Dolomitization often plays a critical role in the pore network development of platform carbonates, with implications for reservoir quality distribution. Understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies. Here, the role of secular variations in seawater Mg/Ca as a control on dolomitization and early porosity modification was evaluated using one‐dimensional reactive transport models and fluids based on modern (aragonite sea), Mississippian and Aptian (calcite sea) seawaters. The sensitivity of dolomitization to a range of extrinsic controls (brine salinity, temperature, fluid flow rate and pCO2) and to intrinsic reactivity of the sediments (effective reactive surface area) was also explored. Simulations suggest faster calcite replacement by dolomite for seawaters with higher Mg/Ca, indicating that dolomitization potential is determined more by Mg/Ca rather than saturation index. Increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density‐driven fluid flux. In addition to brine composition, effective surface area of precursor sediments and temperature exert a critical control on replacement rate, while secular variations of pH and carbonate alkalinity associated with changes in pCO2 are only secondary controls. Above flow rates of 0·01 m yr−1 replacive dolomitization is reaction‐limited rather than flux limited, favouring alteration of fine‐grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low. Post‐replacement dolomite cementation is flux dependent, and thus favoured in areas of high head gradient and high permeability sediments and, contrary to replacement, supersaturation is a more important driver than Mg/Ca. While uncertainties remain regarding low‐temperature dolomitization kinetics, the capability of numerical simulations to decouple individual controls provides new insights which can be used, in conjunction with traditional comparative sedimentology, to generate more rigorous conceptual models for individual reservoir settings.

Bois-Préau, 92852 Rueil-Malmaison Cedex -France e-mail: aleianna@unina.it -marta.gasparrini@ifpen.fr -tatyana.gabellone@unina.it -smazzoli@unina.it Résumé -Dolomitisation tardo-diagénétique dans les calcaires de bassins triassiques de l'Apennin Méridional (Italie) -Les carbonates pélagiques triassiques de l'Unité de Lagonegro, dans l'Apennin méridional hébergent des corps dolomitiques discordants. Ces dolomies montrent les structures typiques connues comme "zébra" ou dolomite "à selle". Dans cette note, on présente les résultats d'observations de terrains et pétrographiques et les données géochimiques obtenues sur trois affleurements. Les données de terrain indiquent que les structures de type "zébra" et bréchifiées sont contrôlées par la stratification très régulière des calcaires micritiques. La dolomitisation a comporté le remplacement du calcaire et de la précipitation de dolomite cristalline dans les vides sous un champ de contraintes extensionnelles. Les températures d'homogénéisation sont comprises entre 80 et 120 °C, avec un mode à (95 ± 10) °C. Après une correction de pression, elles indiquent une température maximale de formation de la dolomie d'environ 110-115 °C. Les températures de fusion de la glace indiquent une salinité comprise entre 2 et 6 wt% NaCl eq, avec une moyenne de 4.2 %. Les valeurs de δ 13 C sont comparables à celles de l'eau de mer triassique, tandis que les valeurs de δ 18 O sont fortement appauvries. Les valeurs du rapport 87 Sr/ 86 Sr sont au contraire plus élevées que celles estimées pour l'eau de mer triassique, mais comparables à celles du Miocène Moyen-Supérieur. Ces résultats indiquent une dolomitisation accomplie par des fluides relativement chauds avec une salinité voisine de celle de la mer et une composition isotopique comprise entre celle de l'eau de mer et celle de saumures de bassin. On propose que cette dolomitisation a été achevée par les eaux expulsées par les formations qui entourent les carbonates pélagiques triassiques ou celles du mélange sous-jacent. L'intégration des données sur l'histoire thermique et de la déformation indique, si l'on assume un équilibre thermique avec les roches encaissantes, que ce phénomène ait eu lieu après le pic d'enfouissement pendant les premiers stades de l'exhumation. Enfin, puisque les dolomies sont assez répandues dans la région, il est bien possible qu'une importante circulation de fluides ait intéressé l'entier fold-and thrust-belt pendant la déformation, y compris les unités de la Plate-forme Apulienne, qui héberge des importants réservoirs d'hydrocarbures. Abstract

