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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. HarrisAuthor:
Harris Cander Carbonate Reservoirs
Published 1995 as
part of Memoir 63
Copyright © 1995 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Chapter 5
*
Interplay of
Water-Rock Interaction Efficiency, Unconformities, and Fluid Flow
in a Carbonate Aquifer:
Floridan Aquifer SystemHarris 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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