A Quantitative Paleogeomorphic Study of the Fluvio-Deltaic Reservoirs in the Atoka Interval, Fort Worth Basin, Texas, U.S.A.

Abstract

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 of its deposition range from wave- to riverdominated to fan deltas. A 3D survey covering 68 km 2 (26 mi 2 ) 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. These facies are tied to their characteristic well log signatures, and that link is used as a template for interpreting facies across the study area solely using of log motifs. Seismic attribute slices were examined but allowed limited mapping of channel morphologies because of limited resolution of the data. These data limitations necessitated implementing a process of defining channel dimensions using point-bar measurements from well logs. Quantitative analysis of channel dimensions in cross section was undertaken and results compared with sparse morphometric data observed in seismic. Results of empirical analysis indicate that channel widths vary from 34 to 456 m (112-1496 ft). Limited observations in seismic data show that channel sinuosities range from 1.09 to 1.32. Calculations of flow characteristics and channel slopes suggest that slopes decreased from early to late Atoka time as the basin filled. The Atoka sediments lack indications of an apparent mixed marine/nonmarine influence and mixed grain sizes. Because of its overly long distance from the highland sources compared with that of other fan-delta settings, the Atoka is not thought to represent a succession of fan-delta deposits. A review and comparison of modern and ancient analogs to Atoka sediments support the interpretation of a river-dominated delta system. This interpretation is corroborated by gamma-ray log motifs, calculated flow characteristics, and channel morphology.