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

John B. Comer
Indiana Geological Survey, Bloomington, IN, [email protected]

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 due to 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 densitystratified water columns, which 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.

AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009