Stratigraphic Geometry of Cavern Collapse and Seismic Modeling - A Predictive Tool for Exploration

Robertson C. Handford

The exploration importance of carbonate paleokarst/cave reservoirs is underscored by large fields, such as Puckett and Brown-Bassett fields in west Texas, Wilburton Field in Oklahoma, and Casablanca Field of offshore Spain. In light of these, there has been some important facies analysis work done comparing modern cave facies with paleocave reservoirs, but there has not been any effort to determine how cave geometries may be imaged by seismic reflection data.

In an effort to fill this gap, modern caves have been examined in order to construct models illustrating the stratigraphic geometry of cavern collapse. As cave chambers in bedded limestones are excavated and enlarged by vadose and phreatic waters, shear stress and tension loads on unsupported ceiling beds increase resulting in ceiling collapse and the deposition of ceiling-derived breakdown. This leads to interbedded breakdown, fluvial clastics, and speleothem deposits. Subsequently, an arch-shaped ceiling geometry forms in an effort to attain a more stable configuration. If the cave chamber is not destroyed from the surface by fluvial entrenchment and karst denudation, it passes into the subsurface during subsidence. However, with increasing burial, stress loads increase and cause to al collapse of cave chambers. Thus, chambers tend to be filled with early breakdown and fluvial deposits followed by burial-related breakdown. Continued loading leads to readjustment and refracturing of the breakdown and the propagation of fractures from the central chamber outward into the country rock. This forms a halo of crackled and mosaic breccia surrounding a chaotically brecciated zone of breakdown and cave-fluvial sediment. The geometry of the buried and collapsed cave chamber depends upon the size of the excavated cave chamber, depth of burial, and the bulk physical properties of the encasing limestone.

Seismic modeling was conducted in order to determine if the cavern collapse geometry could be resolved seismically in a subsurface setting. A geologic model, consisting of layers of differing densities and P-wave velocities, was constructed and convolved with a zero-phase Ricker wavelet at various frequencies. Assuming preservation of significant breccia porosity within the collapsed chamber, the models show a passage from continuous reflections in the undisturbed country rock to discontinuous reflections inclined towards the cavern core. Reflection character in the collapsed cave chamber was variable depending upon model parameters. Pullups and pulldowns are significant where velocity and density contrasts between the country rock and collapsed chamber are important. Similar reflecti n character may be visible in high-resolution 2-d and 3-d data and could help delineate paleocave reservoirs.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995