CHARACTERIZING FRACTURE DISTRIBUTION IN LAYERED ROCKS USING GIS-BASED TECHNIQUES

Geographic Information Systems (GIS) provide a framework to analyze the spatial distribution of linear features such as fracture traces from 2-D maps, photographs and remotely sensed images. We therefore explored a wide range of GIS-based techniques in an effort to characterize the similarities and differences between fracture populations hosted in alternate stratigraphic units. The study was performed on cross sectional exposures of the Monterey Formation in Santa Barbara County, California. Interbedded lithologies of the Monterey Formation often display different styles of brittle deformation, with joints confined to siliceous beds and small faults confined to mudstone beds. Fractures and stratigraphic boundaries were mapped in the field and then digitized as linear traces into the GIS. The map area was then divided into a grid consisting of square cells, where each cell contains the calculated value of the fracture attribute selected for analysis. For example, the fracture intensity was calculated for each cell in the map area by summing the lengths of all fracture traces within a prescribed search radius centered on that cell. Results show that the average small fault intensity in mudstones is approximately 42% less than the average joint intensity in siliceous beds. A second parameter, the fracture cluster size, was calculated for the two populations. A higher cluster size corresponds to greater fracture connectivity, and hence greater conductivity for the overall fracture network. Joints in siliceous beds yield a cluster size of 0.14 whereas small faults yield a value of 0.08, both of which are lower than the threshold of 0.5 required to ensure a continuous pathway for fluid flow. In contrast, a cluster size of 0.8 was measured for throughgoing faults traversing the entire section, emphasizing the important role played by throughgoing fractures in enhancing flow through layered rocks.