Abstract

This paper provides a comprehensive analysis and documentation of a popular outcrop (roadcut) of the Woodford Shale, known as the I-35 outcrop. At the bed scale, based on outcrop weathering responses, the lithology of the Woodford Shale is cyclically represented by two distinctive, intercalated rock types: soft (incompetent, ductile) siliceous shales and hard (competent, brittle) cherts. The detailed geologic characterization integrated results from several laboratory techniques: X-ray diffraction, X-ray fluorescence, total organic carbon (TOC), thin-section petrography, scanning electron microscopy, rock hardness, and uniaxial compressive strength (UCS) tests. Experimentally, soft beds are ductile because they sustained significant plastic deformation before failure on UCS tests; these beds are finely laminated, clay-rich (>15%), have very high TOC content (>6  wt%), very high concentrations of organic proxies (Mo, U) and detrital proxies (Ti, Zr, K, Al), and low Si/Al ratios. In contrast, hard beds are brittle because they sustained little to no plastic deformation before failure; these beds are massive (not visibly laminated), quartz-rich (>85%), and have lower TOC content (<6  wt%), lower organic proxies (Mo, U), and lower detrital proxies (Ti, Zr, K, Al). The results from our study attest the outcrop-based distinction between soft and hard beds, as determined by systematic contrasts in composition, rock fabric, and mechanical properties. In addition, our results provide insights for the prediction of rock properties for the two principal rock types within the Woodford Shale, giving rise to speculation that if a 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, thus helping to reduce the costs/time related to acquiring large data sets. In addition, observations from this and other outcrops indicate the potentially best horizontal targets are those with approximately equal proportions of the two rock types.

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