Abstract

Late Devonian - Early Mississippian black shales are excellent oil source rocks throughout Oklahoma and much of western Arkansas. Black shales were deposited in shallow water shelf or epiric 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 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 aulocogen (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 Radiolaria, 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 predominately associated with the Late Paleozoic Arbuckle and Ouachita orogenesis.

d. Late silicification and mineralization along fractures contemporaneous with

e. Generation and expulsion of petroleum.

The principal expulsion mechanism for these Late Devonian-Early 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.