Fracture Envintories and Transfer of Strain into the Eastern Colorado Plateau as Evaluated by Field, Lidar, and Remote Sensing Techniques
Tim F. Wawrzyniec
Western State Colorado University, Department of Natural and Environmental Sciences, Gunnison, Colorado
The Crested Butte Lineament (CBL) is a topographically distinctive feature with a bearing toward ~315° that extends 82 km from the southeast margin of the Tertiary Piceance Basin of western Colorado to Tomichi Dome, which is a Cenozoic volcanic feature positioned near the flank of the Laramide-age, Sawatch Uplift. Although the CBL is clearly expressed in regional topography, the northern third of the trend is largely buried under glacial debris and along most of its length there are only a few mapped structures that appear to define this lineament. The standing interpretation is that most of the recognized fault structures are Laramide in age. However, there is very little supporting kinematic data or age-constraints that can validate this assertion. The on-going research represented here seeks to use modern remote sensing techniques (e.g., lidar and satellite imagery) to provide a multi-scale, evaluation of fracture inventories associated with a wide range of lithologies and ages, to devise a robust method for comparing large, statistically significant fracture datasets, gather new numerical age determinations of local intrusive bodies, construct new thermal models of the Uinta petroleum system, and to apply both seismic and gravimetric techniques to characterize the poorly understood kinematic significance of the CBL. The data will test the hypothesis that the CBL is an important strain transfer mechanism that couples Late Cenozoic, Rio Grande Rift extension with hydrocarbon expulsion and formation of the Gilsonite Dikes of the eastern Uinta Basin.
Remote sensing techniques have improved at a rapid rate and over the past few years have demonstrated their utility in multi-scale mapping and evaluation of large-scale structures including complex fracture systems (e.g., Sarma, et al., 2005, Cengiz et al., 2006). Commercial providers of satellite imagery can now provide up to 0.5-meter resolution color (RGB) imagery over most of the Earth's surface. Within the US, comparable airphoto imagery and 1m, black-and-white GEOEYE satellite imagery is broadly available and easy to acquire in digital formats. Such imagery combined with digital elevation models provide a new interpretive framework where outcrop expression of structures can be mapped over large regions, relatively quickly. Perhaps the most important attributes that can be extracted from such data are strike and dip, fracture trace trend, trace exposure length, and trace spacing. Given enough ground control, and exposure, geologic contacts can also be easily mapped. Although there is nothing new about such interpretive techniques the recent advances in satellite technology are providing imagery at resolutions at the highest resolutions that can be legally acquired for commercial purposes. In addition to such imagery, terrestrial and airborne lidar scanning technology is revolutionizing our view of topographic expression of geologic structures (e.g., Chan et al., 2007; Engelkemeir et al., 2008; Arrowsmith et al., 2009).
Examples of the application of lidar technology to problems of geologic importance are broad and can be found impacting nearly every subfield of the geologic sciences. Most notably are those studies that have used terrestrial lidar systems to image and map lithofacies variation over large-scale outcrops (e.g., Bellian et al., 2005; Phelps et al., 2008; etc.). Other applications are found in change detection (e.g., Wawrzyniec, et al., 2007; Perroy, 2009; Day et al., in press) where repeat imagery is used to calculate the magnitude and location of volumetric change. In structural applications a great deal of work has been reported where terrestrial lidar systems are used to characterize fracture networks within surface rock exposures (e.g., Roberts and Poropat 2000; Kemeny and Donovan, 2005; Gaich et al., 2008; Haneberge, 2006; Sturzenegger and Stead, 2009; Sturzenegger et al., 2011). In most cases, the application of lidar to capturing fracture networks seeks to characterize key metrics including orientation, size, roughness, and spacing, all of which can be further used to characterize block spacing and flow tortuosity, which are key characteristics in discrete fracture network models of fractured reservoirs.
