NioFracture Initiative at the University of Wyoming: GIS Analysis of Natural Fractures in the Rocky Mountains

Title: NioFracture Initiative at the University of Wyoming: GIS Analysis of Natural Fractures in the Rocky Mountains

Authors: Eric A. Erslev, Adjunct Professor; Laura Kennedy, Research Scientist; Department of Geology and Geophysics, University of Wyoming, Laramie, WY

Publication: The Outcrop, May 2013, p. 9, 11-15

Introduction to NioFracture
Understanding fractures in units that host unconventional resources (e.g., the Niobrara Formation) is vital to predicting the permeability and economic potential of these tight reservoirs. Just understanding fracture timing is critical as the most recent fractures are more likely to be open, and thus control hydrocarbon storage and permeability conduits. Unfortunately, the complex Phanerozoic tectonic history of the Rockies makes predicting fracture orientations and intensities difficult because an orogeny’s minor faults and joints commonly extend far beyond their major structures. For instance, minor fractures with NE-SW to E-W extension during post-Laramide rifting extend well beyond the Rio Grande Rift itself. In addition, localized fracturing mechanisms like syn-Laramide arch collapse and post-Laramide back-sliding on Laramide thrusts can refract regional stress trajectories, greatly impacting fracture orientations.

In 2012, the NioFracture initiative received seed funding by the School of Energy Resources at the University of Wyoming to develop a digital geographic information systems (GIS) database for fractures in the eastern Rockies. The purpose of NioFracture is to integrate diverse sources and representations of minor fault (mode 2 and 3 fractures) and joint (mode 1 opening fractures) data in order to graphically portray the extent of Rocky Mountain fracture provinces. Initial datasets were largely academic surface data from basin margins but it is hoped that the data coverage will be expanded to basin interiors with industry subsurface data.

Our expectation is that the combination of data from diverse sources into a common platform will result in new insights with both academic and industrial importance. For instance, fracture orientations provide critical testsof tectonic models for basement-involved foreland thrust belts like the Laramide Rocky Mountains (Erslev and Koenig, 2009), whose cratonic locations are distant to collisional plate margins. Understanding the tectonic controls on fracturing can then provide constraints for petroleum exploration efforts because natural fracture trends are vital to predicting both unstimulated and stimulated reservoir permeability.

Methods
Laura Kennedy (now at Weatherford Laboratories) designed the NioFracture GIS work flow and input fracture data from various academic sources (see Erslev and Koenig (2009) for references and fracture analysis methods) into ArcGIS (Kennedy et al., 2012). The GIS data tables describe locality information (e.g., location and source of data), attributes of the raw data (e.g., number of fractures measured, average plane/line orientations and degree of clustering), and interpretations (e.g., relative/absolute ages of fracture subsets and calculated stress/strain for each subset). Fracture orientations and  stress/strain data are portrayed using smoothed rose diagrams superimposed on to satellite imagery and/or geologic bedrock maps. To date, 8300 minor faults and 4099 joints from the eastern Rocky Mountain area have been compiled and tabulated.

At the outcrop scale, the rose diagrams are overlain by a point at their actual geographic  location. The point size is scaled to the number of fractures for each locality. Data attributes for each rose diagram include fracture characteristics (number, orientation, clustering, etc.), geologic unit, inferred stress directions, and timing. Stereonets of 3D data, available field photos, and diagrams from the published data source can be linked
to each rose plot (Fig. 1).

MayLS1
Figure 1. An example of local outcrop data (Kennedy, 2012) from the Alcova Reservoir area in Wyoming plotted as 2D rose diagrams with additional 3D data in data tables, outcrop photos and stereonets.

To visualize larger regional trends, outcrop data was lumped into subregional (typically a graduate student thesis area) and regional scales (Fig. 2). The data tables also contain the eigenvector analyses of this data, which quantify their average orientations and clustering. It is apparent in the largely Laramide data plotted in Figure 2 that Laramide compression directions are largely unimodal, and fan from NE-SW orientations at the Wyoming-Montana border to E-W orientations at the Colorado-New Mexico border.

Application Example
At the regional and sub-regional scales (Fig. 2), Laramide minor faulting commonly defines domains where either thrust or strike-slip faulting is dominant. The dominant fault type is revealed by fracture rose diagrams where either thrust fault strikes are perpendicular to the compression directions or strike-slip fault strikes are conjugate to the compression directions (typically about 20 degrees both clockwise and counter-clockwise to the compression directions). The difference in these cases is important to petroleum exploration because near-vertical strike slip faults would be expected to provide a far better vertical permeability and stimulation of overlying strata than low-angle thrust faults.

