--> --> Modelling Laramide Deformation to Predict Fracture Orientations in the Big Sand Draw, Wyoming: Part 2 - Fracture Modelling

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Modelling Laramide Deformation to Predict Fracture Orientations in the Big Sand Draw, Wyoming: Part 2 - Fracture Modelling


The structural evolution of the Big Sand Draw (BSD) Anticline, Wyoming is simulated to predict orientations and distributions of natural fractures in the Tensleep reservoir and surrounding sedimentary packages. A recent CO2 flood suggests that the natural fracture system strongly influences fluid flow. Deformation mechanisms responsible for the formation of the anticline are forward modelled and strain captured to predict natural fracture orientations and intensities. The anticline verges westward and likely formed during the Late Cretaceous – Eocene Laramide Orogeny. Much of the Laramide deformation involves the reactivation of basement structures. Part 1 of this study used structural modelling to identify the processes responsible for deformation, suggesting that a blind N-S to NNW-SSE striking basement fault is responsible for long wavelength folding. Best-fit forward modelling parameters for the basement structure were used to record strain with kinematic and triangular dislocation algorithms. The orientations of the principal strain directions were used to predict the orientations of tensile and shear fractures. Measured fracture data from wells were compared with predicted orientations; the fracture types which best-fit the observed data were found to be joints (90° to e1) and conjugate shear planes (30° to e1). The dominant measured fracture trend is perpendicular to the anticline axis. These fractures are best reproduced by simulated reverse slip on a deep, steep, basement fault. Deeper units had a higher proportion of reproducible fractures than units higher in the stratigraphy; with ~ 90% of the fractures reproduced in the lowest Carboniferous Madison horizon. This supports the finding that the basement structure is the main control on fracture development and suggests there may be localised stress-field perturbations and low ambient stress at near-surface levels. To understand current fluid flow patterns in the BSD Anticline, the fracture sets created were analysed for slip stability and dilation tendency under the present-day stress field. It was found that the central portion of the anticline had the highest dilation tendency values in the Tensleep reservoir interval. This is an area where joints and shear planes, produced from basement involved deformation are very prevalent. The results of this study have important implications for the positioning of future CO2 injection sites and production strategies for the BSD Anticline.