Woodford Shale in Southern Midcontinent, USA - Transgressive System Tract Marine Source Rocks on an Arid Passive Continental Margin with Persistent Oceanic Upwelling

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2008-04

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Abstract

Woodford Shale (Givetian to Kinderhookian) is a prolific hydrocarbon source rock in the southern USA Midcontinent and is locally an unconventional oil and gas reservoir. Woodford sediments were deposited in epeiric seas as anaerobic and dysaerobic biofacies recording widespread bottom-water anoxia and strongly density-stratified water columns. High concentrations of marine organic matter coexist with abundant biogenic silica, indicating that high biological productivity in surface waters was supported by dynamic upwelling. Hypersalinity, recorded as anhydrite in burrows and syneresis cracks, suggests an arid paleoclimate and indicates that density stratification was due in part to accumulation of hypersaline bottom water. Plate-tectonic reconstructions consistent with an arid paleoclimate and dynamic upwelling place this region on the western passive continental margin of North America in the dry tropics near 15 degrees south latitude. Here, southeasterly trade winds and Ekman circulation force surface water westward toward the open ocean and countercurrents with upwelled oceanic water eastward onto the craton. The strong net flow of ocean water into the epeiric seas developed because of the high rate of evaporation during a period of eustatic sea level rise. Transgressive system tracts along west-facing, arid passive continental margins produce marine source rocks because of the steady influx of upwelled oceanic nutrients, which support high biologic productivity, strongly density-stratified water columns that inhibit oxygen re-supply and promote organic matter preservation, and absence of significant rainfall, which precludes large river discharge and minimizes influx of terrestrially derived clastic sediments.

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This poster was presented at the 2008 Annual Convention and Exhibition of the American Association of Petroleum Geologists (AAPG) in San Antonio, Texas, from April 20-23, 2008. John B. Comer won an AAPG Award of Excellence "Top 10" Poster Presentation for the poster.

Keywords

Woodford, Woodford Shale, Devonian, Devonian shale, Devonian black shale, gas shale, Oklahoma, Texas, Arkansas, New Mexico, Southern midcontinent, source rock, petroleum source rock, petrology, shale petrology, sedimentation, shale sedimentation, depositional environment, black shale, unconventional reservoir, shale gas reservoir, fractured reservoir, shale, sedimentary rocks, organic geochemistry, thermal maturity, organic carbon, paleogeography, depositional setting, paleoceanography, paleoclimate, depositional processes, paleocirculation, continental margin, epeiric sea, cratonic basin, organic carbon, production potential, upwelling, marine transgression, sea level rise, eustasy, hypersalinity, silica diagenesis, Radiolaria, Tasmanites, sedimentary structures, laminae, organic productivity, storm sedimentation

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