ABSTRACT

The Late Devonian Chattanooga Formation is a potential hydrocarbon source rock on both the Ozark Uplift and in the adjacent Arkoma Basin. Outcrop and subsurface samples were obtained along a transect that approximates the Arkansas-Oklahoma state line. The organic-rich black shale has a mean total organic carbon (TOC) content of 3.2% on the northern Arkansas structural platform. At depths less than 350 ft. (115m), it is thermally "mature" with a vitrinite reflectance of 0.7% (Ro). In the basin at depths greather than 5000 ft. (1640 m), the shale is "post mature" with vitrinite reflectance values greater than 3.0% (Ro). The highest mean random vitrinite reflectance value that was observed in this study was 5.65% (Ro) from the maximum depth of the transect at 14,690 ft. (4820m).

A linear relationship of depth vs % Ro better predicts the maturity transition from the platform to the basin than the typical exponential relationship of depth vs % Ro. Separate depth vs % Ro plots for the platform and the basin indicate a more rapid increase of % Ro with depth in the basin than on the shelf. Increasing geothermal gradients toward the south are responsible for the linear best fit relationship and the different slopes of the reflectance gradients on the platform and in the basin.

Present depths of burial are inadequate to obtain high maturities in the late stage oil window on the platform and the late stage oil and condensate window to the dry gas window and beyond in the basin with the current geothermal gradients. It is suggested that the temperatures and depths that established these maturities were attained during the Ouachita Orogeny. Although the generation of hydrocarbons has now ceased, some hydrocarbons may have accumulated during this time.