Outcrop Analogs of Multi-Layer Fracture Zones

Gross, Michael R.¹ (1) Florida International University, Miami, FL

The ability of a fracture network to transmit fluids in large part depends upon the connectivity, spacings, lengths and apertures of the fractures. The overwhelming majority of subsurface flow is accommodated by the largest fractures. Though fewer in number, these multi-layer fracture zones have the highest transmissivities and are often in direct hydrologic contact with the more numerous smaller fractures. In order to assess the contribution of multi-layer fractures to reservoir flow, one must evaluate their morphological character (e.g., width, planarity, aperture distribution, roughness, number of segments and how they are linked), the kinematics of their formation (sense of motion and relation to strain), and their position within a multi-scale mechanical stratigraphy. To this end, multi-layer fractures were analyzed for a variety of lithologies and tectonic settings. Results suggest a common process for the formation of multi-layer fractures in layered rocks, namely the coalescence and linkage of pre-existing, bed-confined fractures. Further, geometries of multi-layer fractures change systematically in response to increasing strain. This allows for the mapping of strain based on the geometry and frequency of multi-layer fractures, or conversely, the prediction of fracture intensity based on the spatial distribution of strain. An understanding of the geometries, origin and hydrologic properties of multi-layer fractures will contribute to the characterization of fractured reservoirs and models that more accurately reflect the architecture of natural fracture networks.