What are the Mechanisms and Timing of Joints in the Rockies?

Eric Erslev, Department of Geosciences, Colorado State University, Fort Collins, CO 80523, phone: 970 491-6375, [email protected]

Joints, also known as extensional or mode 1 fractures, form by opening perpendicular to their surfaces and thus provide important fluid conduits in the Rocky Mountains. The resulting directional permeability can enhance petroleum recovery rates and efficiency as well as hasten water breakthrough and cause the bypassing of large reservoir compartments. A better knowledge of joint mechanisms and timing is essential to effective petroleum production and exploration because early pre- and syn-tectonic joints may be sealed by subsequent processes and uplift-related joints may not extend to production depths in the subsurface.

A recent effort to merge traditional, surface-based fracture studies with new seismic anisotropy studies has resulted in an intriguing mismatch of hypotheses. Many surface studies have attributed jointing to syn-Laramide compression, post-Laramide Rio Grande extension and/or a variant of exfoliation where burial strain is released perpendicular to the topographic surface during uplift and erosion. While joints fitting these mechanisms exist, many joints seen in the field and inferred from seismic data are perpendicular to regional compression directions. This suggests that recent interpretations have neglected an important mechanism of jointing, that of rebound in the direction of prior tectonic compression. This mechanism predicts that many Rocky Mountain joints formed during uplift and erosion in orientations that are perpendicular to prior Laramide shortening trends. A more complete appraisal of joint mechanisms promises to help unlock the tectonic history of the Rocky Mountain basins and allow better predictions of permeability anisotropy in petroleum reservoirs.