Sequence Stratigraphy and Organic Richness of the Lower Pennsylvanian Atoka Series, Northwest Anadarko Basin, Texas and Oklahoma Panhandle
Production from Atoka limestones and, less commonly, sandstones in the mature northwest Anadarko Basin has occurred since the late 1940’s (cumulative production 185 Bcfg and 6.7 MMBO in Ochiltree and Lipscomb Counties, Texas). Operators have recently been drilling horizontally and applying hydraulic-fracture stimulations in the thin (6-27 ft, average 11 ft) carbonate Atoka reservoirs to tap adjacent organic-rich shales. To aid active operators in this emerging hybrid shale-carbonate play, we analyzed the Atoka Series centered on Ochiltree and Lipscomb Counties, Texas, and Ellis County, Oklahoma. Our assessment of cores, wireline logs, and production data included evaluation of sequence stratigraphy, depositional facies, and shale organic richness.
The Atoka Series, averaging 390 ft in thickness in the study area, overlies a second-order unconformity at the top of the Morrow Formation. The lower Atoka “Thirteen Finger Lime,” composed of fissile, organic-rich shale interbedded with fossiliferous wackestones, represents a third-order transgressive systems tract. The thickest, most organic-rich shale bed in the Atoka, occurring at the top of the Thirteen Finger, is inferred to represent a third-order marine condensed section. Using a petrophysical model to estimate total organic carbon (TOC) in the shales on the basis of the Exxon “delta log R” method, our preliminary results from 28 wells indicate that average TOC in Atoka shale lies in the 4-6% range. A third-order highstand systems tract downlaps the Thirteen Finger condensed section and contains several progradational parasequences that are capped by limestone and dolomite “conventional” Atoka reservoirs. If horizontal drilling/fracture stimulation is able to successfully tap large hydrocarbon volumes in the organic-rich Atoka shales, the new play should open up a broad, continuous drilling fairway and quickly eclipse past production volumes.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009