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Bulk Solution Disequilibrium in Aqueous Fluids as Exemplified by Diagenetic Carbonates

By
Hans-G. Machel
Hans-G. Machel
Department of Geology University of Alberta EdmontonAlberta, Canada
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Published:
January 01, 1990

Abstract

Recrystallization, and in some cases precipitation, of minerals in aqueous fluids may occur in thermodynamic disequilibrium with the bulk solution. This compromises applications of the concept of cement stratigraphy and several geochemical models such as those commonly used to identify former subaerial exposure surfaces in carbonate rocks. Bulk solution disequilibrium may be a common phenomenon during recrystallization of carbonates as well as other minerals in aqueous fluids, i.e., in diagenetic, hydrothermal, and metamorphic environments.

Bulk solution disequilibrium generally occurs when dissolution and concomitant reprecipitation take place in thin surface-adsorbed or surface-bonded fluid layers that have compositions different from the solution occupying the bulk of the pore space. The elemental and isotopic compositions of the recrystallized minerals are determined by the compositions of these surface fluid layers, as well as by recrystallization rates. Thereby, paragenetically coeval grains, crystals, or crystal growth zones, may obtain heterogeneous trace element compositions, as exemplified by diagenetic carbonates from a Devonian reef in Western Canada. Modeling of recrystallization in bulk solution disequilibrium yields compositions that do not permit unambiguous interpretation of diagenetic environments such as meteoric, brackish, or marine. These conclusions follow from application of chemical microprofiles of crystal surfaces, diffusion models for dissolving and precipitating crystals, and calculations of trace element distributions.

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Contents

AAPG Memoir

Prediction of Reservoir Quality Through Chemical Modeling

Indu D. Meshri
Indu D. Meshri
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Peter J. Ortoleva
Peter J. Ortoleva
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American Association of Petroleum Geologists
Volume
49
ISBN electronic:
9781629811239
Publication date:
January 01, 1990

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