In order to explore the possibility of distinct domains of strike-slip and thrust faulting, locality-scale rose diagrams for fault data (Holdaway, 1998; Larson, 2008) along the NE Front Range between Loveland, CO and the Wyoming border were plotted (Fig. 3a). To more clearly portray the difference between thrust- and strike-slip-dominated localities, bubble maps of percent strike-slip faults were plotted (Fig. 3b), with low % strike slip fault localities (high % thrust faults) shown as green bubbles, high % strike-slip fault localities as red bubbles, and more equal proportions of strike-slip and thrust faulting as yellow bubbles. This bubble map clearly defines 2 wide strike-slip corridors. One is centered on the Livermore embayment, whose NE-striking high-angle faults were proposed to have dextral strike-slip motion by Erslev and Holdaway (1999), Larson (2008) and Tetreault (2008) on the basis of fracture orientation deflections, paleomagnetic pole rotations, and 3D structural restorations. Still, until this GIS analysis, we did not suspect that the zone of pervasive strike-slip shear extended beyond the exposures of the major strikeslip faults. In addition, this analysis also shows that the area around the plunging Milner Mountain anticline west of Loveland is also a shear zone of dominantly strike-slip minor faulting.

The existence of strike-slip corridors and transfer zones in the Rockies, long advocated  by Stone (1969) and others, may have important implications to natural fractures in resource plays. For instance, areas dominated by low-angle thrust faulting may lack large subvertical minor faults that create permeability conduits between units. This can be a  good thing if bounding units contain water, but it can be a bad thing when well economics and fracture stimulation require communication between vertically-adjacent units.

Conclusions
The NioFracture compilation unifies diverse public domain fracture data into one  comprehensive database, aiding predictions of unconventional reservoir fracture permeability. Initial results show a remarkable uniformity of Laramide fractures indicating ENE-WSW compression despite major differences in the trends
of major structures. Zones where major structures are oblique to regional compression  typically are dominated by strike-slip minor faults and ENE-striking joints.

Post-Laramide E-W to NE-SW extension has locally created overprinting fracture sets  roughly orthogonal to Laramide fractures. Thus, if an exploration effort wants to drill  perpendicular to the major open fracture set, it is critically important to know the age of the area’s dominant fractures. Based on the regional patterns shown by the NioFracture GIS initiative, horizontal drilling targeting Laramide joints and minor strike-slip faults legs should be roughly NNW-SSE. If most reservoir permeability is created by NW-SE post-Laramide jointing and normal faulting, NE-SW horizontal drilling may be more  ideal. These regional patterns are complicated by local fracture mechanisms, which the NioFracture initiative is currently investigating.

For information on access to the NioFracture database, please contact Eric Erslev at eerslev@uwyo.edu.


References
Erslev, E.A., and Holdaway, S.M., 1999, Laramide faulting and tectonics of the northeastern Front Range of Colorado, in Lageson, D.R., Lester, A.P., and Trudgill, B.D., eds., Colorado and adjacent areas: Boulder, Colorado, Geological Society of America Field Guide 1, p. 41-49.

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, in Kay, S., Ramos, V., and Dickinson, W.R., eds., Backbone of the Americas: Shallow Subduction, Plateau Uplift and Ridge and Terrane Collision: GSA Memoir 204, p. 125-150.

Holdaway, S.M., 1998, Laramide deformation of the northeastern Front Range, Colorado: evidence for deep crustal wedging during horizontal compression: Unpublished M.S. thesis: Fort Collins, Colorado State University, 146 p.

Kennedy, L.E., 2011, Laramide transpression and block rotation followed by northeast-southwest extension, southeast Wind River Basin area: Unpublished M.S. thesis, Laramie, University of Wyoming, 98 p.

Kennedy, L., Erslev, E., and Aydinian, K., 2012, Mapping Rocky Mountain fractures: GIS methods for resource plays: AAPG Abstracts with Programs, 2012 Rocky Mountain Section Meeting, Grand Junction, CO.

Larson, S., 2008, Laramide transpression and oblique thrusting in the northeastern Front Range, Colorado: 3D kinematics of the Livermore Embayment: Unpublished M.S. thesis, Fort Collins, Colorado State University, 420 p.

Stone, D.S., 1969, Wrench faulting and Rocky Mountain tectonics: Mountain Geologist, v. 6, p. 67-79.

Tetreault, J., Jones, C.H., Erslev, E., Hudson, M., and Larson, S., 2008, Paleomagnetic and structural evidence for oblique slip folding, Grayback Monocline, Colorado: Geological Society of America Bulletin, v. 120, p. 877-892.