Dolomitization models

Lafon, G. M., G. A. Otten and A. M. Bishop, 1992, Experimental determination of the calcite-dolomite equilibrium below 200 °C revised stabilities for dolomite and magnesite support near-equilibrium dolomitization models. Geological Society of America Abstracts with Programs 24, A210-A211. 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Badiozamani K. (1973) The Dorag dolomitization model-application to the Middle Ordovician of Wisconsin. J. Sediment. Petrol. 43, 965-984. 

Carpenter A.B. (1976) Dorag dolomitization model by K. Badiozamani-A discussion. J. Sediment. Petrol. 46, 258-261. 

Morrow D.W. (1982b) Diagenesis 2. Dolomite-Part 2. Dolomitization models and ancient dolostones. Geoscience Canada 9,95-107. 

Tucker, M.E. (1990) Dolomites and dolomitization models. In Carbonate Sedimentology (Eds Tucker, M.E. Wright, V.P.), pp. 365-400. Blackwell Scientific Publications, Oxford. 

Plates of glass, PTFE, dolomite and marble were used. Dolomite and marble were chosen because they represent minerals found in oil reservoirs. Glass and PTFE were investigated because they represent high and low surface energy solids respectively and are good model systems for data comparisons. Liquid/solid wetting cycles were obtained for each of the solids in the liquids listed in Table III. 

Since the deposit contains halite and anhydrite, the brines should be saturated with respect to these minerals and hence provide a good test of the activity models. Table 8.8 shows analyses of brine samples from the deposit. Note that the reported pH values are almost certainly incorrect because pH electrodes do not respond accurately in concentrated solutions. Hence, there is little to be gained by calculating dolomite saturation. 

In this chapter, we develop a mass balance model of the fractionation in reacting systems of the stable isotopes of hydrogen, carbon, oxygen, and sulfur. We then demonstrate application of the model by simulating the isotopic effects of the dolomitization reaction of calcite. 

The mixing calculation is interesting in that it demonstrates a common ion effect by which dolomite precipitation drives feldspar alteration. In the model, dolomite forms because the saline water is rich in Ca++ and Mg++, whereas the fresh water... 

In fact, the choice of CO2 fugacity has little effect on the mineralogical results of the mixing calculation. In the model, the critical property of the Fountain fluid is that it is undersaturated with respect to calcite, so that calcite dissolves when the fluid mixes into the Lyons. Because we assume equilibrium with dolomite and magnesite, the saturation index (log Q/K) of calcite is fixed by the reaction... 

We can use reaction modeling techniques to test the conditions under which dolomite will react with hydrochloric acid to produce gas in the injection zones. 

To configure a model of the reaction of dolomite with 30 wt.% hydrochloric acid, we start REACT and enter the commands... 

Fig. 30.1. Volumes of minerals precipitated during a reaction model simulating the mixing at reservoir temperature of seawater into formation fluids from the Miller, Forties, and Amethyst oil fields in the North Sea. The reservoir temperatures and compositions of the formation fluids are given in Table 30.1. The initial extent of the system in each case is 1 kg of solvent water. Not shown for the Amethyst results are small volumes of strontianite, barite, and dolomite that form during mixing.

Mattes, B. W. and E. W. Mountjoy, 1980, Burial dolomitization of the Upper Devonian Miette Buildup, Jasper National Park, Alberta. In D.H. Zenger, J. B. Dunham and R. L. Effington (eds.), Concepts and Models of Dolomitization. SEPM Special Publication 28, 259-297. 

This model for the system CaC03 MgC03 applies only for ideal ordering of Mg and Ca ions in the dolomite structure. Ideal ordering occurs only in precipitates of dolomite formed at temperatures above about 250°C. Studies in the laboratory (52) show that dolomitization (the development of ordering in the Mg and Ca distribution in the calcite structure) is a very slow process at ordinary temperatures. Therefore, a solid-state chemical model more applicable to precipitated dolomites is ... 

Land LS (1980) The isotopic and trace element geochemistry of dolomite the state of the art. In Concepts and models of dolomitization. Soc Econ Paleontol Min Spec Publ 28 87-110 Lane GA, Dole M (1956) Eractionation of oxygen isotopes during respiration. Science 123 574-576... 

Figure 7.18. Comparison of experimental and predicted phase equilibria in the system CaC03-MgC03 using CVM in the tetrahedron approximation for a trigonally distorted f.c.c. Ising lattice. Semi-quantitative agreement is achieved for the calcite-dolomite segment but the Mg-rich side of the diagram indicates the need to include a more complex model (Burton and Kikuchi 1984b).

The results of various attempts to explain the provenance of the mature Whittle waters in terms of the above precursor waters and mineral phases are outlined below. Assuming that Whittle recharge water is the sole precursor, several attempts were made to model the evolution of the mature Whittle water by reaction of the Whittle recharge waters with various minerals. Where the mineral suite included calcite, dolomite, gypsum, pyrite and K-jarosite, but not ankerite, the resulting models were of poor credibility they invoke coupled precipitation of calcite (although dissolution is far more likely) and substantial dissolution of... 

To a good first approximation, the Great Lakes fit a model involving the equilibrium of calcite, dolomite, apatite, kao-Unite, gibbsite, Na- and K-feldspars at 5°C., 1 atm. total pressure with air of PCo2 = 3.5 X 10" atm. and water. Dynamic models, considering carbon dioxide pressure and temperature as variables (but gross concentrations fixed), show that cold waters contain excess carbon dioxide and are unsaturated with respect to calcite, dolomite, and apatite, whereas warm waters are nearly at equilibrium with the atmosphere but somewhat supersaturated with respect to calcite, dolomite, and apatite. 

We may now examine specific information from chemical analyses of the Great Lakes (7, 8, 13) to determine to what degree the variations of the proposed model fit the actual data. Rather than consider all of the variables at once, it is simpler to consider smaller portions to get a better idea of what actually is happening. We shall look at calcium carbonate equilibria, dolomite equilibria, phosphate equilibria, and silicate equilibria. 

For the calculations, averages of the results of the two 5. -equilibrium models of Ca2+ = 35 p.p.m., Mg2+ = 7 p.p.m., and alkalinity = 1.55 X 10 3 equiv./liter are used. Solubility data of Larson and Buswell (11), carbon dioxide solubility data of Hamed and Davies (2), and the carbonate ionization data of Hamed and Hammer (3) and Hamed and Scholes (4) are used. Linear interpolations are made for dolomite between pK(soly) = 16.3(5°C.) and 17.0(25°C.). Equations outlining the calcite and dolomite calculations are ... 

A classic example of metastability is surface-seawater supersaturation with respect to calcite and other carbonate minerals (Morse and Mackenzie 1990 Millero and Sohl 1992). The degree of calcite supersaturation in surface seawater varies from 2.8- to 6.5-fold between 0 and 25 °C (Morse and Mackenzie 1990). In Fig. 3.18, experimental calcite solubility (metastable state) is approaching model calcite solubility (stable state) at subzero temperatures. In Table 5.1, the difference in seawater pH, assuring saturation or allowing supersaturation with respect to calcite, is 0.38 units. Moreover, in running these calculations, it was necessary to remove magnesite and dolomite from the minerals database (Table 3.1) because the latter minerals are more stable than calcite in seawater. But calcite is clearly the form that precipitates... 

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