Utility of Modern Cave Systems in Interpreting Paleokarst Reservoirs

Ira D. Sasowsky¹, Charles T. Feazel^2
1University of Akron
²ConocoPhillips

Modern cave systems are useful analogs for interpreting, predicting, and simulating paleokarst reservoirs. Interpretation of log character, seismic signature, and map patterns to reveal the distribution of paleocave systems requires a fundamental understanding of the processes and products of the karst environment. Prediction of the location, spatial distribution, or internal geometry of paleokarst reservoirs at the basin, prospect, or reservoir scale involves the ability to reconstruct paleohydrology within realistic stratigraphic and structural bounds. Simulation of paleocave reservoirs can be used to help make significant investment decisions in well design, well spacing, and injection strategies.

Modern karst systems are complex, but not random. Most originate by flow of meteoric water from a recharge area to a discharge area. Others develop by tidal pumping, or from deep acidic sources. The porosity pattern that evolves is a function of structure (including material properties, hydrologic boundaries, and fractures), process (physical & chemical), and time. After burial of the terrane, modification occurs by collapse, compaction, and mineralization. The resulting paleokarst reservoir retains the geometry inherited from its initial phase (or phases). 

Models useful in exploration and production may be obtained by: 1) examining the geologic history of the reservoir – including sea level curves, as appropriate, to identify times of meteoric or mixing-zone exposure, 2) identifying appropriate modern analogs, 3) quantifying porosity distribution by empirical means in the modern analog, 4) developing a process-based model for the analog, and 5) creating an appropriate reservoir conceptual model accounting for post-burial events. Three-dimensional digital models of modern caves show promise for reservoir simulation. The interactive visualizations show a wide range of architecture. Thousands of surveys of these modern systems exist, but they may be unavailable for use or difficult to access. The format of such data is also non-standard, which can lead to problems with manipulation. Another complication involves the variable level of detail in the data: a survey may generate accurate representations of narrow cave passages, but might include only a few azimuth and distance shots across large cave rooms, rather than detailed profiles of the chamber walls. 

Calculated karst porosities for 10 systems that we evaluated range from 0.04 to 3.51% of the total rock volume enclosing the surveyed caves. Permeability enhancement in post-collapse paleocave systems is harder to quantify than porosity. Cave breccia can be simulated as an open-frame conglomerate with moderate to high permeability, but initial cave-floor sediment, speleothems, and both pre-burial and post-collapse cements all increase tortuosity, add flow barriers or baffles, and introduce numerous complexities into flow modeling.