Ronald K. Stoessell

Stoichiometric saturation tests of NaCl1−xBrx and KCl1−xBrx

Stoichiometric saturation is examined as a possible control on Br contents of halite and sylvite during precipitation from binary salt solutions of NaCl-NaBr and KCl-KBr, respectively. Experimental data at 25°C, assumed to represent Stoichiometric saturation, were used to predict mole fractions of NaBr in halite and KBr in sylvite in thermodynamic equilibrium with fluids as a function of aqueous activity ratios of Br−:Cl−. The predictions are based on the additional assumption that the aqueous activity product of the major salt component in the precipitated salt was independent of the trace Br content in the salt lattice. The extension of the predictions to diagenetic pressures and temperatures is discussed. The predicted equilibrium Br content of halite at initial halite saturation of evaporating seawater is in close agreement with that computed from the distribution coefficient of Lutz (1975), measured in slowgrowth single crystal experiments. Fluid recrystallization of halite and sylvite at near-surface temperatures is predicted to generally deplete the Br contents in the solids. Bulk Br contents in halite in cap-rock of Gulf Coast salt domes generally agree with those predicted by the recrystallization of halite in the presence of evaporative-concentrated seawater. At a constant solution composition, increasing temperature results in increasing the equilibrium Br content of halite, making less efficient Br depletion in halite by recrystallization.

Chemical Constraints and Origins of Four Groups of Gulf Coast Reservoir Fluids

Pitzers method for computing activity coefficients has been applied to reported fluid compositions in four groups of Gulf Coast reservoirs: the Mississippi and Arkansas Smackover Formation, the Texas Edwards Group, The Texas Frio Formation, and the Louisiana Miocene sequence. The activity quotients in fluids from carbonate reservoirs imply a reaction relation between calcite and disordered dolomite, in which ordering increases at higher temperatures. This relationship indicates that an increasing Mg:Ca molality ratio with decreasing temperature does not necessarily imply dolomite dissolution by an updip-moving fluid. Instead, it may reflect an approach to equilibrium between calcite and metastable dolomite. Activity quotients in fluids from clastic reservoirs imply a rea tion relation between chlorite, illite, potassium feldspar, and quartz at temperatures above 100°C (212°F). Present fluid compositions in the Smackover and the Edwards appear to be related to the updip movement of Louann brines from the Gulf Coast basin, accompanied by dolomitization. In particular, bromide plots for the Edwards fluids indicate simple dilution of Louann brines with interstitial fluids. Similar plots for fluids from the Smackover show the complicating effects of illite formation, sylvite dissolution, and halite recrystallization. Albitization may have occurred during fluid movement through clastics prior to entering the Smackover and Edwards. Present fluid compositions in the Frio and Miocene are apparently not related to Louann brines. The compositional data reflect the effects of halite dissolution, the subsequent albitization of plagioclase and potassium feldspar, and the dissolution of carbonates during secondary porosity formation.

Secondary porosity revisited; the chemistry of feldspar dissolution by carboxylic acids and anions

Carboxylic acids in subsurface waters have been proposed as agents for dissolving feldspars and complexing aluminum to create secondary porosity in sandstones. Previously published experimental work indicated high aluminum mobility in the presence of carboxylic acid solutions. In order to further evaluate aluminum mobility, alkali feldspar dissolution experiments were run at 100 degrees C and 300 bars in the presence of mono- and dicarboxylic acids and their anions. Experimental results imply that under reservoir conditions, aluminum-organic anion complexes are insignificant for acetate and propionate and possibly significant for oxalate and malonate. Propionate appeared to inhibit alkali feldspar dissolution and, hence, may retard aluminum mobility. Dissolution of feldsp r in the presence of oxalic and acetic acid can be explained by enhanced dissolution kinetics and greater aluminum mobility under low-pH conditions. The general absence of such low-pH fluids in subsurface reservoirs makes this an unlikely mechanism for creating secondary porosity. Also, the thermal instability of oxalate and malonate limits their aluminum-complexing potential in reservoirs at temperatures above 100 degrees C.

Effects of sulfate reduction on CaCO 3 dissolution and precipitation in mixing-zone fluids

ABSTRACT Sulfate reduction in the sea-water end member of mixing-zone fluids changes the fluid-saturation indices for carbonate minerals. Mixing-zone fluids were modeled quantitatively using Atlantic sea water and a fresh-water end member from Barbados for sulfate reduction in the presence of goethite, in the absence of goethite, and with back-oxidation of aqueous sulfide to sulfate. Minor amounts of sulfate reduction enhance dissolution of CaCO3 in brackish to saline mixing-zone fluids, whereas major amounts of sulfate reduction enhance precipitation in all mixing-zone fluid compositions. Secondary reactions produce either CaCO3 dissolution or precipitation, which depend in magnitude on the amount of sulfate reduction. Dissolution results from the back-oxidation of aqueous sulfide to sulfate, and precipitation occurs from the transformation of iron oxides to iron sulfides. pH buffering, due to sulfate reduction, prevents a linear correlation of calcite-saturation indices with the moles of calcite necessary to dissolve or precipitate to reach equilibrium. Without iron sulfide precipitation or the back-oxidation of aqueous sulfide, the moles of calcite that can be dissolved continue to increase up to about 25% sulfate reduction, even though the degree of undersaturation begins to decrease with greater than 5% sulfate reduction. The effects of iron sulfide precipitation and the back-oxidation of aqueous sulfide are greatly enhanced by increasing the amount of sulfate reduction, being generally more than 0.1 g of calcite per kg of water dissolved or precipitated, respectively, at 50% sulfate reduction.

Dedolomitization in NaCaCl brines from 100° to 200°C at 300 bars

Dedolomitization experiments were run at 300 bars for 7 to 8 weeks at 100°, 150° and 200°C in chloride brines containing 2 m (molal) Na, 0.5 m Ca, and (except for 1 experiment) 0.02 m Sr. The “ordered” dolomite dissolved and low-Mg calcite back precipitated as a replacement and/or rim cement of equant spar calcite crystals 10 to 50 μm in diameter. Between 100° and 200°C the mode of calcite formation changed from direct precipitation into void space produced by dissolving dolomite to pseudomorphic replacement of dolomite. The dedolomitization textures formed in these experiments are similar to those found in the rock record. Solution compositions were monitored as a function of experimental time and imply that calcite-dolomite equilibrium was approached at 100°C and reached at 150° and 200°C. The final aqueous molality ratios of Ca:Mg were 14.2, 23.5, and 35.4, respectively, at 100°, 150°, and 200°C. The respective activity ratios, computed using Pitzers model, were 14.3, 23.6, and 37.0, representing estimated equilibrium constants for the reaction: Mg2+ + 2CaCO3 = CaMg(CO3)2 + Ca2+. The estimated standard state enthalpy of reaction is 3.6 kcal between 150° and 200°C. The empirical Sr distribution coefficient, (SrCa)solid(SrCa)liquid, for the precipitated calcite was determined from micro-probe analyses of the calcite and solution data. The average coefficient values increased with increasing temperature, being 0.027 (±0.008 = s.d. of 19 measurements), 0.048 (±0.017 = s.d. of 19 measurements), and 0.063 (±0.013 = s.d. of 32 measurements), respectively, at 100°, 150°, and 200°C.