University of Bristol -Explore Bristol Research General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms Reactive transport modelling of hydrothermal dolomitisation using the CSMP++GEM coupled code:Effects of temperature and geological heterogeneity AbstractReactive transport simulations using our CSMP++GEM coupled code were applied to study the major controls on replacement dolomitisation and the development of dolomite geobodies in a hydrothermal setting. A series of 2D simulations show how elevated temperature and reactive surface area increase the rate of dolomitisation, and result in a dolomite replacement front that is both sharper and inclined at a higher angle from vertical. This inclination, an effect of gravity segregation, is apparent in thick homogeneous units, but in layered systems the lithological contrast determines the shape of the dolomite front. The increase in permeability resulting from porosity generation upon replacement of calcite by dolomite has a major effect on accelerating the overall progress of dolomitisation. In contrast, the changes in fluid density due to chemical reactions and the pressure dependence of thermodynamic data have a minor influence under simulated conditions. Primary dolomite forms slowly after complete replacement of host calcite, leading to porosity decrease, and is only locally important around the source of the hydrothermal fluid.For a simple layered system, our model results are in excellent agreement * Corresponding author. Present address: ETH Zurich, Institute of Geochemistry and Petrology. E-mail address: alina.yapparova@gmail.comPreprint submitted to Chemical Geology July 6, 2017with those obtained using TOUGHREACT code. They do, however, show the advantage of unstructured triangular over structured rectangular meshes for resolving complex curved/inclined front shapes. Such meshes also offer benefits in simulating fault-controlled hydrothermal dolomitisation. Our simulations predict dolomite geobodies comparable in scale and morphology to natural examples documented at outcrops, and underline the importance of understanding the permeability structure within and around the fault zone.

A regional study on late burial syntectonic dolomites and syntectonic calcite and quartz veins occurring in the hemipelagic Triassic-Jurassic succession of the Lagonegro Basin (southern Apennines) was carried out by integrating fieldwork with structural, fluid inclusion, and geochemical investigations on both surface and subsurface samples. The main goal was to characterize the parent fluids – partly dolomitizing – channelled along a major, out of sequence thrust zone. Calcite veins from the deeper tectonic mélange zone that occurs at the base of the allochthonous wedge – as evidenced by numerous oil wells – were investigated as well. The results point out that the fluids at peak burial conditions, achieved during the first deformation stage, were characterized by temperatures of 120-140 °C and salinities averaging 2 wt % NaCl eq, whereas those channelled along the late (out of sequence) thrust zone and responsible for partial dolomitization of the succession had slightly lower temperatures (Th=95±10 °C) and salinities in the range of slightly modified to normal marine seawater (mean at 3.7 wt % NaCl eq). Similar low salinities are encountered in late dolomites from other Fold and Thrust Belts of the Italian peninsula. The most probable source for the fluids channelled along the investigated out of sequence thrust zone is represented by Miocene marine pore-waters expelled from the tectonic mélange zone. A role on fluid salinities appears to be played by the different lithologies of the Fold and Thrust Belts décollement surface

Reactive transport modelling (RTM) is a powerful tool for understanding subsurface systems where fluid flow and chemical reactions occur simultaneously. RTM has been widely used to understand the formation of dolomite by replacement of calcite, which can be an important control on carbonate reservoir quality. Dolomitisation is a reactive transport process governed by slow dolomite precipitation and cannot be correctly simulated without a kinetic rate model. The new CSMP++GEM coupled RTM code uses the GEMS3K kernel for solving geochemical equilibria by the Gibbs energy minimization method with the CSMP++ framework that implements a hybrid finite element-finite volume method to solve partial differential equations. The unique feature of the new coupling is the mineral reaction kinetics, implemented via additional metastability constraints. CSMP++GEM is able to simulate single-phase flow and solute transport in porous media together with chemical reactions at different pressure, temperature and water salinity conditions. This RTM assures mass conservation which is crucial when simulating transport of solutes with low concentrations over geological time. A full feedback of mineral dissolution/precipitation on the fluid flow is provided via corresponding porosity/permeability evolution and two source terms in the pressure equation. First, the mass source term accounts for the mass of solutes released during mineral dissolution or taken from the solution by mineral precipitation. The second source term attributes to the fact that the solution density is affected by mineral dissolution/precipitation, too. This effect is included