Seismic Geomorphology and Architecture of the Atoka Interval Channels, Fort Worth Basin, Texas, U.S.A

Vishal Maharaj and Lesli Wood
Bureau of Economic Geology, Jackson School of Geosciences University of Texas at Austin, Austin, TX

The lower Atoka Group (Lower to Middle Pennsylvanian) of the Fort Worth Basin (FWB) forms a significant (~2 to 3 Tcf), and as yet underexploited, domestic gas resource. Although thousands of wells penetrate the Atoka, the origin and character of this unit are still being debated. Historical models include wave- and river-dominated and fan deltas, as well as various braided, meandering, and mixed-load stream systems. The actual influence of deeper Ellenburger karsting and syndepositional faults on depositional system orientations and depositional element character is widely debated as well.

A 3-D survey covering the 26-mi² area of the FWB has been integrated withextensive subsurface logs and core from five wells through the Atoka for detailed mapping of the interval. In seismic, the Atoka is represented by a ~200-msec interval showing geometrical relationships of lapout (onlap, offlap, toplap, downlap, and truncation) and the existence of imageable geomorphologic features. Five key seismic surfaces were mapped to provide a framework within which to image spatial morphology of the Atoka’s depositional elements. Seismic slices show channeling at numerous levels, with occurrence increasing deeper in the section. In seismic, channel forms appear moderately sinuous (1.09 to 1.32), with orientation varying temporally from north-south and southwest-northeast to east-west and back to southwest-northeast. Their widths range from 67 to 583 m. Wells show that the stacking patterns and log motif of the Atoka elements and whole core provide a lamination level of detail on the environments of deposition. Cored channels show a thickness of ~5 m. All of these parameters can be used to calculate flow properties.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas