AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
(1) Geology, University of Oklahoma, Norman, OK.
Understanding the mechanisms under which shales deform is fundamental to improving exploration success in unconventional resource plays. An outcrop of the Woodford Shale has been chosen to characterize fracture patterns at the outcrop level, and relate them to the structural regime of the study area and internal stratigraphy of the formation. The outcrop is located along a syncline in the Arbuckle Mountains of Oklahoma (Sec 34,T. 1 S., R. 2 E.) between Vines Dome and Dougherty Anticline. Gamma ray logs of the outcrop were obtained, the section was measured, samples were collected and fracture orientations along various points were recorded. A unique fracture pattern was mapped, where fracture orientations were different at the various points along the structure. Also, natural fractures were abundant and perpendicular to bedding in brittle cherty layers, and scarce and slanted in ductile, thin-bedded mudstones. The recognition of these patterns is in agreement with measurements obtained in the laboratory of the mechanical properties of the Woodford Shale. Alternating sequences of laminated mudstones are rich in organic content. They are also mechanically ductile. Interbeded cherty layers are less organic rich, and mechanically brittle. Therefore, the Woodford Shale can be defined as an axially anisotropic material in the direction perpendicular to the laminations and an isotropic material in the direction parallel to the laminations. An analog model constructed in the laboratory is used to examine the regional relationship between folds and the development of natural fractures in a brittle-ductile material. Another analog model examines small-scale deformation in materials with varying mechanical properties. The hypotheses for these experiments based on field observations are: 1) laminations in shale formations are an important control in the propagation of natural fractures along with bed thickness and mineralogy, and 2) fracture patterns in shales are predictable along folds assuming the stress field is understood. A 3D computer model will also be constructed with the data from the analog models, to refine a conceptual model that can be applied to predicting fracture patterns in brittle-ductile materials and other shale formations.