GEOLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF THE CHATTANOOGA SHALE IN WEST-CENTRAL ARKANSAS

The Chattanooga Shale is a Late Devonian euxinic marine unit characterized by thin beds (typically several tens of feet in thickness) in the west-central portion of Arkansas. The Chattanooga Shale was deposited as a deepening southward wedge in the study area, with well penetrations ranging from 6,435 ft to 14,030 ft. It is a potential shale-gas resource in this region of the Arkoma basin with very limited production to date. In order to facilitate the research project Chattanooga Shale well cuttings were collected for geochemical analysis at the Norman F. Williams Well Sample Library in Little Rock, Arkansas. The organic geochemical data generated from this study includes Total Organic Carbon content (TOC), Rock-Eval pyrolysis, vitrinite reflectance analysis, and kerogen type identification. Structure contour and isopach maps were developed from subsurface correlation of well logs. Vitrinite reflectance (Ro) values range from 1.51% at a depth of 7,705 ft to 5.05% at 14,695 ft, with a statistical mean value of 3.31%. Vitrinite reflectance values appear to increase southward which implies that thermal fluid flow from the Ouachita thrust belt was directed northward during the Ouachita orogeny. TOC values generated from this study range from a minimum of 0.25% to a maximum of 7.29%. The mean TOC value is 2.88%. The main kerogen type of the Chattanooga Shale is amorphous and herbaceous, which suggests an oil-prone marine organic matter origin with an influx of terrestrial supply. Rock-Eval pyrolysis in the Chattanooga Shale indicates that almost all of the samples have present-day values that fall within the dry gas window.