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.

Pore morphology effect on elastic and fluid flow properties in Bakken formation using rock physics modeling

“…Previous field observations, experimental, and numerical studies have demonstrated that fractures propagating from a stiff interval towards a less-stiff one tend to propagate more efficiently and commonly penetrate the contacts (Laubach et al 2009;Gudmundsson et al 2010;Slatt and Abousleiman 2011;Ferrill et al 2014;Galvis et al 2018;Kolawole et al 2021b). These suggest that induced fractures from target lithofacies characterized by low aspect ratios, and stiffer rocks with higher bulk and shear modulus, will tend to generate a coherent and connected fracture network that can connect with other pre-existing natural fractures or reactivate faults.…”

Fracture Properties of Nash Point Limestone and Implications for Fracturing of Layered Carbonate Sequences

“…Carbonate sequences, including that at Nash Point, are commonly composed of interbedded layers of limestone and calcareous shales (Galvis et al 2017;Sosa et al 2017;Forbes Inskip et al 2018;Hosseini et al 2018). In such sequences, there are likely significant contrasts in the fracture properties of the different strata and this will affect how fractures propagate across the sequence as a whole.…”