Application of New Seismic Attributes to Mega karst in the Fort Worth Basin

SULLIVAN, CHARLOTTE and KURT MARFURT Allied Geophysical Laboratories, University of Houston, Houston, Texas, AL LACAZETTE, Naturalfractures.com, and MIKE AMMERMAN, Devon Energy, Oklahoma City, OK

3-D seismic volumes of the central Fort Worth Basin display fractures and collapse features that extend vertically some 800 m from the Ordovician Ellenburger Formation to the middle Pennsylvanian Caddo Limestone. These features have been attributed to collapse of subaerial mega karst features in the Ellenburger. We have used dip derivative and coherence seismic attributes to investigate the origin of these features. Although time slices through the Ellenburger contain features that can be interpreted as cockpit karst, dolines and frying pan valleys, the alignment of many sinkhole features in the Ordovician Ellenburger coincide with deep basement faults and Pennsylvanian and younger lineaments.

In addition, the length of the vertical collapse features is excessive for simple Ellenburger cavern collapse, and horizon slices on the Pennsylvanian Caddo limestone lack exposure features of significant magnitude to produce 800+m of top -down karst. By incorporating the timing constraints provided by better imaging of subtle lineaments in the seismic volume, we hypothesize that subaerial karst was less important than “bottoms up” tectonic-induced collapse on the formation of the vertical chimneys. We speculate that although the vertical collapse chimneys have likely been affected by burial fluid diagenesis, the main consequence of burial fluid flow may be the cementation of macrofractures. The presence of tectonically controlled karst and collapse, coupled with burial fluid flow has basin- wide implications for distribution of fractures, calcite cement, and reservoir compartmentalization.