Analysis of anomalous thicknesses of the Hunton Group (Upper Ordovician–Lower Devonian) using image and standard open-hole wire-line logs

ABSTRACT Correlating the internal stratigraphy of the Upper Ordovician to Lower Devonian Hunton Group carbonates in the subsurface is problematic without paleontological data. Adding to this challenge are the seemingly rare occurrences of drastic changes in the thickness of the underlying Sylvan Shale, the Hunton Group itself, and the overlying Woodford Shale over short distances. Many workers have invoked the use of faults that intersect the wellbore or preferential erosion to explain these anomalies without taking a closer look at patterns on wire-line logs that accompany these anomalies. By integrating data from resistivity image logs, wire-line log curves, and isochore maps, anomalous thickness patterns can be explained by depositional variation and erosion, without relying on interpretations of faults intersecting the wellbores. Patterns in isochore maps strongly suggest that vertical movement on fault blocks created differential accommodation, with increased deposition along downthrown fault blocks and nondeposition and erosion on uplifted fault blocks. The most significant consequence of this study is the realization that any attempt to understand the fault movement and paleotopography during the Hunton Group must also include the underlying Sylvan Shale, as Hunton deposition was influenced by the post-Sylvan paleotopography.