Hans-G. Machel, 1990. "Bulk Solution Disequilibrium in Aqueous Fluids as Exemplified by Diagenetic Carbonates", Prediction of Reservoir Quality Through Chemical Modeling, Indu D. Meshri, Peter J. Ortoleva
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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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Prediction of reservoir quality ahead of the drill is one of the most complex problems facing exploration geologists, especially when they are exploring in frontier basins, where rock and water data are minimal or non existent. Although useful descriptive models of diagenesis have existed in the past, they cannot be applied in the areas where rock and water data do not exist. This volume comes out of a 1987 conference oand contains 10 chapters that document the substantial progress made toward the goal of modeling reservoir quality. One facet of chemical modeling, namely porosity prediction, is the thrust of this book. However, chemical modeling has contributed heavily in the field of environmental geochemistry, nuclear waste disposal, and in the thermal recovery of heavy oil and the like, thus one such chapter is included in this memoir.