MULTIPLE AGES AND ORIGINS OF EXTENSIONAL FRACTURES IN THE ROCKY MOUNTAINS

Both shear (faults) and extensional (joints) fractures provide critical tests of tectonic hypotheses and solutions to industry problems related to reservoir anisotropy. Our industry-supported fracture studies show complex patterns indicating multiple fracture mechanisms and stages in the Rocky Mountains.

Early NE- to E-striking, syn- to late-Laramide joints are typically partially to totally sealed with vein-fill mineralization. They strike sub-parallel to uni-directional Laramide compression directions defined by over 20,000 minor fault measurements, which indicate an average horizontal compression trending N67E-S67W. This orientation is nearly perpendicular to average Laramide basement-cored arch and secondary fold trends, indicating formation by splitting during thrust-dominated Laramide deformation.

Sets of younger, roughly NW-striking joints cut rocks as young as Miocene, and thus post-date Laramide deformation. Some NW-striking joint sets cross-cut Laramide folds, consistent with NE-SW post-Laramide regional extension. Associated post-Laramide normal and strike-slip faults show N-S to NE-SW extension, commonly reactivating pre-existing faults with oblique slip. Other joint sets parallel thrust-faulted arch margins, indicating localized post-Laramide back-sliding on Laramide master thrusts, especially those with more E-W strikes. Locally, these joints are consistent with current day stresses, paralleling modern S_Hmax from drilling-induced fractures and microseismic studies.

Joints adjacent to folded arch margins and major river drainages are more difficult to understand. They may be generated by unfolding of continental lithosphere due to isostatic and erosional unloading. Continued research to define the patterns and mechanisms of jointing is needed to successfully predict subsurface permeability anisotropy in conventional and resource petroleum plays.