A Quantitative Paleo-Geomorphic Study of the Fluvio-Deltaic Reservoirs in the Atoka Interval, Fort worth Basin, Texas, U.S.A
The Atoka Group (Lower-Middle Pennsylvanian) of the Fort Worth Basin (FWB) forms a significant (~2-3 Tcf), and as yet underexploited, domestic gas resource that is often considered a secondary target for operators drilling the deeper Barnett Shale. Although thousands of wells penetrate the Atoka in the FWB, the origin and character of this unit are still debated. Current models for its deposition range from wave- to river-dominated, to fan deltas.
A 3-D survey covering 68 km2 of the FWB has been integrated with wireline logs from 226 wells and core from 3 wells for detailed analysis of the Atoka. Well log mapping reveals that the Atoka can be subdivided into 12 parasequences that stack to form: (a) a lower, regressive; (b) a middle, transgressive; and (c) an upper, highstand parasequence set. Seven facies are identified in core, and include channel fill, proximal delta front, delta-plain, fluvio-estuarine, distal delta front, prodelta, and shelf carbonate facies. They are tied to log signatures as a template for interpreting facies using log motifs across the study area. Limited resolution of channelized reservoir elements in seismic necessitated implementing a process for defining channel dimensions using point bar measurements from well logs. Quantitative analysis of channel dimensions in cross-section was undertaken and results compared to sparse morphometric data observed in seismic. Results indicate that channel widths vary from 34 to 456 m. Channel sinuosities range from 1.09 to 1.32. Calculations of flow characteristics and channel slopes suggest that slope changed over time decreasing from lower to upper Atoka as the basin filled. A review and comparison of modern and ancient analogs to Atoka sediments support the interpretation of a river-dominated delta system. On the basis of the lack of mixed marine/non-marine influence, lack of mixed grain sizes and distance from the highland sources, the Atoka is not believed to represent a succession of fan delta deposits. Gamma-ray-log motifs, calculated flow characteristics, and channel morphology suggest the Atoka to represent a simple river-dominated delta system.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009