Abstract

The Pennsylvanian Lower Atoka Formation along US Highway 5, northwest of Jacksonville, Arkansas, has been described as part of an ancient submarine fan complex. Three facies are identified from this outcrop: lowstand and transgressive tract deposits in a channelized mid-fan to outer-fan setting, channel levee or overbank deposits, and highstand or avulsionary shales. This paper deals with the complexities within the numerous shale facies found at this locality and attempts to understand the subtle differences that exist between them and how they change laterally in a morphological and structural sense. A variety of changes occur in the shales, and it is important to understand how these changes affect the clay mineralogy, seal stability, and overall reservoir architecture, and how these deposits should be interpreted and classified.

We classified these shales into specific categories by using laser particle size analysis and a variety of other petrographic techniques as they may relate to fluid flow. We employed analytical techniques such as X-ray diffraction, X-ray radiography, laser particle size analysis, scanning electron microscopy, total organic carbon, mercury injection capillary pressure, thin section petrography, and previous studies to ensure a comprehensive evaluation of these shale deposits.