Laboratory-Based Lithological and Geomechanical Characterization of the Woodford Shale: A Tale of Two Rock Types

Abstract

Perhaps one of the most remarkable features of the Woodford Shale is its high frequency cyclical interbedding between soft (incompetent) and hard (competent) beds. In this work, we thoroughly investigated the main rock characteristics of these couplets (soft-hard) across several outcrops of the Woodford Shale in south central Oklahoma. To document such typical high-frequency cyclicity, two samples were collected at every foot (one soft and one hard) from the stratigraphic sections. Then, a comprehensive multi-scale characterization was performed combining results from several laboratory techniques, including: X-ray diffraction, X-ray fluorescence, total organic carbon (TOC), thin section petrography, scanning electron microscopy (SEM), rock hardness tests, and uniaxial compressive strength tests (UCS).

Laboratory results corroborated the outcrop-based distinction between soft and hard beds, as determined by systematic contrasts in composition, rock fabric and mechanical properties. Experimentally, soft beds are ductile since they sustained significant plastic deformation before failure on UCS tests; these beds are usually finely laminated, clay-rich (>15%), have very high TOC content (>6%), very high concentrations of organic proxies (Mo, U), low Si/Al ratios, and high detrital proxies (Ti, Zr, K, Al). Hard beds on the other hand are brittle since they sustained little to no plastic deformation before failure; these beds are massive, quartz-rich (>85%), and have lower TOC content (<6%), lower organic proxies (Mo, U), and low detrital proxies (Ti, Zr, K, Al).

The high-resolution results from this work provide insights for the prediction of rock properties of the two most dominant and distinctive rock types within the Woodford Shale strata, soft and hard beds; giving rise to speculate that if proper physical distinction is made between soft and hard beds in cores or outcrops, few samples would work well for upscaling rock properties within larger intervals with incomplete sets of data. On the other hand, results of this work identifies potential subsurface target zones as composed by high frequency interbeddings of soft and hard beds. The soft beds have higher TOC and thus contain potential hydrocarbons acting as local source rocks. Whereas, the hard beds contain abundant natural fractures acting as conduits for hydrocarbons flow during hydraulic fracturing treatments.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018