Title: Rocky Mountain Fractures: Academic-Industry Collaborations Reveal Their Complexity and Potential
Author(s): Eric Erslev, Adjunct Professor, University of Wyoming
Publication: The Outcrop, January 2011, p. 4-6
Article type: Lead Story
There can be no doubt that fractures play a huge role in determining the viability of Rocky Mountain petroleum plays. Tight shear fractures (faults) can form seals that compartmentalize reservoirs and form reservoir pressure boundaries (e.g., the southern boundary fault or “line of death” of Jonah Field, WY). Where open extensional fractures (joints) and jogs on faults provide enhanced permeability, production can reach the levels of the EOG Jake 2-01H well in northern Weld County, CO, which reportedly produced 90,000 bbl of oil in the first 90 days.
Thus, predicting fracture orientations and intensities is crucial to many Rocky Mountain petroleum plays, whether it be exploring for the next Jonah Field or siting an expensive horizontal well in the Niobrara Formation. The current importance of fracture knowledge to companies is shown by the fact that I have been asked not to reference some of my students’ work by name in this article.
Fracture prediction remains a huge challenge to industry because fracture mechanisms are inherently complex. It is my opinion that this challenge is best met by a collaborative approach integrating industry tools (e.g., image logs, micro-seismic studies, P- and S wave azimuthal anisotropy) with academic student energy (needed to measure and analyze fractures) and theoretical insights from kinematic and dynamic modeling.
Our program has shown the efficacy of this collaborative and integrated approach. Past and current student research has effectively tested Laramide tectonic hypotheses by measuring the shear sense and slip directions on over 20,000 minor faults (Erslev and Koenig, 2009). These reveal remarkably uniform, sub-horizontal ENE-WSW slip and compression directions that are almost perfectly perpendicular to the average trends of both Laramide arches and smaller reservoirs calefolds. These observations are contrary to the predictions of vertical tectonic, largescale transpressive, and multi directional compressional models for the Laramide Orogeny.
Recent minor fault studies done as part of the Bighorn Project (Bighorns.org) have revealed an intriguing sequence of deformation. The Bighorn Project, a collaborative project with co-P.I.s Anne Sheehan (University of Colorado), Kate Miller (Texas A&M) and Christine Siddoway and Megan Anderson (Colorado College), is part of the N.S.F. EarthScope Program. Over the last 2 years, we have completed a set of passive and active (with up to 2000 seismometers and 16 blasts) seismic experiments to map the full crustal structure of the Bighorn Arch and determine the tectonic mechanisms causing Laramide basement-involved deformation. As part of this project, minor fault analyses from strata encircling the Bighorn Mountains have revealed discrete domains of strike-slip and thrust faulting as well as a dual-stage Laramide development for the arch. An initial stage of uniform ENEWSW shortening was followed by a second stage of deformation which contained an additional component of gravitational spreading away from the arch culmination. This new model can explain some of the diversity of Laramide structures and fractures in the Rocky Mountains.
In contrast to the relatively simple Laramide faulting patterns in the Rockies, the mechanisms and timing of extensional fractures (joints) remain highly controversial. Since joints in an outcrop could have formed during the previous day, more data is needed to determine their mechanisms and timing. Documenting jointing patterns in pre-, syn- and post-orogenic strata can be very helpful to determine joint timing. For instance, a current controversy as to joint timing – whether NW-SE joints are pre-Laramide or post-Laramide – can be effectively tested by seeing if they cut syn- and post-Laramide strata, which they do in several locations throughout the Rockies.
While there appears to be domains of the Rockies where the primar y joints formed by splitting during E N E – W S W Laramide compression (Erslev and Larson, 2006), most of my students have shown the over whelming importance of post- Laramide joining. In their work, industry collaborations have been vital, providing access to subsurface image logs and seismic attributes that identify which surface fractures are important in the subsurface. Numerous jointing mechanisms have been indicated, including regional post-Laramide NESW to N-S extension, localized back-sliding on Laramide master thrusts (Thompson, 2010), and regional multi-directional detachment within overpressured strata.
One thing is clear – Rocky Mountain fractures are complex, with multiple ages and mechanisms of formation, and simplistic unified models will fail to predict them. Creating accurate models for individual areas will require extensive resources, and I think this will be best done through collaborations between industry and academia. Current collaborative shale-gas/oil efforts at the Colorado School of Mines and the Enhanced Oil Recovery Institute of the University of Wyoming provide possibilities for effective industry-academic collaborations, as do arraignments between companies and individual faculty members.
Erslev, E.A., and Koenig, N.B., 2009, 3D kinematics of Laramide basement-involved Rocky Mountain deformation, U.S.A.: Insights from minor faults and GIS-enhanced structure maps: GSA Memoir 204, p. 125-150.
Erslev, E.A., and Larson, S.M., 2006, Testing Laramide hypotheses for the Colorado Front Range arch using minor faults, in Raynolds, R., and Sterne, E., eds., Mountain Geologist special issue on the Colorado Front Range, v. 43, p. 45-64.
Thompson, R., 2010, Two-stage development of the Wind River Basin, Wyoming: Laramide shortening followed by post-Laramide regional extension, localized back sliding and arch collapse: Unpublished Colorado State University thesis, 116 p.