Lithofacies and stratigraphy of a complete Woodford Shale outcrop section in South Central Oklahoma: Geologic considerations for the evaluation of unconventional shale reservoirs

Abstract

We have developed a detailed rock-based documentation of a previously undescribed Woodford Shale outcrop in South Central Oklahoma. The complete exposed section provided the opportunity to investigate lithologic attributes across the complete Woodford Shale thickness, as well as on its under- and overlying formational contacts. Within the Woodford Shale strata, seven lithofacies were recognized honoring textural and compositional attributes, and they were grouped based on their weathering response into soft (incompetent) and hard (competent) beds. Internally, across the Woodford interval, there is an overall upward increase in quartz content, represented by higher proportions of siliceous shales and chert around the middle and upper members, whereas the lower member is mostly dominated by organic and clay-rich shales interbedded with minor proportions of cherty beds. However, most notable was the rhythmic cyclicity between hard (brittle) and soft (ductile) lithofacies throughout the Woodford, from which systematic measurements of bed thickness and soft-to-hard ratios are examined to illustrate multiple scenarios of stratigraphic anisotropy. The geologic assessment of reservoir quality was assessed using the vertical arrangement of lithofacies, from which we hypothesized that potential target zones are interpreted to be composed by high-frequency interbeddings of organic-rich “soft” beds (acting as a local source) and “hard” brittle beds (acting as more frackable or fractured rocks). According to this model and the vertical stratigraphy, a potential target zone is interpreted to lie in between the upper half of the middle Woodford and lower half of the upper Woodford member, where the soft-to-hard ratio is approximately 50/50 and the beds are thinner.