An Integrated Sedimentological and Geochemical Analysis of the Woodford Shale, Permian Basin

Nicholas B. Harris, Nikki T. Hemmesch, and Cheryl A. Mnich
Department of Geology and Geological Engineering, Colorado School of Mines

We have undertaken detailed, core-based analyses of the Woodford Shale in Pecos County, Texas, with the objectives of: defining lithofacies, characterizing depositional settings and processes, and establishing a sequence stratigraphic framework for this classic black shale. Our methods include core description and interpretation of several long Woodford cores, correlation to modern log suites, and detailed sampling for organic and inorganic geochemical analysis. A parallel study investigates the relationship between shale mineralogy and mechanical properties of the shale.

Sedimentological analysis demonstrates that the Woodford Shale in Pecos County contains at least five, distinctly different lithofacies: massive (structureless, homogeneous, bioturbated) carbonate facies, black (structureless, laminated) mudstone facies, grey (structureless) mudstone facies, siltstone facies, and a laminated (planar-parallel, ripple) carbonate facies. Different processes were active during deposition, including sediment gravity flows, turbidites and pelagic deposition. Carbonate-rich lithofacies were sourced from a shallow-water platform, possibly the Diablo Platform to the west. These lithofacies are stacked systematically, suggesting that their deposition was controlled by cyclic variations in relative sea level, i.e. they represent the basinal equivalent of classic shallow marine stratigraphic sequences.

The stratigraphic cycles have geochemical expression, indicated by variation in TOC content, redox indicators and sedimentation rate. Longer-term cycles are also evident in the geochemical data, including a systematic upward increase in biogenic silica, decrease in clay content, and sharp discontinuities in TOC content at the boundaries between the upper, middle and lower Woodford. These longer-term variations suggest changes in climate and / or ocean circulation patterns that are superimposed on the shorter-term sea level cycles. Some geochemical patterns, for example the variation in biogenic silica, have a direct impact on the shale’s mechanical properties and therefore on it’s response to fracture stimulation.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009