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The Sevier-Knox/Trenton Total Petroleum System (Hypothetical)

 

Milici, Robert C. and Ryder, Robert T.

U.S. Geological Survey, Reston, VA 20192

       

Black Ordovician shales, up to 7000 feet thick (Athens and Sevier Shales in Tennessee, and the Paperville Shale and Liberty Hall Formation in Virginia), occur along the eastern side of the Valley and Ridge Province.  The Sevier and Paperville Shales constitute the largest mass of these potential source rocks and are generally over mature with respect to hydrocarbon generation (Sevier CAI ~ 3-4; Paperville CAI ~ 2.5-4).  TOC analyses of Sevier and Paperville outcrop samples (n = 8) range from 0.04% to 1.08% (average = 0.37%). 

Oil and gas are produced from Ordovician strata in Alleghanian structural traps in the western part of the Valley and Ridge, from the Swan Creek field in Tennessee, and from the Rose Hill and Ben Hur fields in Virginia.  Furthermore, hydrocarbon occurrences have been documented along the top-of-Knox unconformity within the nearby Mascot-Jefferson City zinc district in Tennessee, and within biohermal buildups along the Middle Ordovician basin margin in Virginia.  It has been postulated that the source of both the zinc ores and associated hydrocarbons in eastern Tennessee was the Sevier Shale, and that the source of the hydrocarbons in the Ordovician buildups in Virginia is the Paperville Shale.  Given the large volume of these shales and the documented potential for early generation (Devonian?), it is possible that considerable amounts of hydrocarbons were generated from these strata and have migrated long distances westward to where they are preserved in Lower Paleozoic stratigraphic traps and in large extensional structures in the deep, relatively undeformed Plateau region of the Appalachian basin.