57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Modern carbonate platforms are characterized by heterogeneously distributed secondary permeability and porosity, which ranges from small (mm to cm) vugs to humanly traversable macropores. Understanding the processes that control formation of secondary porosity and permeability may improve predictive models of porosity and permeability distributions in subsurface carbonate reservoirs, depending on their preservation during burial. Secondary porosity and permeability form through dissolution reactions that depend on flow of water undersaturated with respect to carbonate minerals. Flow is critical to formation of the macropores because it maintains contact between undersaturated water and carbonate rocks and removes reaction products. The link between flow and dissolution complicates modeling of distributions of secondary porosity.

Flow of undersaturated water must be concentrated within discrete areas of carbonate platforms to form macropores. While processes that concentrate flow to form macropores are well-constrained in continental settings, less is known about how flow is concentrated to form macropores in modern carbonate platforms. In continental settings, flow is concentrated where adjacent low permeability siliciclastic rocks deliver runoff, which commonly is undersaturated, to the upstream ends of conduit-dominated flow systems. Runoff from large rain events can increase heads in conduits faster than groundwater heads increase, resulting in exchange of water as gradients become oriented from conduits to the matrix. Modern carbonate platforms, however, typically lack impermeable siliciclastic catchments. Consequently, meteoric precipitation diffusely recharges the aquifer, limiting head gradients that could drive exchange and associated dissolution of macropores. This diffuse recharge rapidly equilibrates with aquifer rocks minimizing the ability of recharge to drive macropore formation.

One process that could drive exchange of undersaturated water in modern carbonate platforms capable of macropore formation is tidal pumping. If tides change heads in high permeability regions of aquifers faster than low permeability regions of aquifers, resulting head gradients would drive exchange of water between these two regions. To determine the impact of tides on exchange, we measured water elevations at high temporal resolution in the ocean, blue holes (the local name for sinkholes) and wells on San Salvador Island and Rum Cay, Bahamas (Martin et al., 2011; 2012a). The lag and dampened amplitude of the tidal variations provide information about the diffusivity values (transmissivity/storativity) at the wells. Diffusivity was found to average around 1.3 x 10⁶ m²/day for the wells and around 76.9 x 10⁶ m²/day at blue holes, assuming dampening results only from head loss. These diffusivity values were used to estimate hydraulic conductivity that averages around 6 x 10⁴ m/day for the wells and 2 x 10⁷ for the blue holes based on an assumed aquifer thickness of 10 m and a storativity of 0.3. We assume low values obtained from wells represent hydraulic conductivity of matrix permeability and elevated values from blue holes represent flow that occurs primarily in conduits. These differences in permeability cause hydraulic heads in the aquifer to lag heads in the conduits and blue holes through a tidal cycle. The resulting differences in heads result in gradients between conduit and matrix porosity that vary semi-diurnally and drive exchange of water between blue holes and the matrix porosity at tidal frequencies. Assuming observed tidal dampening is caused by exchange, we estimate the average exchange for one blue hole on San Salvador Island to be about 0.9 m³ of water per half tidal cycle, or about 1% of the complete tidal change in volume of water in the blue hole. Depending on the water composition, tidally driven exchange could thus cause dissolution within the aquifer matrix.

For dissolution to occur, water must be undersaturated with respect to calcite. Water in blue holes commonly has elevated organic carbon (OC) concentrations at the pycnocline as a result of deposition of terrestrial organic carbon and primary productivity within blue hole water columns. When this OC is oxidized to CO₂, carbonic acid forms and causes undersaturation with respect to calcite. To determine how organic carbon oxidation affects calcite saturation state, we assessed variations in water compositions at tidal, diel and seasonal frequencies at Ink Well Blue Hole on San Salvador Island a total of six times in October 2010 following the rainy season, and in May 2012 following the dry season (Martin et al., 2012b). In Ink Well Blue Hole, carbon isotope values of the dissolved inorganic carbon pass through a minimum at intermediate values of SpC reflecting localized mineralization of OC at the pycnocline regardless of tidal stage, time of day, or season. In contrast, saturation indices (SI) of the water do not display minimum values at intermediate SpC values and instead are inversely linear with SpC, with values decreasing from around +0.4 to -0.2 in water with low to high SpC, respectively. Similarly, Ca concentrations in excess of those expected from simple mixing between water with low and high SpC are also inversely linear with SpC and elevated by 2 to 3 mM in the low SpC water, reflecting dissolution in the fresh water above the pycnocline. Differences in indicators of dissolution (excess Ca and SI) are most pronounced at seasonal timescales, indicating that sufficient mixing occurs within blue hole water columns and into the aquifer to disperse chemical variations that occur at tidal and diel frequencies. This dispersion could distribute the potential for dissolution that is generated through OC mineralization in the water column of blue holes throughout the aquifer matrix.

This grant has supported field collection opportunities and laboratory analyses for two of my PhD students, Amy Brown and John Ezell, who are working on cycling of metals in carbonate systems and on carbonate dissolution rates, respectively. John has also been supported as a Graduate Research Associate on the grant. My work has been impacted through submission of a major research proposal to the NSF, in collaboration with Jason Gulley (lead PI), who is a recent PhD graduate and has been heavily involved in this research. The NSF proposal, which is based in part on the results of this study, is designed to investigate how inland lakes in the Bahamian Archipelago vary in size and salinity with sea level rise.