A NEW SPECTRAL GAMMA-RAY PROFILE OF UPPER DEVONIAN/LOWER MISSISSIPPIAN WOODFORD SHALE ON THE SOUTH FLANK OF THE ARBUCKLE ANTICLINE, ARBUCKLE MOUNTAINS, OKLAHOMA (Invited Presentation)

A spectral gamma-ray (SGR) profile from a Woodford Shale exposure near Springer, Oklahoma has been constructed for the purpose of improving stratigraphic and paleoenvironmental interpretation of an economically important gas shale. The Springer site is an abandoned quarry located on the south flank of the Arbuckle anticline. The quarry exposes the entire thickness of the Woodford Shale (80 meters) from the basal contact with the Silurian/Devonian Hunton Group to the upper contact with the Mississippian Sycamore Limestone. SGR data were collected using a handheld spectrometer to measure concentrations of potassium, uranium, and thorium. Uranium in black mudstone is commonly a proxy for total organic carbon content while potassium and thorium are indicators of sediment provenance. As most petrophysical properties of mudstone sequences vary spatially, an SGR profile of the Woodford Shale at this location provides an opportunity for comparison to (1) SGR profiles of nearby, previously studied Woodford Shale outcrops and (2) subsurface gamma-ray well logs in southern Oklahoma.

This new profile contains gamma-ray patterns observed at an outcrop section located about four kilometers to the west. Notably, a major SGR “marker” with about 100 ppm of uranium is present in both locations. In the greater southern Arbuckle Mountains area, this SGR marker occurs at the top of the informal middle member of the Woodford Shale and is used to subdivide the mudstone on a sub-regional basis.

Historically, identifying the contact between the Woodford Shale and the overlying Sycamore Limestone using geophysical well logs or cuttings has been problematic (e.g., the contact may be abrupt or gradational). The contact in the Springer quarry is clearly gradational with intercalated beds of Woodford Shale lithofacies and poorly lithified gray-green mudstone characteristic of the overlying Sycamore Limestone. The gray-green mudstone is clay-rich and ductile in comparison to the siliceous and brittle Woodford Shale. The first occurrence of gray-green mudstone in the quarry is about four meters from the top of the exposure. Above this first occurrence, the gray-green mudstone changes back to Woodford Shale lithofacies. The gray-green mudstone interval contains the highest concentration of potassium and thorium observed in the SGR profile.