Fault-fracture networks and related fluid flow and sealing, Brushy Canyon Formation, west Texas

We describe fault-fracture networks and associated cementation in excellent three-dimensional exposures of the Permian Brushy Canyon Formation in the central Delaware Mountains of west Texas. Faults and fractures are present in two main, nearly orthogonal sets (NNW and NE trending). A third set is present adjacent to some faults, or between two closely-spaced faults. Spacing of fault-parallel fractures decreases near some faults, in some cases parabolically. Fault core zones contain banded carbonate veins, complex breccias with banded vein clasts, and hydrocarbon material. Fault damage zones contain carbonate veins, and iron oxide and carbonate matrix alteration. These features are interpreted to record a complex history of slip, multiple fluid flow events and multiple cementation events along the fault-fracture network. Outcrop velocity probe data and porosity data suggest that matrix cementation halos have formed around the faults. The width of such halos controls the effectiveness of fault seals and the volume of reservoir rock that may be damaged by porosity occlusion. Fracture orientation and spacing are dissimilar in the hanging wall vs. footwall of some faults, suggesting that cementation-related fault sealing and reservoir damage may be asymmetric around such faults. Fracture-fault networks thus focused fluid flow at times, and were seals at times. In analogous reservoirs, open fractures would allow fracture-assisted production, and sealed fractures would compartmentalize the reservoir into semi-independent production zones.