Precision Volumetric Calculations of Modern Karst Voids

Alexander Pachos¹, Ira D. Sasowsky¹, and Eric S. Wright^2
1Dept. of Geology & Env. Sci., University of Akron, Akron, OH
²Department of Mathematics, The University of Montana - Western, Dillon, MT

Modern karst systems are useful analogs for petroleum reservoirs, providing information on possible architecture, boundaries, and porosity values. In order to evaluate cave survey errors, we conducted a study of 4 epigene telogenetic karst conduits. In each conduit we established closely spaced stations for a tripod-mounted, rotating laser rangefinder. The resulting data set is dense both radially and longitudinally. Selective removal of measurements allowed comparisons of both area and volume based on data density to evaluate whether a point of diminishing returns existed. Data deletion caused area to diverge from the 32 radial point standard by as much as ±40%. The effects on calculated volume were compared when both radial point data and entire intra-transect stations were omitted from the data set. The crudest possible volume uses only the endpoint survey stations and 4 radial points. When this combination was applied to the 4 cave passages, the volumes were underestimated from -2% to -30% in 3 cases, and overestimated by +12% in one. When 32 radial points were used with the 2 end stations, values were underestimated in 3 cases (-4% to -40%) and overestimated in 1 (+1%). Using all stations but only 4 radial measurements resulted in 2 underestimates (-22% and -32%) and 2 overestimates (+3% and +4%). Girth and shape of the passages in the four surveys were relatively uniform. Overall results of this experiment suggest that where there is high variability of passage girth and shape, dense spatial data are required for accuracy similar to a more uniformly shaped passage. Surveys can be designed to achieve a required degree of accuracy. Traditional cave surveys can at least provide order-of-magnitude estimates of human scale karst porosity. Even if porosity is redistributed by burial and collapse, it provides opportunity for petroleum accumulation.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas