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Metrics of Karst: Quantitative Analyses of Caicos Platform Dolines and Implications for Improved Geologic Modeling of Diagenetic Features

Sean A. Guidry1, Aram N. Derewetzky2, and Franciszek J. Hasiuk3
1ExxonMobil Development Co., Spring, TX
2ExxonMobil Exploration Co., Houston, TX
3University of Michigan, Ann Arbor, MI

Geologic modeling of zones of early cavernous porosity development in carbonate reservoirs can be especially challenging beyond existing well control. As a result, dolines on Providenciales, Turks and Caicos Islands, were analyzed in order to evaluate the spatial density and metrics of these early diagenetic features, as well as their relationship to local fracture orientations. Dating of the adjacent host rock material indicates that these diagenetic features are well established in rocks deposited ~122,000 ybp (U/Th). In the dataset of 116 dolines, mean length is 5.16 ± 4.38m, width = 3.40 ± 2.72m, and area = 21.27 ± 32.57m2. Regression analyses of aspect ratios reveal a good, predictable relationship between doline length and width. Approximately 60% of the dolines are less than 10 m2 in area. Numerical modeling of observed dolines using “change of support” spatial statistics establishes a density of 1.13 dolines/acre; a testament to the abundance of small heterogeneities in carbonates that can potentially impact flow. Bivariant plots of area versus exceedance probability indicate three doline populations (0.1m2 to <4m2, 4m2 to 44m2, and >44m2), and calculations of associated fractal dimensions indicate some degree of scale invariance. Analyses of doline long axis azimuth reveal a Gaussian distribution with strong directional controls on doline development within the area of study, thereby establishing an early diagenetic anisotropy template. Early diagenetic alteration in these young carbonates is far more complicated than simple meteoric lenses superimposed on topographic highs. Based on these observations, early cavernous porosity development preserved in ancient carbonate successions is likely more widespread than would be resolved by traditional seismic methods, and the bulk of fluid flow may be controlled by numerous, small dissolution features.

 

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