AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
New Insights into Fault-Bend Folding Revealed by Area-Depth Relationships in Physical Models, the Rosario Oil Field, Venezuela, and an Outcrop Example from the Canadian Rockies
(1) consultant, Tuscaloosa, AL.
(2) Earth & Planetary Sciences, Rutgers University, Piscataway, NJ.
Traditional methods for the interpretation of compressional fault-bend folds, namely restoration, forward modeling, and fault-displacement measurement, are usually based on the assumption of constant bed length and bed thickness (constant BLT). Internal small-scale deformation is considered to be absent or small enough to be neglected. The constant BLT assumption is tested here with experimental models and field examples and found to miss important aspects of the structure that are significant from the reservoir scale to the regional scale. The experimental models, consisting of either dry sand or wet clay above two overlapping plates, simulate deformation above a thrust fault with a flat-ramp-flat geometry. As the plates converge, a ramp, dipping ~30°, forms within the sand or clay layer. With additional convergence, the hanging wall moves up the ramp and onto the model surface (the upper flat). Cross sections from the models interpreted using the traditional constant-BLT techniques underestimate the boundary displacement by half and neglect the internal deformation. The area-depth relationship predicts the correct value of displacement and, in addition, gives the depth to detachment and the internal strain. Strain magnitudes are substantial, ranging from 0 to 20%. The Rosario oil field, Venezuela, is a fault-bend fold that formed with both upper and lower detachments buried beneath a thick overburden. Constant BLT restoration underestimates the fault displacement, whereas the area-depth relationship gives displacements that nearly match those measured on the profile. Of importance to possible reservoir-scale fracturing, layer-parallel extension is predicted for beds below the upper detachment. The plausibility of this seemingly unlikely prediction is verified with measurements from a fault-bend fold observed in an outcrop west of Calgary, Alberta, Canada. In both field examples, the layer-parallel extension is inferred to be the result of area-constant vertical thinning associated with the anticline having been forced to lift a thick overburden.