Regular solution site-mixing model for chlorites

Activity expressions are presented for a six end-member, regular solution, site-mixing model for chlorites. The end members are ideal 14-Å chlorites for which experimental stability data are lacking. Estimates were made of the standard state 25°C and 1 bar molar 3rd law entropies, volumes, and the Maier-Kelley heat capacity coefficients. Experimental stability data from the literature for 14-Å chlorites were used with different sets of exchange energies, for cations on adjacent sites, to compute estimates of the standard state chemical potentials of the end members at 25°C and 1 bar. More experimental data are needed for an adequate definition of the exchange energies and the standard state chemical potentials. The model was applied to diagenesis in clastic reservoirs. Aqueous activity ratios of Mg2+:Fe2+ were computed as an equilibrium function of the corresponding molar ratios in authigenic chlorites. The aqueous activity ratio was independent of the chlorite Al content at a constant molar ratio of Mg2+:Fe2+ in the chlorite. The model predicts a wide range of molar Mg2+:Fe2+ ratios in authigenic chlorites in equilibrium with reservoir fluids. Trends in these molar ratios should be independent of the Al content in the chlorites. The model can be applied directly to 7-Å chlorites when experimental data become available to estimate the various thermodynamic parameters of the 7-Å end members.РезюмеПредставлены выражения для активности для модели хлоритов, основанной на шести конечных членах и регулярном растворе. Конечные члены являются идеальными 14 Å хлоритами, для которых экспериментальные данные по стабильности не имеются. Оценивались молярные энтропие (3 закон), объемы и коэффициенты теплоемкости Майера-Келлея для стандартного состояния 25°С и 1 бара. Экспериментальные данные по стабильности, имеющиеся в литературе для 14 Å хлоритов, использовались с различными энергиями обмена соседних катионов для расчета химических потенциалов конечных членов при стандартном состоянии 25°С и 1 бара. Однако для соответствующего определения энергий обмена и химических потенциалов при стандартном состоянии необходимы дополнительные экспериментальные данные. Эта модель применялась для диагенеза в кластических резервуарах. Рассчитывались отношения водных активностей Mg2+:Fe2+ как функции равновесия соответствующих молярных отношений в аутигенных хлоритах. При постоянном молярном отношении Mg2+:Fe2+ в хлорите отношение водных активностей не зависило от содержания Al в этом минерале. Эта модель предсказывает широкий диапазон значений молярных отношений Mg2+:Fe2+ в аутигенных хлоритах в равновесии с жидкостями резервуара. Тенденции изменений этих молярных отношений не должны зависеть от содержания Al в хлорите. Модель может непоердственно применяться к 7 Å хлоритам, если имеются экспериментальные данные для определения различных термодинамических параметров 7 Å конечных членов. [E.G.]ResümeeAktivitätsausdrücke werden für ein sechs Endglieder enthaltendes, reguläres Lösungs-Platz-mischungsmodell für Chlorite angegeben. Die Endglieder sind ideale 14-Å Chlorite, für die es keine experimentellen Stabilitätsdaten gibt. Es wurden Schätzungen für den Standardzustand der molaren (3. Hauptsatz-)Entropie bei 25°C und 1 Bar, das Volumen, und die Maier-Kelley Wärmekapazitätskoeffi-zienten gemacht. Experimentelle Stabilitätsdaten aus der Literatur für 14-Å Chlorite wurden zusammen mit verschiedenen Austauschenergien für Kationen auf benachbarten Plätzen verwendet, um Schätzungen für die Standardzustände der chemischen Potentiale der Endglieder bei 25°C und 1 Bar zu berechnen. Es werden mehr experimentelle Daten für eine angemessene Definition der Austauschenergien und der Standardzustände der chemischen Potentiale benötigt. Das Modell wurde auf die Diagenese in klastischen Bereichen angewandt. Das Aktivitätsverhältnis in wässriger Lösung war bei einem konstanten molaren Mg2+:Fe2+-Verhältnis im Chlorit unabhängig vom Al-Gehalt des Chlorites. Das Modell sagt einen großen Bereich von molaren Mg2+:Fe2+-Verhältnissen in authigenen Chloriten voraus, die im Gleichgewicht mit den Vorratslösungen sind. Trends in diesen molaren Verhältnissen sollten unabhängig vom Al-Gehalt der Chlorite sein. Das Modell kann direkt auf 7-Å Chlorite angewendet werden, wenn experimentelle Daten zur Vergügung stehen, um die verschiedenen thermodynamischen Parameter der 7-Å Endglieder abzuschätzen. [U. W.]RésuméOn présente des expressions d’activité pour un modèle à 6 membres terminaux, à solution regulière, et à sites mélangés pour des chlorites. Les membres terminaux sont des chlorites idéales de 14 Å pour lesquelles des données expérimentales de stabilité manquent. On a estimé les entropies de 3eme loi, les volumes, et les coefficients de capacité de rétention de chaleur de Maier-Kelley pour l’état standard à 25°C et 1 bar molaire. Les données expérimentales de stabilité trouvées dans la littérature pour des chlorites de 14 Å ont été utilisées avec différents ensembles d’énergies d’échange, pour des cations sur des sites adjacents, pour computer des estimations de potentiels chimiques à l’état standard des membres terminaux à 25°C et 1 bar. Le modèle a été appliqué à la diagénèse dans des reservoirs élastiques. On a computé les proportions d’activité aqueuse pour Mg2+:Fe2+ en tant que fonction d’équilibre des proportions molaires correspondantes dans des chlorites authigéniques. La proportion d’activité aqueuse était indépendante du contenu en chlorite Al pour une proportion molaire constante de Mg2+:Fe2+ dans la chlorite. Le modèle prédit une large gamme de proportions molaires Mg2+:Fe2+ dans des chlorites authigéniques en équilibre avec les fluides du réservoir. Les tendances dans ces proportions molaires devraient être indépendantes du contenu en Al des chlorites. Le modèle pourra être directement appliqué à des chlorites de 7 Å lorsque l’on aura des données expérimentales pour estimer les paramètres thermodynamiques variés de membres terminaux de 7 Å [D.J.]

