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Calculating Regional Diffusivity of Bahamian Carbonates by Analysis of Tidal Oscillations in Wells, Boreholes and Karst Features

Abstract

Isolated carbonate platforms are accreted, eroded and overprinted through alternating cycles of sea-level high and low stands, creating packages of carbonate rocks with highly variable primary depositional porosity and permeability. Depositional porosity and permeability are modified across multiple scales through dissolution and precipitation processes. Although controls on matrix-dominated porosity and permeability are well established, understanding how non-matrix properties impact flow in aquifers and reservoirs remains challenging due to biases associated with sample size. For example, porosity and permeability measurements are commonly measured at the core and thin section scale, even though fluid flow in carbonate rocks occurs across multiple scales ranging from sub-mm to km. Consequently, capturing effective properties associated with large scale non-matrix pore types, including touching vugs, caves, and conduits, becomes critical in understanding regional scale connectivity. Our study evaluates regional-scale diffusivity values from San Salvador Island, Bahamas, to evaluate how matrix and non-matrix connected pore types impact flow across an isolated carbonate buildup. Values were calculated from tidally-driven water level oscillations in boreholes, wells, blue holes and caves that ranged from 2.06x104 m2/day in public supply water wells completed in Holocene rocks to 2.3x108 m2/day in recently cored boreholes drilled into Pleistocene rocks deposited before MIS-5e (~125 ka), and from 1.56x107 m2/day to 2.33x108 m2/day in blue holes and flank margin caves. Using subsets of these data, we investigate the relationship between age, pore types, pore system distribution, and degree of pore connectivity. Our results indicate an efficiently connected system with nearly equivalent tidal response in deep boreholes, blue holes, and in cave pools. The spatial distribution of our sampling locations has lead us to conclude that this system is laterally extensive, of variable pore volume, and has become well connected due to dissolution related to the freshwater lens position. This would imply that non-matrix horizons can be predicted in the reservoir setting in part by understanding the number and duration of freshwater lens emplacements.