Fracture characterization in tight carbonates: An example from the Ozark Plateau, Arkansas

Abstract

Outcrop studies of fracture development, used as analogs for subsurface fracture patterns, are critical because of the importance of fractures as fluid flow pathways and the fact that most fracture networks exist at a smaller resolution than current seismic data can resolve. Fracture networks in carbonate units are typically controlled by the mechanical properties of the unit, indicating that the mechanical stratigraphy, as well as the fracture stratigraphy, should be considered. This study presents the results of a fracture analysis in the Mississippian carbonates of the Ozark Plateau, considering both mechanical and fracture network characteristics. Mechanical characteristics of the succession were defined using a combination of rebound values and thin section petrography. Fracture characteristics included orientation and intensity, together with abutting relationships. Results indicate that fracture orientations show a distinct evolution throughout the measured succession, including the appearance of early systematic sets, followed by pervasive systematic fracture sets related to existing basement features. Fracture orientation changes do not correspond to changes in mechanical stratigraphy. Fracture intensity, however, is related to the thickness of the mechanical unit instead of the bed thickness and is greatest in less competent units. Mechanical control influences the fracture network on a smaller scale than that of regional tectonic stresses. Thus, evaluations of carbonate reservoirs must account for both the large-scale and the small-scale investigations into fracture characteristic controls. Outcrop evaluations are of critical importance to properly assess characteristics that are challenging to recover from conventional subsurface data sets such as core and seismic reflection volumes.