Abstract

Three-dimensional seismic volumes from the central Fort Worth Basin display roughly circular collapse chimneys that extend vertically about 800 m from the Ordovician Ellenburger Formation to the Atokan (middle Pennsylvanian) Caddo Limestone. Collapse chimneys in carbonates may be caused by subaerial karst, hydrothermal, or tectonic extensional processes. We use 3D multitrace geometric attributes including coherence, volumetric curvature, and energy-weighted, coherent amplitude gradients to investigate details of the origin of these structures. The Ordovician Ellenburger surface resembles a subaerial karst landscape of cockpits, dolines, and frying-pan valleys, while resistivity-based wireline image logs record 50 m of karst breccia facies. However, images from coherence and long-wavelength most-positive and most-negative-curvature volumes show many of the 800-m collapse features are associated with basement faults or with subtle Pennsylvanian and younger tectonic features, rather than with intra-Ellenburger collapse. We hypothesize that although the Ellenburger surface does contain a subaerial karst overprint, the first-order control on the formation of the vertically extensive collapse chimneys is bottoms-up tectonic-induced extensional collapse. Although these collapse chimneys have been affected by burial fluid diagenesis, the main consequence of burial fluid flow may be limited to the documented cementation of macrofractures. The apparent dominance of tectonic extension processes over subaerial karst and hydrothermal processes has basinwide implications for distribution of fractures, late-stage cements, and reservoir development and compartmentalization.

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