Date of this Version
Peppers, M.H., 2015, Unraveling controls on fracture stratigraphy in carbonates: the influence of regional stress, mechanical properties, and diagenesis. [M.S. Thesis]: Lincoln, University of Nebraska, 101 p.
Fracture characteristics analyzed from outcrops provide key insights into the migration pathways of subsurface hydrocarbons, and allow for a detailed understanding of the tectonic history in an area. This study looks to assess the impacts that various controlling factors have on the development of fracture characteristics. To complete this objective, a succession of Ordovician to Mississippian rocks was examined. The logged section includes the Cotter Dolomite, Chattanooga Shale, St. Joe Formation, and the Boone Formation (subdivided into informal Upper and Lower members). Located in northwestern Arkansas and southwestern Missouri, data were collected from roadcut exposures along Highway 71. Collected fracture orientation data were used to determine the evolution of regional stress affecting the area of interest. Using hardness data measured from outcrops via a Schmidt Hammer, a generated mechanical stratigraphy was compared to lithology and diagenetic alteration. Photographs taken of fractures from the field were used to determine Fracture Intensity (FI) values, and used to ascertain the effect that bed thickness and mechanical property variation had on the development of FI.
Fracture orientations indicate 4 main deformation events: 1) folding of the Ordovician strata, 2) oblique compression and indentation during the Ouachita Orogeny, 3) effect of far-field stresses from Ancestral Rocky Mountain uplifts, and 4) far-field stresses as a result of the late stage of the Alleghanian Orogeny. FI varies with mechanical contrasts, but is not constrained by bed boundaries. The mechanical stratigraphy observed in the studied succession is a direct result of lithological changes, with minor impact from diagenetic chert within some units. This work indicates that in this study area, regional stresses exert the primary control over fracture orientation, while large-scale mechanical variations control the FI. These findings contribute to the development of a suite of “best practices” for future studies by demonstrating that a thorough regional stress investigation and careful mechanical stratigraphy analysis are critical to understanding fracture development.
Advisor: Caroline M. Burberry