From Bureau of Economic Geology, The University of Texas at Austin (
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Bureau Seminar, April 13, 2007

Reservoir-Scale Depositional Facies, Trends, and Controls on Sandstone Distribution of the Lower Atoka Group ("Bend Conglomerate"), Fort Worth Basin, Texas

Tucker Hentz

Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin.


Study of the largely siliciclastic lower Atoka Group established detailed reservoir-facies distribution of four chronostratigraphic units in a 900-mi2 area comprising all of Wise and adjacent easternmost Jack Counties (Boonsville field). Net-sandstone maps (~1-mi well spacing) representing the basal, medial, and upper parts of the lower Atoka, supplemented by core descriptions, reveal that (1) the unit comprises depositional facies that are far more heterogeneous than previously described, (2) syndepositional regional faulting was a significant control on sandstone distribution, and (3) primary source areas and related principal sediment-transport pathways vary systematically with age within the lower Atoka. These elements form a predictive model for locating primary sandstone fairways and for generally assessing the unitís additional infill and stepout potential. Such an effort is timely because of the aggressive development of the deeper Barnett Shale play in most of the Fort Worth Basin.

The lower Atoka records the initial phase of major post-Mississippian siliciclastic influx into the Fort Worth Basin and has long been described as consisting of fan-delta deposits. However, sandstone distribution and core data indicate that bed-load and mixed-load braided-river, braid-plain/braid-delta, prodeltaic, estuarine, and peat-swamp deposits characterize the unitís progradational-retrogradational cycles. Regional, NE-oriented faults controlled macroscale (tens of miles) incised-valley and fluvial- and distributary-channel orientations, elongate intrasystem paleohighs, and pronounced sand accumulations. Primarily SE-NW sandstone trends characterize the lower part of the unit, reflecting sourcing from the Ouachita Foldbelt (=40 mi distant), whereas the NE-SW trends in the upper part identify a shift to a primary source from the nearby Muenster Arch.