Characterizing the Three-Dimensional Architecture of a Coalesced, Collapsed-Paleocave System in the Lower Ordovician, Ellenburger Group by Integrating Ground-Penetrating Radar, Shallow-Core, and Outcrop Data

Robert G. Loucks, Ph.D.

A three-dimensional study of a coalesced, collapsed-paleocave system near Marble Falls, Texas, was completed by integrating a closely spaced grid of ground-penetrating radar (GPR) data (11.2 km of line) and 29 15-m cores. An adjacent 1,000-m long quarry face was also mapped in detail.

The data from the GPR lines were characterized in the context of the continuity, regularity, and strength of the reflections. Three classes of GPR reflections are apparent: (1) strong, continuous, undisturbed reflections imaging the undisturbed strata/undisturbed host, (2) relatively continuous reflections up to tens of meters long, characterized by faults and folds, imaging the disturbed strata/disturbed-host, and (3) chaotic reflections having little to no perceptible continuity imaging cave related breccias. Integration of data from the cores, GPR survey, and outcrop displays the three-dimensional architecture of the paleocave system. Large-scale facies and structural patterns are readily apparent from the GPR data, and rock textures and pore types are provided by the core and outcrop data.

The study volume was divided into three 3-m-depth slices to map out the geometry and areal extent of the collapsed-paleocave facies. Each slice displays a series of paleocave breccias trending to the northeast. The breccias are separated by disturbed- and undisturbed-host rock. The breccia bodies that outline the trend of former passages are as much as 345-m wide. The intervening area between the breccias is up to 200-m wide. The best reservoir quality is within the collapsed paleocave trends, and consists of chaotic and crackle breccia porosity.