3-D CHARACTERIZATION AND ANALYSIS OF FOLD-FRACTURE RELATIONSHIPS WITH APPLICATION TO RAPLEE MONOCLINE, UTAH

Folding and fracturing can be intimately related processes in which the overburden stress, the remote tectonic stress, local folding related stresses and stress perturbations due to pre-existing fractures all influence new fracture formation. The development of tools for the prediction of fracture orientations would have important consequences for reservoir and aquifer assessment as fractures can act as conduits channeling fluids into reservoirs or break sealing capacity and disperse accumulated fluids. As a case study, the relationship between the current fold geometry and fractures of Raplee Monocline are examined. Raplee is a Laramide aged, ~14-km long fold exposed in the Monument Upwarp of south-eastern Utah and deforms the Pennsylvanian and Permian Paradox, Honaker Trail, Rico and Cedar Mesa Formations. The study involves three distinct parts: 1) Field based characterization of the fractures on and around the fold, 2) development of accurate models of the fold’s geometry and 3) analysis of the fold’s shape using the concepts of differential geometry.

Fracture orientations and spatial densities (number of joints per meter measured orthogonal to joint set strike) were obtained in multiple stratigraphic units. Three distinct bed perpendicular joint sets (I-III, numbered by age) are seen, with approximately orthogonal sets (I, 75-105° unfolded strike; II, 000-050°) present on the fold and in flat-lying strata away from the fold. The third set (III 115° to 140°) is seen only on the fold and increases in spatial density with increased fold amplitude and fold limb dip, suggesting it is folding induced. Global Positioning Systems (GPS) data were collected to constrain the fold shape. Geostatistical interpolation, including kriging and Discreet Smooth Interpolation (DSI), was used to create geometric models of the fold where spatial data are absent. Using algorithms based on differential geometry, the shape characteristics of the fold models can be precisely quantified. For example, the maximum curvature magnitude and direction at any point on the surface can be calculated. By combining the fracture data, fold shape models and geometric analysis (curvature as a proxy for strain), correlations between fold geometry and fracture characteristics can be examined and eventually predicted.