blanca region is ~5.8% with ~290 meters of E-W horizontal extension. The throw across this region exceeds the throw of the rift-bounding Cañones fault. The width of the PBF damage zone vs fault throw is consistent with previous studies. Sampling techniques at the PBF provided fracture information in three dimensions at various locations and at scales ranging from micrometer to kilometer. A 2 m-diameter circular window is used in the field to sample fracture populations on bedding planes. Fracture density, scale, and geometry vary with respect fault core proximity. At this fault splay the damage zone becomes more complicated as two major faults interact. Several observations about fracture density are made from the sample windows: fracture density in the damage zone increases with proximity to the fault core, fracture density is highest at the fault core and splay point, the higher the fracture density the smaller the fracture length per area, and the more fractures per area the more irregular the fracture orientation. Basic connectivity is a factor in understanding potential transmissivity through a network. Identifying three fundamental connective facets (I, X, Y) are part of the window analysis. These connective facets are then plotted on a ternary diagram where connections per branch (CB) can be quantified. The higher fracture density, the more connected the network is. There is a quasi-linear correlation between the sum of connective facets versus the fracture density per window. Using a Gigapan image of the PBF, nearly 30,000 fractures are annotated. This detailed annotation provides geometry, density, and connectivity at a high resolution. These methods, and others, enhance the understanding of how permeability is affected by fracture damage zones.