A Case of Mistaken Identity-The Atoka Shale /Barnett Shale in the Central Midland Basin.

Clark Osterlund, Dr. Helge Alsleben, Dr. John Breyer, Dr. Nowell Donovan, Stonnie Pollack, and Dexter Harmon
Department of Geology, Texas Christian University, Ft. Worth Texas, and Fasken Oil and Ranch, Ltd, Midland, Texas

Fine-grained units underlying the Strawn Formation in the Central Midland Basin have historically been interpreted as Pennsylvanian (Atokan) or Mississippian (Chesterian) strata. Recent palynology data suggest that the shale was deposited during the late Mississippian (Osagean-Chesterian), making it equivalent to the Barnett Shale. The trend of the Barnett was studied in a 20-mile2 (~30 Km2.) area straddling the corners of Andrews, Ector, Martin, and Midland Counties in West Texas where, depths to the Barnett range from 9,900 feet (3,000 meters) to 11,500 feet (3,500 meters). In the study area, siliceous-calcareous shales dominate the Barnett shale; however, resistivity changes allow division of the shale into an upper and lower unit. The upper unit can be further subdivided into six subunits by distinct log curve markers, which are interpreted as gravity flow deposits consisting of silty bioclastic debris. Resistivity logs show an elevated response from the gravity flow deposits, while gamma ray and neutron density logs show a funnel shaped, upward coarsening electrofacies. Isopach maps of the flow deposits suggest a source to the north, coinciding with the Chesterian platform margin located in northern Andrews County. Preliminary TOC analyses, using the inverse relationship between the gamma ray and resistivity logs, suggest that the upper Barnett section is relatively TOC lean, whereas the lower Barnett has higher values. Exploration focus can be enhanced with the detailed mapping of flows for the upper Barnett, and mapping high resistive, calcareous units in the lower Barnett.

AAPG Search and Discovery Article #90152©2012 AAPG Southwest Section Meeting, Fort Worth, Texas, 19-22 May 2012