Upper Devonian-Lower Mississippian black shales are excellent oil source rocks throughout Oklahoma and much of western Arkansas. Black shales were deposited in shallow-water shelf or epeiric sea environments in the north and deep basins in the south (i.e., Arbuckle province). Silty black shale is more common in the north whereas silicified black shale increases

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to the south. Overall low rates of clastic sedimentation and high planktonic organic productivity prevailed over the entire region. The small amounts of clastic silt and clay came from the north and northeast with local derivation from the Nemaha ridge and Ozark dome. Some silt was probably contributed from the northwest along the axis of the southern Oklahoma aulacogen (Anadarko basin). Primary carbonate deposition occurred locally on or near a distal southern platform between the Arbuckle and Ouachita provinces (Pauls Valley uplift).

Diagenesis in the Woodford-Chattanooga source rock section proceeded through the following relative time sequence: (a) silicification, chiefly by recrystallization of radiolarians, which probably followed the reaction conversion of amorphous opal-A to opal-CT to chert; (b) dolomitization of deep basin opal or chert and shallow-platform carbonate laminae; (c) tectonic faulting, folding, and associated fracturing and stylolitization predominantly associated with the late Paleozoic Arbuckle and Ouachita orogenies; (d) late silicification and mineralization along fractures contemporaneous with (e) generation and expulsion of petroleum.

The principal expulsion mechanism for these Upper Devonian-Lower Mississippian oil source rocks is "whole oil" migration through coarser grained matrix pores, stylolites, and fractures, rather than diffusion on a molecular scale. Diffusion migration does occur but appears only to affect internal migration over a few millimeters within the source rock, and thus cannot account for expulsion of large volumes of oil.

Preliminary calculations based on source rock extract data indicate that approximately 147 billion bbl of oil have been generated within Woodford shales in the 23,000 sq mi (598,000 sq km) geographic area of southern and western Oklahoma underlain by the Woodford formation.

Minimum relative oil expulsion efficiency appears to have been approximately 18 to 19% of the oil generated within the Woodford. Thus, at least 27 billion bbl of oil have been expelled into adjacent formations in southern and western Oklahoma while 120 billion bbl of oil remain in the source rock.

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