Abstract: The Paradox of Porosity and Fluid-Rock Interaction in Carbonate Platforms

Harris Cander, Jay L. Banner

As carbonate sequences undergo diagenesis, mineralogic stabilization, and compaction, both porosity and fluid flux generally decrease. Intuitively, evolved carbonate systems should more easily buffer the trace element and isotopic compositions of the progressively smaller volumes of fluid they transmit. As well, late-stage diagenetic phases should have compositions that reflect this host-rock buffering. However, examples from the Paleozoic, Tertiary, and Quaternary with differing degrees of mineralogic maturity, reveal that as carbonate systems evolve and lose porosity, the ability of the host rock to buffer pore fluid compositions decreases.

Eocene pore fluids from the upper Floridan aquifer system (⁸⁷Sr/⁸⁶Sr = 0.7082-0.7085; Sr = 0.1-1 ppm; ^dgr¹³C = -14 to 0 ^pmil PDB) are in isotopic and elemental disequilibrium with host marine limestone and dolomite (avg: ⁸⁷Sr/⁸⁶Sr = 0.70775-0.7078; Sr = 275 ppm; ^dgr¹³C = +2). Late stage calcite cements that line cavities are in equilibrium with the pore fluids, indicating precipitation at very high fluid/rock ratios.

Saline groundwaters from Mississippian carbonate rocks of central Missouri have ⁸⁷Sr/⁸⁶Sr compositions (0.7105-0.7155) that are more radiogenic than host rocks (avg. 0.7075-0.7090), indicating limited fluid-rock interaction. Nd isotopic disequilibrium between the pore fluids and carbonate rocks further documents that the groundwaters have preserved extraformational compositions during transmission through the aquifer. As in the Floridan aquifer, late carbonate cements are not in equilibrium with the bulk rock. While most diagenetic phases have marine rare earth element (REE) signatures, only late carbonate cements bear non-marine signatures. This is consistent with the extremely high fluid/rock ratios required to alter REE signatures.

In a less mature system, Pleistocene reef limestones of Barbados, some groundwaters are not in equilibrium with the host rock (water: ⁸⁷Sr/⁸⁶Sr = 0.70905; limestone = 0.70915). Thus, isotopic disequilibrium is maintained even in a high-porosity aquifer that still contains aragonite.

These different systems reveal a paradox that the ability of a carbonate aquifer to modify its pore fluids can decrease as porosity decreases. This paradox may result from the development of systems of focused flow that transfer permeability from the matrix to conduits and limit interaction between pore fluids and the bulk rock. Three factors can work in concert with focusing of flow to inhibit host rock interaction: (1) attainment of carbonate saturation in fluids before or soon after entry of into the aquifer; (2) lining of conduits with late diagenetic phases of different isotopic composition from the host rock, such that successive fluids acquire the signature of late diagenetic minerals rather than the bulk rock; (3) flow rates in conduits may be too high to permit reactions to p ogress. As diagenesis proceeds and porosity decreases, the host rock exerts progressively less control on its pore fluids and mineral precipitation occurs at higher fluid/rock ratios. In carbonate platform aquifers, the type of flow network is more important than total porosity or fluid flux in determining the potential for fluid-rock interaction.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994