Internal Stratigraphy, Composition, and Depositional Setting of the Woodford Shale in Southern Seminole County, Oklahoma

Abstract

A core to log analysis was conducted to examine the relationship between lithology and wire-line log characteristics of the Woodford Shale. Shale cores were analyzed to determine if laterally correlative patterns in wire-line log signatures reflected compositional heterogeneity. Cores were described, sampled and analyzed to determine lithofacies and internal composition. Maps were constructed to determine if paleotopography or syndepositional tectonics affected Woodford Shale thickness. The composition of the Woodford Shale in the study area was compared to compositions and interpretations of the shale from studies conducted in the Ozark Uplift of Missouri and Oklahoma, and the Arbuckle Mountains, Lawrence Uplift and Criner Hills Uplift of southern Oklahoma. Four (4) laterally correlative subunits were established for the Woodford Shale. These subunits have distinct wire-line log signatures and were traceable across and beyond the study area. Thinning and thickening of the Woodford Shale occurs mostly at the expense of the lower subunits and reflects localized and regional paleotopography. Subunit composition, in particular the relative abundances of silt, chert, TOC and carbonate, may serve as an indicator of sea level. The compositional reflection of sea level was mirrored by changes in spectral gamma-ray measurements. Interpreted zones of relatively high sea level are rich in uranium and have higher U-Th ratios. Intervals interpreted as representing relatively lower sea level are rich in carbonate and silt; consequently these zones have lower uranium and U-Th ratios. A depositional model was developed for the Woodford Shale that places the Woodford Shale in southern Seminole County in a mid-shelf setting between the proposed inner shelf of the Noel Shale in the Ozarks and the distal shelf/slope of the Arbuckle Mountain and Criner Hills Uplifts.