USE OF SPECTRAL GAMMA RAY PROFILES FOR DISCRIMINATING DEPOSITIONAL AND STRATIGRAPHIC SUCCESSIONS IN UPPER DEVONIAN-LOWER MISSISSIPPIAN GAS SHALE INTERVALS, NORTH AMERICA

A hand-held spectrometer was used to systematically collect K, U, and Th gamma-ray (GR) data from outcrops of the Woodford Shale (Oklahoma), Chattanooga Shale (Oklahoma and Tennessee), and Cleveland Shale (Ohio). Vertical stratigraphic profiles of these data suggest that the proportions and amounts of K, U, and Th provide insight to provenance, deposition, and diagenesis of gas shale. In most instances, the GR response in gas shale is dominated by U. In some instances, however, GR responses are dominated by K and/or Th rather than U.

Based on comparisons to modern marine settings, the abundance of U in gas shale is related to accumulation of U through diffusion from seawater into a muddy substrate under conditions of anoxia and slow sedimentation. Periods of extremely slow sedimentation yield U-dominated GR peaks that provide local to regionally significant stratigraphic markers in the subsurface. In contrast, other strong GR markers in gas shale are controlled by the abundance of K and Th rather than U. These K- and Th-rich markers are suggestive of more rapid sedimentation and inheritance of minerals with terrestrial affinity. The base of both the Woodford and Chattanooga in Oklahoma is rich in K and Th. This observation is compatible with mud deposition across a regionally significant unconformity surface (the Acadian Unconformity). At Henry House Creek in south-central Oklahoma, an additional major K- and Th-rich GR peak occurs internal to the Woodford Shale (about 43 m from the base of the formation). This marker can be correlated to another Woodford exposure located 36 km to the south. Above this marker at both locations, the K and Th content of the Woodford systematically decreases to the top of the formation (upper 27 m).

Well-defined vertical cycles in K, U, and Th occur throughout the stratigraphic succession of Upper Devonian-Lower Mississippian gas shales of North America. From the perspective of Walther's Law, these vertical cycles provide evidence for systematic lateral changes in depositional conditions as related to tectonics, climate, and sea level. Assuming these successions are conformable (no regionally significant internal unconformities), shale intervals must contain correlative conformities that will prove invaluable for unraveling technical issues associated with sequence stratigraphy and burial history analysis.