FRACTURE INTENSITY, FREQUENCY, AND PERMEABILITY OF A NORMAL FAULT SPLAY

Field studies show that fracture intensity decreases with distance from the fault core. Several studies attribute this pattern to the magnitude of fault throw whereby the width of the damage zone increases with throw. At a fault splay the damage zone becomes more complicated due to the interaction of two major faults. This study focuses on an exposure of an extensional fault splay in the Abiquiu Formation in New Mexico. This study aims to characterize fracture system in a normal fault splay zone.

In order to determine the net throw across the fault system several stratigraphic sections are measured to correlate specific packages of rock with estimates of 20 to 30 meters. We employ the circular window method to determine fracture variation along and across the splay zone. In each 2-m diameter circle, we measure the orientation, length, and aperture of fractures. We also obtain gigapixel imagery from a 200-meter wide, 50-meter high cliff to determine fracture patterns.

Preliminary results from the data reveal 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 spaly, 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 show higher fracture intensity, frequency, and thus higher connectivity. There also appears to be a logarithmic scaling relationship with fracture length and aperture.

Observed groundwater issues affected by the fault / fracture relationship include varying mineral composition, artesian wells, water seeping from fractures, oxidized fault cores, and variation of cementation.

This study is important to understanding the stress and strain relationships in a normal fault splay zone. Identifying the relationship between fracture intensity, frequency and permeability have applications to fluid transportation in fault zones.