Remote sensing evaluation of fractures in support of kinematic models can be completed using a variety of methods to capture a range of attribute distributions. Information from satellite imagery and photos is limited to trace trend, exposure length, and spacing. Combined with high-resolution digital elevation models, true orientation can be captured. The true advantage of evaluation of large-scale imagery is that it provides fracture metric distributions that span the decameter to kilometer scale. In fact, such trends can be difficult to recognize in the field, which makes large-scale imagery the only method to capture fracture data at these scales. The high-resolution imagery provides fracture trend data that spans the outcrop allowing such data to inform fracture properties not directly captured by this type of imagery. We can further bridge this gap in scale by capturing lidar imagery of the outcrop.
Terrestrial lidar data has a wide range of uses in geologic investigations. The technology is particularly useful in capturing metrics about the spatial distribution of lithofacies or the spatial distribution of changes that have affected an outcrop between scans. In structural applications, the technology is used within the mining industry to capture fracture parameters to inform complex and detailed discrete fracture network models to evaluate fluid flow (e.g., Sturzenegger et al., 2011). Specifically, key advantages of the technology include the ability capture of fracture orientation, size metrics (e.g., area), roughness, aspect ratio, fracture density and intensity, length of share edges between fracture sets, and angles between factures that share a common edge. Data can be classified based on relationships between visible bedding, structural position, bed thickness, angle from mean slope, and lithology. Lidar technology can capture the trace length of a specific fracture, but provides no information regarding the mode of the fracture or the types of fracture termination associated with an individual fracture. Given that these are important properties for any potential kinematic evaluation of a regional structure, lidar data should always be considered a supplement to standard field techniques. Lidar data that is generated in the context of adequate field studies provides fracture metrics that overcome the sampling bias that is created by only focusing on those outcrops attainable to the field geologist and it provides metrics that span the scales of investigation between field studies and the regional picture captured in air photos or high-resolution satellite imagery.
A critical component to this study is to further characterize features associated with the Crested Butte Lineament. In particular gravimetic and seismic techniques are being applied to determine if there is sub-surface expression of a master structure that defines the lineament. Gravity measurements may demonstrate a density structure that reflects basement offset or perhaps a modified porosity structure that is indicative of a near surface brittle structure. Given the distribution of thermal springs along the Lineament, there is some suggestion that the latter may be important. The application of shallow seismic techniques may further elucidate differences in near surface rock properties. Along most of the CBL in the upper Gunnison Valley the degree of mineralization and metamorphism of Cretaceous shale lithologies is not restricted to intrusive contact aureoles. Rather, silicification of these weak shales varies along the Lineament with no clear connection to nearby intrusive complexes. Additional characterization of the lineament includes providing new uplift and emplacement ages on some of these intrusive complexes in an effort to segregate fracture inventories between Laramide-age deformation and the considerably younger deformation associated with intrusion emplacement and fractures that may represent the regional stress field influenced by the Late Tertiary, Rio Grande Rift.
Do date, the results of the characterization work of the CBL are being considered in the context of 1D thermal models based on freely available subsurface data from petroleum systems of the Uinta Basin. Work completed in classroom projects at WSCU, and published reports have demonstrated many of the Tertiary petroleum systems of the Uinta Basin entered the "oil-window" sometime between 35-30Ma, which coincides with major phases of intrusive and volcanic activity along the CBL. The models based on known geologic constraints also demonstrate that as the rocks generated a massive hydrocarbon expulsion event that resulted in the formation of the Gilsonite Dikes of Eastern Utah. Positioned at about the same latitude, these dikes have a mean orientation of about ~310°, which is parallel to the trend of the CBL is sub-parallel to the extension direction associated with early Rio Grande Rifting in southern Colorado (Wawrzyniec et al., 2001; Bader, 2009).
The work represented here reflects a project that is on going. However, a successful conclusion of the research program will demonstrate the following: 1) the value of a multi-scale approach to evaluating fracture networks associated with a regional structure; 2) demonstration of a regional structure expressed in the subsurface along the CBL; 3) evaluate an important test that addresses the transfer of late Tertiary extensional strains associated with Rio Grande Rift into the eastern Colorado Plateau; 4) and how this strain may have modified the pattern and timing of hydrocarbon generation expulsion from the Tertiary Uinta Basin.
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AAPG Search and Discovery Article #120140© 2014 AAPG Hedberg Conference 3D Structural Geologic Interpretation: Earth, Mind and Machine, June 23-27, 2013, Reno, Nevada