Characterizing Fracture Networks in a Normal Fault Splay Zone

Abstract

The Plaza Blanca fault is located NE of Abiquiu, New Mexico. The fault splays into two SE-dipping, synthetic normal faults. Several studies attribute increased damage zone width and decreased fracture density from the fault core to the magnitude of fault throw. At this fault splay the damage zone becomes more complicated as two major faults interact. A fundamental issue in the field area is the interaction of groundwater with the Abiquiu formation. Hypothetically ground water will utilize the fracture network associated with faulting as a conduit for fluid transportation. To fully understand such implications several studies are conducted. We employ the 2m diameter circular window method and measure fracture orientations, lengths, apertures and connective facets to determine fracture variation along and across the splay zone. There is a positive correlation among fracture frequency, intensity, and structural position. Fracture frequency (FF) is defined as the number of fractures per unit area (L0/L2 = L-2), and fracture intensity (FI) is the cumulative length of fractures per unit area (L1/L2 = L-1). Background fracture has low frequency (FF = 1 – 9 m-2) and intensity (FI = 1 – 3 m-1). In the damage zone surrounding the splay, both indices increase (FF = 12 – 24 m-2, FI = 5 – 9 m-1). Within the splay zone, FF and FI are even higher (FF = 10 – 53 m-2, FI = 6 – 12 m-1). The fault core of one splay displays very high values (FF = 128 m-2, FI = 10.8 m-1), while the highest frequency occurs exactly on the splay point (FF = 195 m-2). Proximity to the fault core shows higher fracture intensity, frequency, and thus higher connectivity. Results reveal a logarithmic scaling relationship with fracture length and aperture. Three connective nodes types within each circle are plotted on a ternary diagram: isolated tips (I-nodes), abutting fractures (Y-nodes), and crossing fractures (X-nodes). Background fractures generally have little connections (I = 30-70%, Y = 30-70%, X = 0%); those from the outer part of the damage zone show an increased connectivity (I = 25-40%, Y = 55-75%, X = 0-5%). Fractures are highly linked within the splay zone with elevated proportion of Y- and X-nodes (I = X = 8-16%); and the highest connectivity is found at the splay point (I = 8-9%, Y = 55-56%, X = 35-36%). Further characterization of the fracture networks are represented in gigapixel imagery. Porosity and cementation studies are forthcoming.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016