Middle and Late Paleozoic Organic-Rich Gas Shales of the North American Midcontinent

Darwin R. Boardman, James Puckette, and Ibrahim Cemen
Geology, Oklahoma State Univerisy, Stillwater, OK

Black organic-rich gas producing shales are abundant in outcrop and subsurface in the North American Mid-Continent. The middle Paleozoic Woodford and Chattanooga black shales (Frasnian-Tournisian) 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. Faunal elements consist of pelagic forms including radiolarians, conodonts, ammonoids, and fish debris and are largely devoid of any benthic organisms.

Midcontinent Late Mississippian black shales (Visean-Serpukhovian) include the Barnett Shale of Texas, Caney Shale of central and southern Oklahoma, and Fayetteville Shale of Arkansas and northeast Oklahoma. These shales contain 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 but, whereas the benthic faunas of low diversity include acrotretids and Leiorhynchoidea brachiopods, bivalves (Caneyella), and gastropods (archaeogastropods). These suggest that times of anoxia and dysoxia alternated possibly related to water masses changes related to the onset of Gondwana glaciation.

Late Carboniferous (Moscoivan-Gzhelian) and Lower Permian (Asselian) black fissile organic-rich phosphatic shales are numerous (>30) in the Midcontinent and are typified by being thin (1-2 meters) and associated with maximum marine flooding during cyclothemic sedimentation. These black shales are overlain and underlain by gray dysoxic shales.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas