Browsing by Subject "Sedimentation and deposition--Texas--Fort Worth Basin"
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Item Depositional element morphology and architecture of the Atoka interval, Fort Worth Basin, TX, U.S.A.(2008-12) Maharaj, Vishal Timal; Wood, Lesli J.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 seen as a secondary target as operators make their way to the deeper Barnett Shale. Although thousands of wells penetrate the Atoka in the FWB, the origin and character of this unit are still debated. The influence of deeper Ellenburger karsting on depositional morphology and element character has also been insufficiently addressed. Current depositional model interpretations vary from wave-dominated, to river-dominated, to fan deltas, comprising various braided, meandering, and mixed-load stream systems. An understanding of the facies in the Atoka and their spatial distribution can be enhanced by utilizing the most effective tool in describing each level of resolution (seismic/low, well log/medium, core/high) and producing methods of correlation that accurately define the geometries that exist. A 3-D survey covering 68 km² of the FWB has been integrated with data from 226 wells and core from 3 wells for detailed mapping of the Atoka. In seismic, the Atoka is represented by a ~200-ms interval showing geometric relationships of lapout (onlap, offlap, toplap, downlap, and truncation), which suggest the existence of imageable geomorphic features. Both seismic and well log mapping show 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. Core analysis reveals that eight main facies exist in the Atoka, which suggest the interval comprises a series of coarse-grained, fluvio-deltaic and shallow marine deposits. Criteria are outlined for defining channel dimensions using point bar measurements from well logs, and a detailed quantitative analysis of the variability of these dimensions in cross-section and plan-view was undertaken. Point-bar analyses indicate that channel widths range from 34 to 456 m, given the variability in both local and regional accommodation space created during deposition. A review and comparison of modern and ancient analogs to Atoka sediments support the interpretation of a river-dominated delta system. The argument is reinforced by observations from facies components identified in core, gamma-ray-log expressions, and the distant location of the study relative to highland sources. Previous fan- and wave-dominated delta interpretations are therefore discounted and do not apply to the study area