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3-D Seismic Evidence of the Effects of Carbonate Karst Collapse on Overlying Clastic Stratigraphy and Reservoir Compartmentalization

B. A. Hardage
Bureau of Economic Geology, University of Texas at Austin, Austin, TX

A study was done across a portion of Boonsville gas field in the Fort Worth Basin of North-Central Texas to determine how geophysical, geological and engineering techniques can be combined to understand the mechanisms by which fluvio-deltaic depositional processes create reservoir compartmentalization in a low- to moderateaccommodation basin. An important geologic influence on stratigraphy and reservoir compartmentalization that was discovered in this basin was the existence of numerous karst collapse chimneys over the area covered by 3-D seismic data. These cylindrical, near-vertical karst collapses originated in, or near, the deep Ordovician-age Ellenburger carbonate section and created vertical chimneys extending as high as 2,500 ft (760 m) above their point of origin, causing significant disruptions in overlying clastic strata.

Seismic imaging shows that these karst features affect strata above the Ellenburger and that they created a reservoir compartment in the Upper Caddo, an upper Atoka valley-fill sandstone that occurs 2,000 ft (610 m) above the Ellenburger. By correlating 3-D seismic images with outcrops of Ellenburger karst collapses, the physical dimensions (height, diameter, cross-sectional area) of the seismic disruptions observed in the 3-D data can be equated to karst dimensions seen in outcrops. This Ellenburger carbonate dissolution phenomenon appears to extend over at least 500 mi (800 km). By inference, similar karst models may occur in any basin that has a deep, relatively thick section of Paleozoic carbonates that underlie major unconformities.

AAPG Search and Discovery Article #90905©2001 AAPG Southwest Section Meeting, Dallas, Texas