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Abstract


 
Chapter from: M 63:  Unconformities and Porosity in Carbonate Strata 
Edited By
D.A. Budd, A.H. Saller, P.M. Harris

Author:
Harris Cander

Carbonate Reservoirs

Published 1995 as part of Memoir 63
Copyright © 1995 The American Association of Petroleum Geologists.   All Rights Reserved.

 
Chapter 5

*
Interplay of Water-Rock Interaction Efficiency, Unconformities, and Fluid Flow
in a Carbonate Aquifer: Floridan Aquifer System

Harris Cander
Amoco Production Company
Houston, Texas, U.S.A.


*
ABSTRACT

The middle Eocene Avon Park Formation comprises shallow subtidal skeletal limestones and dolomitized peritidal limestones that underwent several periods of unconformity-related exposure during the Cenozoic. The porous limestones are typical of the Tertiary Floridan aquifer system, which has both high interparticle, matrix porosity and a conduit flow system comprising karst zones, caves, vugs, channels, and bedding planes. Avon Park limestones have retained most primary porosity (F = 20 to 30%) and Eocene marine-like geochemical compositions, despite being exposed to flushing by meteoric groundwater during these long-lived unconformities. The marine-like geochemical compositions indicate low water/rock ratios during mineralogical stabilization to calcite. The most common diagenetic product in the limestones is isopachous bladed calcite cement that precipitated during intraformational unconformities or immediately after deposition. The limestones are in oxygen, carbon, and strontium isotopic disequilibrium with modern Floridan aquifer groundwater (limestone: d18O = -1.0 to +1.0, PDB; d13C = 0 to +2.0; 87Sr/86Sr = 0.70777 ; Sr = 400 ppm; dilute groundwater: d18O = -0.5, SMOW; d13C = -5 to -14; 87Sr/86Sr = 0.7081 to 0.7089). Based on geochemical modeling, quantitative estimates of the number of pore volumes that have reacted with Avon Park limestone compared to the number of pore volumes that have flowed through the rocks indicate that the long-term efficiency of water-rock interaction is less than 0.002%.

In contrast to the matrix limestone, late-stage, conduit-lining coarse calcite cements in the Avon Park are in isotopic and elemental equilibrium with modern Floridan aquifer groundwater, indicating precipitation at extremely high water/rock ratios and interaction efficiency (late calcite: d18O = -3.3, PDB; d13C = -7.0; 87Sr/86Sr = 0.70875; Sr = 15-20 ppm). The radiogenic 87Sr/86Sr composition of these calcite cements indicates that they contain Sr of Middle Miocene age or younger. The contrasting data from the matrix limestones and the conduit-lining cements indicate that the two fluid-flow

 

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