Lacazette, Alfred¹, Charlotte Sullivan², Mike Ammerman³ 
(1) Consulting Geologist, Houston, TX 
(2) University of Houston, Houston, TX 
(3) Devon Energy Corporation, Oklahoma City, OK

ABSTRACT: These “Karst” Features in the Ellenburger Are Really Pull-Apart Basins

The Ellenburger carbonates are an important fractured reservoir or aquifer in Texas, New Mexico and Oklahoma. We used seismic attributes based on a new method of vector dip computation by volumetric coherent energy correlation to investigate changes in fault, joint, and lineament orientations in a 3-D seismic data set from the Fort Worth Basin and to determine the structural and temporal framework for the development of collapse systems that extend vertically through the Ordovician Ellenburger carbonates and a 2300 ft (700 m) thick Mississippian-Pennsylvanian siliciclastic sequence to the Pennsylvanian Caddo Limestone. The collapse features developed during deposition of the overlying section and have a rounded, sinkhole-like appearance in this growth section. Such features have been called karst-collapse features. We find that in this area they cannot be Ellenburger karst because: 1. They developed after deposition of the overlying shales. 2. They affect too great a vertical thickness of section, i.e. the thickness of the collapse section is too great relative to the thickness of the Ellenburger. 3. The entire Ellenburger section is involved in the collapse. Tracing the features downwards in the seismic data reveals that in the pre-growth section they are sharply rombohedral pull-apart basins developed at stepovers between oblique-slip extensional faults that extend into the deep basement. The rounded shape in the growth section results from the geometry of syntectonic deposition and because the sediments collapsed prior to lithification.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.