ORIGIN, SOURCE-ROCK, AND RESERVOIR CHARACTERISTICS OF NOVACULITIC CHERT (DEVONIAN-MISSISSIPPIAN) IN THE SOUTHERN MIDCONTINENT

Novaculitic chert in the Woodford Shale and Arkansas Novaculite of Oklahoma is associated with prolific unconventional mudrock reservoirs, yet limited analysis of this chert as a petroleum source rock and reservoir rock has been performed. Outcrop, core, thin section and SEM/EDS analysis was used to characterize and interpret the chert. Abundant siliceous microfossils, phosphatic nodules, and organic matter (up to 6% TOC) indicate that the chert formed in a region of marine upwelling and high organic productivity associated with an oxygen minimum zone. On the Oklahoma Shelf, novaculitic chert was deposited atop a major Cambrian-Devonian carbonate bank. In the Ouachita Embayment, by contrast, similar chert was deposited in an oceanic setting above Cambrian-Silurian turbidite fan deposits of the continental rise. Paleogeographic analysis indicates that the paleoclimatic setup for upwelling and oxygen minimum zone formation differs significantly from modern examples. Novaculitic chert within the Woodford Shale is an important petroleum system component not only because of brittleness, but also because of organic content and hydrocarbon storage potential. Brittleness contributes to hydrofracture propagation, and natural fractures contribute to hydrocarbon storage and producibility. Rock matrix is composed largely of peloidal silica with interstitial amorphinite. Organic-walled microfossils and remnants of microbial films also are common. SEM analysis indicates that the silica-organic matrix is nanoporous and stores hydrocarbons mainly by sorption.