Constraints on bed scale fracture chronology with a FEM mechanical model of folding: The case of Split Mountain (Utah, USA) W. Sassi a, ⁎, M.L.E. Guiton b , Y.M. Leroy c , J.-M. Daniel a , J.-P. Callot d a IFPEN, Rueil-Malmaison, France b IFPEN, Lyon, France c CNRS, Ecole Normale Supérieure, Paris, France d LFC-R, CNRS, Université de Pau et des Pays de l'Adour, Pau, France abstract article info Article history: Received 13 November 2011 Received in revised form 11 July 2012 Accepted 30 July 2012 Available online 9 August 2012 Keywords: Fracture sets Folding Elasto-plasticity Finite element method Mechanical modeling of fold related fractures Chronology of fracture sets development A technique is presented for improving the structural analysis of natural fractures development in large scale fold structures. A 3D restoration of a fold provides the external displacement loading conditions to solve, by the finite element method, the forward mechanical problem of an idealized rock material with a stress-strain relationship based on the activation of pervasive fracture sets. In this elasto-plasticity constitutive law, any activated fracture set contributes to the total plastic strain by either an opening or a sliding mode of rock failure. Inherited versus syn-folding fracture sets development can be studied using this mechanical model. The workflow of this meth- odology was applied to the Weber sandstone formation deformed by forced folding at Split Mountain Anticline, Utah for which the different fracture sets were created and developed successively during the Sevier and the syn-folding Laramide orogenic phases. The field observations at the top stratigraphic surface of the Weber sandstone lead to classify the fracture sets into a pre-fold WNW–ESE fracture set, and a NE–SW fracture set post-dating the former. The development and relative chronology of the fracture sets are discussed based on the geomechanical modeling results. Starting with a 3D restoration of the Split Mountain Anticline, three fold-fracture development models were generated, alternately assuming that the WNW–ESE fracture set is either present or absent prior to folding process. Depending on the initial fracture configuration, the calculated fracture patterns are markedly different, showing that assuming a WNW–ESE joint set to predate the fold best correlates with field observations. This study is a first step addressing the complex problem of identification of fold-related fracturing events using an elementary concept of rock mechanics. When tight to complementary field observations, including petrography, diagenesis and burial history, the approach can be used to better constrain fractured reservoir characterization. Published by Elsevier B.V. 1. Introduction Many sub-surface reservoirs (e.g. Asmari carbonate in South-West of Iran, Frontier sandstone Formation (Fm) in Colorado Plateau, Ordovi- cian quartzite in South Algeria) are characterized by a low matrix per- meability which could be compensated by the presence of natural pervasive fracture sets of “bedding scale” and heterogeneously distrib- uted in space (Nelson, 1985). The knowledge of the distribution of these fractures is certainly crucial for the development strategy of the fractured reservoirs. For example, Hennings et al. (2000) illustrate the compartmentalization of a reservoir due to the fracture distribution within a fold. Moreover, since the pioneering works of Stearns (1964), Stearns and Friedman (1972), and Price and Cosgrove (1990), it is rec- ognized that the small scale fracture networks could vary according to the structural location in a fold. For similar fold geometry, different frac- ture patterns are found depending on the deformation scenario, for example thick-skinned versus thin-skinned folding (Hennings et al., 2000; Cooper et al., 2006), and suggesting a dependency on the stress state evolution (Engelder et al., 1997; Engelder and Peacock, 2001; Silliphant et al., 2002; Mynatt et al., 2009). Because this heterogeneous distribution of fractures should be taken into account in addition to the heterogeneity in sediment facies distribution, the design of a production scheme for a fractured reservoir, either for hydrocarbon, CO 2 or water flow requires assessment by dedicated numerical modeling. The numerical models require information on the distribution of fractures in terms of orientation, size, conductivity and density (e.g. Bourbiaux et al., 2002). Obtaining such fracture parameters remains difficult because many fractures are discontinuities at sub-seismic scale, and because their distribution in space does not allow a simple mathemati- cal expression to extrapolate the fracture parameters between data from different wells. To compensate for the lack of direct information on fracture populations, the reduction of uncertainty in the fracture characterization is currently obtained through geostatistical methods, which correlate the fracture distribution with available information on a large scale. This information can include sedimentary facies, as given Tectonophysics 576–577 (2012) 197–215 ⁎ Corresponding author. Tel.: +33 147526369. E-mail address: william.sassi@ifpen.fr (W. Sassi). 0040-1951/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.tecto.2012.07.025 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto