Sasowsky, Ira D.1
(1) University of Akron, Akron, OH
ABSTRACT: Detailed Analogs for Paleokarst Reservoirs: Promise and Problems
Modern karst systems are complex, but not random. Understanding the genesis of these systems leads to more accurate prediction of paleokarst porosity patterns and results in improved reservoir models.
Most modern karst conduits transmit meteoric water from a recharge area to a discharge area. Others develop by tidal pumping or from deep acidic sources. The initial porosity pattern that results is a function of structure (including material properties, hydrologic boundaries, and fractures), process (physical & chemical), and time. After burial of the karst, modification by processes such as collapse, compaction, and mineralization occurs. The resulting paleokarst reservoir retains the overall pattern inherited from its initial phase (or phases).
The parameters of recharge type and dominant initial porosity allow prediction of porosity development (general analogs) on a regional scale (1,000-10,000 meters). It is more difficult to develop precise analogs, which, for example, might identify drilling targets on a scale of 10-100 meters. However, studies of present karst systems are reaching the stage where this sort of prediction is feasible. For example, in the Appalachian Plateaus porosity distribution is asymmetric and controlled by structure and relief.
Precision models for exploration targets may be obtained by: 1) examining the geologic history of the reservoir, 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 models of present-day karst porosity can be useful in visualizing potential targets.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.