Fracture Pattern Prediction Using Geomechanical Models Incorporating Diagenesis, with Comparison to Outcrop Data (Cambrian Eriboll Group sandstones, Northwestern Scotland)

Effective and accurate characterization of fracture populations is a key for hydrocarbon reservoir assessments. Outcrop studies have been an important source of information on fracture patterns providing better knowledge of distribution, connectivity and geometry. By establishing an analogy to outcrop observations, core data can be used to better understand subsurface fracture patterns, leading to predict more accurate fracture permeability and flow patterns. Using geomechanical models to simulate fracture pattern development is one way to systematically generalize the detailed observations of outcrop fractures in a specific locality to the general problem of prediction of fractures in the subsurface. I use a combined methodology for fracture pattern description, which includes macro-microscale observations of fractures and cements in Eriboll Formation outcrops (Focused on aperture and length attributes). I use a geomechanical model that simulates fracture growth under the influence of concurrent diagenesis.

The Eriboll Formation outcrops are unusually well-preserved examples of opening-mode fractures (veins and joints) with a great exposure of fracture size distributions. Using photographic and conventional mapping techniques, I generated fracture trace maps. Combining macrofracture observations with measurements along scanlines, petrography, fluid-inclusion analysis and high-resolution scanning electron microscopy (SEM), I found three preferred strikes oriented sets of fractures (outcrop level): N-S, NW-SE and E-W. From microstructural observations, I found that the current fracture pattern is the result of superimposed deformations that produced N-S striking fractures of different ages that share a common strike but that differ in dip and in cross section show consistent crosscutting relations. Rock mechanical properties, subcritical crack index, diagenesis, mechanical layer thickness and strains measured from outcrop and subsurface rocks were used as an input for a geomechanical fracture pattern simulator (Joints simulator, Olson, 2007). My results show how geomechanical models that incorporate fracture and diagenesis can help make outcrop more germane to subsurface fracture prediction.