RELATING FOLD GEOMETRY AND FRACTURE CHARACTERISTICS IN SEDIMENTARY ROCK USING ALSM AND FIELD MAPPING AT RAPLEE RIDGE, UT

Folded sequences of sedimentary rock at depth can act as reservoirs for both hydrocarbons and water. This trapping potential of folds can be strongly effected by fracturing, and therefore understanding and predicting fold-fracture relationships can improve reservoir forecasting. Although folding and fracturing present a common and well studied geologic system, studies of the relationships between fundamental characteristics such as fold geometry and fracture density are difficult in producing fields as measurements of fold shape are hampered by the low resolution of seismic surveying and measurements of fractures are limited to sparse well-bore locations.

To overcome these limitations, we present a case study of an exhumed analogue reservoir, develop methods to quantify both fold geometry and fracture characteristics, and summarize their relationships. The field area is Raplee Ridge, a Laramide age, N-S oriented, ~14-km long monocline in south-eastern Utah. The investigation involves three distinct parts: 1) field based characterization of the fractures on and near the fold; 2) development of accurate models of the fold geometry using high resolution data including ~3.5e7 x, y, z topographic points collected using Airborne Laser Swath Mapping (ALSM); and 3) analysis of the fold shape and fracture patterns using the concepts of differential geometry and fracture mechanics. Field documentation of fracture characteristics enabled the classification of distinct pre- and syn-folding fracture sets and the development of conceptual models of multiple stages of fracture evolution. Numerical algorithms, visual methods and field mapping techniques were used to extract the geometry of specific stratigraphic bedding surfaces and interpolate fold geometry between topographic exposures, thereby creating models of the fold geometry at several stratigraphic levels. Geometric characteristics of the fold models, such as magnitudes and directions of maximum and minimum normal curvature and fold limb dip, are compared to the observed fracture characteristics to identify the following relationships: 1) initiation of folding related fractures at ten degrees limb dip and increasing fracture density with increasing dip and; 2) no correlation between absolute maximum fold curvature and fracture density.