PLT - Fracture Networks in Mississippian Carbonate Rocks: Investigation of Dam Bedrock Foundations in Northeastern Oklahoma

This study focuses on three dam facilities in NE Oklahoma where the foundations' underlying carbonate bedrock experiences post-flooding seepage events, undermining the foundations and possibly diminishing their containment efficacy. The internal erosion related to natural discontinuities, such as joints and faults, threatens the safety of the dams, as they indicate a potential for changes in the structural conditions of the foundation. In carbonate rocks, these discontinuities often consist of complex fracture networks due to their heterogenous origin, evolution, and geometry. As a result, these networks often employ controls on fluid flow, where the fractures can act as conduits. From the geologic perspective, the three facilities have minimal historical information; hence, characterizing the bedrock at each facility is essential. This study aims to provide new geologic data and identify the characteristics of the fracture networks present and the role of those fractures in fluid migration within the bedrock foundations of the dams.

New photogrammetry surveys of each facility were conducted to create high-resolution digital elevation models and orthophotos. These models are used as base maps in a geographic information system to map the geology and structures at each facility. Detailed structural mapping using the augmented circular scanline method, a fracture sampling method, was applied to several exposures chosen based on the best representation of fractures available in outcrop.

The formations exposed at all three facilities comprise of the Mississippian-aged Keokuk and Reeds Spring Formations and consist primarily of interbedded limestone and chert. The geologic and structural mapping revealed that joints along bedding planes, systematic joint sets, fractured chert beds, and dissolution surfaces contribute the most to the permeability of the rocks. Detailed structural mapping was performed in the lower portion of the Reeds Spring Fm., and it was found that the fracture intensity varies depending on lithology. The chert beds have a higher fracture intensity, with an average of 26 m/m2, while the limestone beds have an average fracture intensity of 14 m/m2. Six fracture networks are present: oriented N-S, E-W, WNW-ESE, NW-SE, NNE-SSW, and ENE-WSW. The orientation of the fractures depends on the lithology of the bedding surface measured, with the joints in the limestone-dominated bed predominately oriented in the ENE-WSW direction. These structures intersect at high angles to form fracture networks that allow fluid passage.
Future monitoring of structural conditions is being developed through geologic maps, core logging, cross-sections, downhole geophysical logging, and additional geophysical surveys.