Both geothermal convection and brine reflux drive circulation of sea-water-derived fluids through carbonate platforms during early burial, but dynamic interactions between heat and solute transport and resulting diagenesis are at present poorly understood. This paper describes high-resolution reactive transport model (RTM) simulations that suggest that reflux of 85 ppt brines rapidly restricts geothermal convection to the platform margin, with flow focused in the more permeable shallow carbonates. In a baseline simulation, involving an elongate, 25-km-wide grain-dominated packstone platform, brine reflux resulted in complete dolomitization beneath the 5-km-wide brine pool in 335 ky. The dolomite body then extends downward at c. 22 m/100 ky into an underlying broad area of partial dolomitization. This process enhances porosity at shallow depth, but beneath the dolomite body precipitation of anhydrite occludes porosity and limits the depth of reflux. In contrast, geothermal convection at the platform margin forms a smaller partially dolomitized body over a longer time (, 60% dolomite after 1 My), with very minor associated anhydrite cementation. Reflux diagenesis is sensitive to platform geometry, with higher rates of fluid flow increasing the depth of alteration beneath the brine pool for a circular platform compared to the linear baseline.Fluid flow across thermal gradients enhances reaction rates, and ignoring heat transport by 85 ppt brine reflux underestimates the extent of reflux dolomite by 25% and associated anhydrite by 90%. The depth and rate of anhydritization is sensitive to the geothermal heat flux, whereas platform-top temperatures affect dolomitization rate. Reflux diagenesis is also sensitive to brine density, which affects both fluid flow and reaction rates. Sediment permeability and reactive surface area (RSA) are key intrinsic controls on diagenesis. Where the permeability structure permits sufficient fluid flow, diagenesis preferentially affects more reactive fine-grained sediments. However, as flow rates decline, reactions become focused in the more permeable but less reactive sediments. Simulations thus shed light on why in some settings reflux preferentially dolomitizes muddy sediments, but elsewhere favors grainstones. Once active reflux ceases, brines continue to flow in the subsurface, but this ''latent reflux'' causes only minor dolomitization due to prior Mg 2+ consumption at shallow depth.

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The accurate prediction of the geometry of subsurface dolomite geobodies, their connectivity, and the distribution of reservoir properties is a fundamental challenge in carbonate reservoir characterization. Reactive Transport Models (RTM) couple geochemical reactions with fluid flow to facilitate both 2D and 3D quantitative, process-based investigations of dolomitization and related carbonate diagenetic reactions. The paper will highlight new results and key conclusions from simulations of dolomitization mechanisms in four different hydro-geological systems:Brine reflux,Mixing zone and sub mixing zone,Geothermal circulation andFault controlled hydrothermal circulation. Simulations provide new insights on the spatial distribution and dynamic behavior of:Geometry and distribution of dolomite bodies generated by different styles of subsurface fluid flow and their dynamic interactions;Regional versus local controls on dolomite occurrence and connectivity;Sensitivity and hierarchy of geological controlling parameters;Spatial and temporal relationships between dolomitization and associated diagenetic minerals including anhydrite cements and Mississippi Valley Type(MVT) mineralization;Effect of hydrothermal fluid induced dolomite recrystallization and anhydrite dissolution;Criteria to help identify the distribution of reservoir quality including high permeability dolomite "sweet spots". When integrated with conventional subsurface data and stratigraphic, geochemical, and structural framework, Reactive Transport Models of dolomitization provide fundamental and robust predictive concepts and reservoir quality models for exploration and new / mature field developments. In particular, the state-of-the-art simulations allow the analysis and 3Dvisualization of dolomite body spatial and temporal evolution that can translate into alternative "process-based" well correlation methods and strategies for populating diagenetic bodies and their petrophysical properties in geological models for reservoir flow simulations.

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