Abstract

Understanding mudrocks and shale reservoirs has become a significant area of interest within industry and academia in recent years. Of particular interest is understanding the pervasive variability present within these units. This variability became apparent when conventional approaches, such as lithostratigraphic analysis and well log correlation, were coupled with recent developments in palynostratigraphy and chemostratigraphy. A single shallow Woodford Shale research core in the western Arkoma Basin from Pontotoc County, Oklahoma, was used to identify three scales of stratigraphic cyclicity. By comparing the relative abundances of continental sourced pollen and spores to marine-derived acritarchs over a stratigraphic interval, it was possible to extrapolate the overall trends in shoreline trajectory. Conventional well log analysis, such as gamma ray logs, provided a balanced understanding of the interplay between localized changes in sedimentation and regional shifts in the stratigraphic base level, in addition to providing a means to tie these analyses to extensive subsurface data sets. Chemostratigraphic correlations resolved subtle, but stratigraphically significant, shifts in localized patterns in sedimentation. Using these approaches, the Lower and Middle Woodford Shale can be divided into four chemostratigraphic parasequences within a transgressive systems tract defined by well log and core analysis. The Upper Woodford can be separated into an additional four chemostratigraphic parasequences within a highstand systems tract. Chemostratigraphic data also revealed the changing bottom water conditions present at the time of deposition, with a period of localized anoxic conditions recorded in the Lower and Upper Woodford in this part of the basin. These localized changes in sedimentation and environmental conditions can be nested into two longer term regional transgressions and regressions.

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