The application of outcrop analogues to seismic modeling of fractured reservoirs has been limited to fracture orientation for seismic anisotropy. This study presents a method to translate fracture spacing data into relative values of Hudson's crack-density parameter, and a method to evaluate the effect of stress and fracture orientation on crack density distribution.
Fracture roughness impedes immediate fracture closure under any confining stress. Fracture roughness can be expressed as a population of aligned cracks with given crack density and aspect ratio density distribution. We have assumed that one of these distributions describes the average roughness of a fracture set. Based on that assumption, we can translate fracture spacing into the crack-density parameter.
In order to evaluate the effect of confining stress, we applied Mavko's solution for closure of nonelliptical cracks. We have generated a random gamma density distribution to describe a population of cracks with specific crack densities and aspect ratios. After calculating the applied normal stress for each fracture orientation, Mavko's solution determines the aspect ratios that will be closed under the applied stress. Closure of cracks with those aspect ratios reduces the crack density, allowing us to evaluate the effect of stress and fracture orientation on crack density distribution.
These two methods allow us to generate synthetic seismograms using outcrop data, to model the seismic signature of small faults. The results help to explain the signature of subseismic faults in gas-filled fractured limestones in the Lower Cretaceous of Texas.
AAPG/SEPM: From Outcrop to Simulation II: Using Outcrop Data to Build Reservoir Models