--> ABSTRACT: Hydrocarbons in an Overmature Basin: I, Thermal Maturity of Atoka and Hartshorne Formations, Arkoma Basin, Oklahoma and Arkansas, by Lori A. Hathon and David W. Houseknecht; #91039 (2010)

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Hydrocarbons in an Overmature Basin: I, Thermal Maturity of Atoka and Hartshorne Formations, Arkoma Basin, Oklahoma and Arkansas

Lori A. Hathon, David W. Houseknecht

With exploration expanding into deeper portions of the Arkoma basin, it has become important to determine whether the basin's thermal maturity is a factor that must be integrated into exploitation strategies.

Lateral thermal maturity patterns can be detailed by analysis of the Hartshorne coal bed. From west to east, the Hartshorne increases systematically from 0.7 to over 2.0% vitrinite reflectance (Ro). In the west, Ro contours cross the basin nearly perpendicular to structural strike. To the east, Ro contours are concave eastward, indicating higher thermal maturities in the center of the basin than along its northern and southern margins.

Vertical thermal maturity patterns can be locally detailed by analyzing dispersed organic matter concentrated from Atoka cores and cuttings. Depth versus Ro profiles display a constant slope through the basin, and absolute values of thermal maturity display a lateral variation similar to the Hartshorne coal bed. Mean Ro values in productive Atoka beds range from about 2 to nearly 5%, depending on depth and location within the basin.

The observed thermal maturity of the basin probably resulted from the combined effects of overpressured conditions, gravity-driven fluid flow related to the Arbuckle uplift, and facies-dependent dewatering pathways. Surprisingly, observed levels of thermal maturity indicate that much of the Atoka Formation is overmature, and would be considered nonprospective according to most published thermal maturity-hydrocarbon window relationships. Nevertheless, significant gas reserves exist in patterns that do not appear to be influenced by either lateral or vertical trends in thermal maturity.

AAPG Search and Discovery Article #91039©1987 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma, September 27-29, 1987.