Stratigraphic Relationship Between Shelf and Deep-Water Reservoir Sandstone Units in the Middle Atoka Formation, Arkoma Basin, Arkansas

Jacqueline L. Myers and Doy L. Zachry
University of Arkansas, Fayetteville, AR

The Arkoma basin of north-central Arkansas evolved from a stable shelf to a rapidly subsiding foreland basin adjacent to the Ouachita orogenic belt during deposition of the Atoka Formation (Pennsylvanian). Subsidence occurred along east-trending normal faults that developed as the Ouachita orogen was obducted onto the margin of North America. The normal faults progressively developed from south to north; each defining a shelf margin as subsidence and concurrent sedimentation proceeded. The Tackett and Borum intervals within the middle Atoka Formation contain significant natural gas reservoirs. The units differ strikingly in terms of well log signatures and geographic distribution. Tackett intervals are composed of shale with sandstone units concentrated at the top. Log signatures of Tackett sand units commonly display thickening-upward patterns and correlation of the units is usually possible. Borum intervals occur south and basinward of the Tackett and are characterized by thick sandstone units that display blocky signatures. The sandstones grade laterally into shale and exist as thick, isolated packages. The Tackett sand is interpreted as a shelf deposit that accumulated in a variety of neritic environments with south-directed delivery systems playing a critical role. The Borum sands accumulated in off-shelf environments as discrete lobes fed by west-directed turbidity currents flowing along the base of a slope formed by normal fault systems.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005