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Reports: UNI852300-UNI8: A Field-Based Geomechanical Study of the Formation, Deformation, and Internal Structure of Reservoir-Scale Sandstone Dikes, Sheep Mountain Anticline, WY

W. Ashley Griffith, PhD, University of Texas Arlington

This grant has supported a mix of field work, travel to national conferences, tuition, and stipend for a M.S. student, Jennifer Beyer, and an undergraduate student, Monet Alvarado. Together we have completed field work and are processing field data. Initial results were presented at the 2013 GSA National Meeting in Denver, and more results will be presented at the upcoming 2014 GSA National Meeting in Vancouver. Additional results have been presented at the 2014 AAPG Student Expo in Houston and the ACES student research symposium at UT-Arlington in the spring of 2014. We are in the process of completing data analysis and compiling our results into two individual manuscripts intended for publication during the following year. A summary of our results follows.   Thus far, we have conducted a total of three field campaigns (two supported by this PRF grant) in order to characterize large, up to 1km long, sandstone injectites which intrude the Cretaceous Mowry Formation in the vicinity of Sheep Mountain Anticline (Bighorn Basin, WY). Sand injection was aided by pre-existing joints in the Mowry formation before the folding of Sheep Mountain Anticline. Most of the injectites restore to vertical (dike) and horizontal (sill) orientations when unfolded around bedding (Figure 2). Downward injection of sand from the Peay member of the overlying Cretceous Frontier Formation has been interpreted based on lithologic comparisons of injectite material and parent material, as well as multiple structural relationship which consistently show that injectites propagated downward and laterally, but never upward. This downward injection is evidence of a highly stratified stress field resulting from the deposition, burial, and lithification history of the rock units in the area (Figure 3). The internal structure of the injectites is dominated by two sets of mutually offsetting deformation bands (Figure 4). The deformation bands have shear and compaction components, exhibiting significant porosity loss, as well as minor cataclasis and pressure solution. After formation of the deformation bands, subsequent faulting localized along edges of compound deformation bands (Figure 1B), evidenced in the field by slickensided surfaces. A detailed kinematic analysis of slickenline lineations yield shortening and extension axes consistent with deformation band formation associated with the initiation of Laramide–oriented shortening, and continuing through the folding of Sheep Mountain Anticline.   We were not able to determine the sense of slip for most of the deformation bands where a slip direction could be determined. However, in the few localities where we obtained a sense of slip, most indicated reverse slip, consistent with Laramide contraction. For processing, it was assumed that the deformation bands exhibited reverse slip where the sense of slip could not be determined. Using graphical methods outlined by Marrett and Allmendinger (1990), the orientation of the fault plane, slip direction, and the sense of slip were used to determine the shortening and extension axes for each fault surface. These axes were consistent along the length of each injectite. Fourteen of the eighteen injectites showed sub-vertical extension axes and NE/SW shortening axes, suggesting formation during Laramide contraction and the folding of Sheep Mountain (Figure 5). The other four injectites did not share this NE/SW trend of axes orientation. They showed a NW/SE alignment of their principal shortening axes. We were only able to determine the sense of slip on two of the four injectites, and they exhibited normal slip. The two injectites with known normal sense of slip showed sub-vertical shortening axes and NW/SE extension axes, suggesting their formation occurred during post-folding extension. We could not determine the sense of slip on the other two injectites, so reverse slip was assumed. Under this assumption, our analysis produced sub-vertical extension axes and NW/SE shortening axes, suggesting their formation was pre-folding during the Sevier stress regime. While this could make sense, a closer analysis shows that our assumption of reverse slip does not hold for these two injectites.   The injectites showing NW/SE shortening in their deformation bands have a NE/SW orientation themselves. The Mowry joints that aided this intrusion orientation formed during early stages of folding, when Laramide contraction began. If the injectite formed during early folding, it is impossible that the deformation bands within the injectite to have been formed pre-folding. It is also very unlikely that a NW/SE shortening was produced during Laramide contraction. The deformation bands of these injectites likely do not follow our reverse slip assumption. If we therefore assume they are normal slip, NW/SE extension suggests post-folding deformation.   The outcome of this analysis is that the sandstone dikes were formed early-to-syn folding, whereas deformation bands were forming during and after folding. While the injectites represent potential fluid pathways for hydrocarbon migration, the deformation bands may act as fluid flow barriers. Continued study of these deformation bands will provide a better understanding of their petrophysical implications and how they are related to the kinematic development of the Sheep Mountain area.