Characteristics of Devonian-Mississippian Strata in the Southern Midcontinent: Lithologic, Biogenic and Geochemical Indicators of Processes Influencing Woodford/Arkansas Novaculite Sedimentation
Oklahoma State University
The Devonian-Mississippian (Frasnian-Tournaisian) Woodford Shale is a premier siliciclastic mudrock play in North America. Often thought to be a deep water mudstone, our studies show Woodford facies on-lap pre-existing paleotopography including the Nemaha Ridge and locally preserving sedimentary features and trace and body fossils that indicate nearshore deposition above storm wave base. In the southern Midcontinent region, the Woodford and Arkansas novaculite were examined in core and outcrops to document changes in compo-sition and geochemical signatures in order to better understand con-ditions during deposition of these coeval shelfal-basinal strata. Out-crops examined include the McAlister Cemetery shale pit near Over-brook, OK on the Criner Hills Uplift, Scratch Hill novaculite outcrop near Atoka, OK in the frontal fault zone of the Ouachita, Mountains, and the Jane, MO outcrop on the Ozark Uplift. Cores examined in-clude the Newfield, Poe, western Arkoma Basin and the KGS-OGS Current core from the Lawrence Uplift near Ada, OK. The high shelfal expression consists of the Chattanooga Shale and is typified by black fissile shale with minor silt but generally lacking in non-skeletal phos-phate and bedded cherts. Distal shelf and slope equivalents mapped as the Woodford Shale include interbeds of radiolarian cherts and black fissile shales with locally abundant non-skeletal phosphate. The basinal area deep Ouachita trough contains interbeds of radiolarian cherts (mapped as Arkansas Novaculite) and thin black shale beds that contain scarce nodular phosphate. Conodonts contained in the Arkansas Novaculite indicate that only the upper part of the Novacu-lite is stratigraphically equivalent to the Woodford Shale with a con-siderable thickness in the middle part that is Middle Devonian with no shelfal equivalents. Chert beds appear to be markers for deep upwelling currents and dominate the succession in the more distal facies but pinch out cratonward. The phosphate-rich interval is gener-ally restricted to a relatively short stratigraphic interval in the upper Famennian part of the Woodford Shale or near the Devonian-Mississippian (Famennian-Tournaisian) boundary. Locally, the Wood-ford Shale has large early diagenetic carbonate concretions that con-tain abundant conodonts and radiolarians (McAlister Shale Pit).
Properties of the Woodford Shale that affect reservoir performance were documented including the relative percentage of chert, organic matter, phosphate, carbonate cement, silt and clay. Furthermore, close examination revealed evidence for cyclic deposition, alternating current energy, fluctuating levels of oxygen and biotic activity and changing bottom- and pore-water chemistry. Spectral gamma-ray derived values of U, Th and K were used to estimate marine vs terres-trial sediment input and rates of deposition. Normalized concentra-tions of metals Zn, Cu, U, V and Mo, degree of pyritization (DOP) and total organic carbon (TOC) measurements were determined and used as proxies to estimate water geochemistry and explain the high per-centages of organic matter (OM) preserved in the shale. In general, intervals containing evidence of euxinic bottom water during deposi-tion have increased Mo, TOC and DOP. In contrast, intervals with evidence of bottom water oxygenation such as inarticulate brachio-pods and burrowing have lower values of TOC, Mo and DOP and represent disruption of the water stratification that was prevalent during Woodford time.
AAPG Search and Discovery Article #90184 © AAPG Woodford Shale Forum, Oklahoma City, Oklahoma, April 11, 2013