Evaluation of Volume Estimates for Modern Hypogene Karst Voids
Modern karst voids may provide insight on porosity magnitude and distribution within carbonate reservoirs. Numerous surveys of modern caves have been published, and could be used in a statistical approach for such characterization. The accuracy of such maps is uncertain, though, with regard to volume calculations. A previous study (Pachos 2008) addressed this question for caves formed in telogenetic rocks and found that standard surveying methods give volume values that are within +/- 22% of the most accurate ones. The present study evaluates hypogene caves formed in eogenetic rocks. These "Flank Margin" caves may be more representative of petroleum reservoirs.
Four separate passages on San Salvador, Bahamas were surveyed using a high-density radial surveying technique. Using the entire data set, cross-sectional area and total passage volume were computed and then compared to values computed with reduced data sets. Stations at ~1-2 m spacing effectively divided the passage into slabs; summing the volume of all slabs provides total passage volume. A total of 2,688 measurements were collected.
Data sets of 32 points are the most accurate. The depleted data sets reveal highly inaccurate area and volume estimations. Using 4 and 8-point radial data sets, volume errors as high as +/- 400% were present. The experiment also considered alternative shapes to model cross sectional area from measured values. Using a rectangle, quadrilateral, and ellipse, cross sectional area at each station was calculated. This was more accurate than the initial technique in calculating cross sectional area and total volume.
The experimental results can be used to characterize or predict karst mega porosity. In a situation where data are limited, the results of this study provide upper and lower bounds for errors associated with characterizing channel-like voids. Based on this study, large overestimations may be present using depleted data. Depending on the goal of the work, over or underestimations may be justified, but understanding the nature of these errors can be important in successfully exploring the subsurface.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California