LATE DEVONIAN-EARLY PERMIAN ORGANIC-RICH GAS SHALES OF THE NORTH AMERICAN MIDCONTINENT

Dark gray-black organic-rich gas producing shales are abundant in outcrop and subsurface in the North American Mid-Continent. Late Devonian-earliest Mississippian Woodford and Chattanooga black shales consist of fissile black shales in shelfal settings, abundant chert and black shale interbeds in distal shelf and slope settings, and novaculite with black shale interbeds in basinal settings. No benthic fauna is present. Faunal elements consist only of pelagic forms including radiolarians, conodonts, ammonoids, and fish debris.

Midcontinent Late Mississippian black shales include the Barnett Shale of Texas, Caney Shale of central and southern Oklahoma, and Fayetteville Shale of Arkansas and northeast Oklahoma. These shales differ from those of the Woodford and Chattanooga in lacking significant chert beds or novaculite but are similar in containing black fissile organic-rich shale and localized phosphate. Ammonoid-bearing diagenetic carbonate concretions (bullion) typify these black shales. Faunal elements include both pelagic and benthic components. The pelagic components include radiolarians, conodonts, ammonoids, and fish. Bethic faunas are localized and consist of low diversity including acrotretids and Leiorhynchoidea brachiopods, bivalves (Caneyella), and gastropods (archaeogastropods). The presence of localized benthic faunal elements suggest dysoxic conditions alternated with anoxia.

Late Carboniferous (Moscovian-Gzhelian) and Lower Permian (Asselian) black fissile organic-rich phosphatic shales are numerous (>30) in the Midcontinent, and are thin (1-2 meters) and are typically underlain and overlain by gray dysoxic shales. Abundant benthic faunal elements are present in the gray shales.