Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Houston, Texas, April 9–12, 2006
Depositional Facies, Reservoir Distribution, and Infield Potential of the Lower Atoka Group (Bend Conglomerate) in Boonsville Field, Fort Worth Basin, Texas: New Look at an Old Play
A pilot study assessing additional infield potential of the lower Atoka Group established a framework of 11 reservoir-scale chronostratigraphic units in a 165-mi2 area of southwestern Wise County and adjacent easternmost Jack County to determine details of depositional settings and reservoir-facies distribution. Units are bounded by mostly fourth-order flooding surfaces within prodeltaic shales and delta-plain coals. Although the lower Atoka has long been characterized as consisting of fan-delta deposits, net-sandstone maps (1-mi well spacing), supplemented by core descriptions, indicate that depositional facies are more heterogeneous. Bed-load and mixed-load braided river, braid plain/braid delta (river-dominated), interdeltaic, estuarine, and peat-swamp deposits characterize the progradational-retrogradational cycles composing the lower Atoka. Reservoir units represent primarily fluvial-channel-fill and proximal-delta-front facies. Their regional depositional trends shift from SE-NW to NE-SW midway in the succession, recording, respectively, a change in primary source area from the distant Ouachita Foldbelt to the closer Muenster Arch.
Hundreds of wells with a spacing density of ~80 acres penetrate the lower Atoka in the area, but completions commonly appear to be nonoptimal with respect to reservoir-sandstone trends. Many wells appear to be candidates for recompletion, assuming that original frac jobs were too small to provide necessary vertical connections. Log analysis shows that potential recompletion intervals contain hydrocarbons and have sufficient reservoir quality, suggesting bypassed pay. A simple porosity cutoff of ~10% traditionally determines completion choices. However, core data indicate that these lower Atoka sandstones are productive with porosities as low as 6%. Drainage area varies from tens of acres to ~100 acres. These factors suggest that both recompletion and infill-drilling possibilities exist.