Predicted Retardations of Concentration Fronts Using a Mass‐Balance Approach

Regardless of the chemical controls for a pollutant on sediment sorption or release from an immiscible organic liquid, a simple mass balance can be used to predict the retardation of the ground water pollutant front moving downstream from a source of constant composition. The retardation is the ratio of the total molar difference of pollutant across the front to the molar difference in ground water. The mass-balance approach presented here uses a flushing factor for the entire front. It is implicit in the commonly used retardation factor for pollutants undergoing linear sorption, but does not appear to have been used to predict retardation of concentration fronts involving nonlinear sorption or partitioning in hydrochemical studies. Yet the computations require only limited experimental or field data or the use of an algorithm describing sorption or partitioning of the pollutant. Close agreement was obtained between retardations predicted using the mass-balance approach and those reported from numerical modeling and column experiments in the literature.

The quasi-lattice site-mixing model: The binary, symmetrical case

Abstract The quasi-chemical approximation in the quasi-chemical model was replaced with an empirical algorithm which satisfies mixing constraints not satisfied by the approximation. Unlike the quasi-chemical model, this “quasi-lattice” model will predict unmixing within the miscibility gap rather than a metastable composition with a positive free energy of mixing. For a solid solution (in which the exchange energy in each model is fixed by the critical temperature ) the two different models can predict similar miscibility gaps and values of thermodynamic parameters on the boundaries, indicating a lack of sensitivity to the site-mixing algorithms for these compositions. Temperature and compositional dependencies of the exchange energy are proposed to provide modifications of the predicted miscibility gap compositions.

Kaolinite Formation in Clastic Reservoirs: Carbon Dioxide Factor: ABSTRACT

Thermodynamic modeling of diagenesis in clastic reservoirs was used to quantify geochemical and geohydrologic constraints on the formation of kaolinite. The formation of significant amounts of kaolinite depends upon four conditions: (1) a source for aluminum; (2) the presence of acidic pore fluids; (3) the pH buffering capacity of the fluids; and (4) the quantities of such fluids moving through the reservoir. The first condition is usually satisfied by the unstable mineral assemblage. The second and third conditions can depend upon CO2 released during hydrocarbon maturation, and the final condition requires an open system. The presence of dissolved CO2 buffers the pH so that the pore solution is undersaturated with respect to illites, chlorites, and smectites while kaolinite is being precipitated. Minimum limits of fCO2 in equilibrium with fluids forming appreciable amounts of kaolinite were computed as a function of unstable End_Page 998------------------------------ mineral assemblages, temperature, and fluid compositions. Volumes of fluid necessary to form 5 wt. % of kaolinite are presented as a function of fCO2 and the above variables. Formation of kaolinite can be related to hydrocarbon maturation. Decrease in O/C atomic ratios of different types of kerogen during maturation initiates the eventual release of oxygen as CO2 into the pore fluids. Subsequent precipitation of kaolinite is thus an indicator of hydrocarbon maturation. To quantify this indicator, more research is needed to delineate the controls on the formation of CO2 during maturation. End_of_Article - Last_Page 999------------