UPPER ATOKA OUTCROP TO SUBSURFACE CORRELATION AND SEDIMENTATION HISTORY USING MAGNETIC SUSCEPTIBILITY

The Upper Atoka Formation is Pennsylvanian in age and is shale-rich with interbedded sandstone units. A significant portion of one shale-rich unit is exposed in Hackett, AR in the western Arkansas River Valley region of the state. The purpose of this study was to determine the sedimentation history of the exposure using magnetic susceptibility properties of the shale-rich strata, and link these findings to the regional stratigraphy. Samples were taken from the outcrop at fixed increments of 0.1 and 0.05 m and later processed and analyzed in the lab.

Magnetic susceptibility values correlate well with gamma-ray data from well-logs in the area. These findings will assist in future work linking outcrop and well-log data to solve correlation and structural problems for shale units in the Upper Atoka and throughout the Arkoma Basin, the sedimentary basin the formation comprises.

The magnetic susceptibility data also track Milankovitch-scale depositional cycles in the exposure. The detected cyclicities are eccentricity at 0.129 cycles/meter and 0.5 cycles/meter, and precession at 2.4 cycles/meter and 3.167 cycles/meter. These are the highest frequency sedimentation cycles found in the Atoka Formation.

Magnetic susceptibility is a vital tool in understanding and interpreting sedimentation history and stratigraphy when paired with numerous well-logs already present from Arkoma Basin oil and gas exploration.