Abstract: Fracture Density Modeling Using GIS: Implications for the Development of Throughgoing Fracture Zones and Subsurface Fluid Flow
Two dimensional fracture density maps were created from surface exposures of the Monterey Formation, California, using Geographic Information Systems technology. The rocks underwent an early phase of extensional faulting followed by a late phase of compression. A prominent set of vertical bed-confined joints trending 010 developed in conjunction with late phase compression, followed by a later set of cross joints striking 308. Large, multilayer fracture zones cut through the outcrops, extending vertically and horizontally for tens of meters. These fracture zones strike NNW-SSE, have apertures ranging from 10 - 50 cm, contain blocks of fractured rock within tar matrix, and are the dominant permeability conduits for offshore production in the Santa Maria basin. One may initially misinterpret the fracture zones as faults due to their brecciated appearance and geometries resembling pull-aparts. However, close inspection reveals the fracture zones are extensional features that developed by the linkage of pre-existing bed-confined joints.
GIS software was used to digitize fractures from photographs and then to perform a spatial density analysis of each fracture set as well as the total fracture network. Spatial fracture densities vary dramatically according to fracture set, lithology, and proximity to fracture zones. Most importantly, high contour intervals of fracture densities are often aligned in linear patterns parallel to the trend of large fracture zones, even in areas devoid of apparent fracture zones. Therefore, spatial analysis of fractures may serve as a valuable technique to predict the orientation and density of hydrologically significant fracture zones.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana