Field characterization and analog modeling of natural fractures in the Woodford Shale, southeast Oklahoma

Abstract

Understanding the mechanisms under which shales deform is fundamental to improving exploration success in unconventional resource plays. Several outcrops of the Woodford Shale have been chosen to characterize fracture patterns at the outcrop level and relate them to the principal stresses of the of the study area and rock types of the formation. The outcrops are located in the Arbuckle Mountains of Oklahoma. This region is composed of several northwest-trending folds and regional fault systems. Excellent outcrop exposures of the Woodford Shale are used as reservoir analogs.

Field analysis consisted of outcrop gamma-ray logs, facies descriptions and fracture characterization. Key interpretations include : 3rd order sea level cycles from gamma-ray parasequences, five distinct lithofacies, and abundant natural tensile fractures.

Alternating sequences of laminated claystone and siltstone are rich in organic content and are mechanically ductile. Bedded chert is less organic rich , and mechanically brittle. Therefore, the Woodford Shale can be defined as an axially anisotropic material in the direction perpendicular to the bedding and an isotropic material in the direction parallel to the bedding. Fractures were observed in the field to be planar and perpendicular to bedding in cherts , and inclined with respect to bedding in laminated clay-rich layers.

A simple analog model constructed with a layered sequence of Play-Doh and clay was used to examine the relationship between alternating brittle / ductile sequences and the development of fractures. The conclusions of these experiments are : 1) laminations in shale formations can be an important control on the propagation of natural fractures and 2) fracture patterns in shales are broadly predictable based on the correct identification of brittle and ductile layers using gamma-ray logs. Decreasing gamma-ray and spectral gamma-ray log patterns may indicate brittle zones which would be most favorable for horizontal-well placement and fracture stimulation. Zones with high gamma-ray and high levels of uranium may be problematic for horizontal drilling due to likelihood of higher clay content and may serve as fracture stimulation barriers.