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Controls on the Distribution of Reflux Dolomitisation: Insights From Modelling the Marvellous Madison


Many dolomite reservoirs occur in subtidal carbonates, and pore networks are strongly controlled by early reflux of platform-top brines. In reflux systems the highest porosity is commonly observed in the distal part of the fluid-flow pathway, while primary dolomite cements tend to occlude the porosity close to the brine source. However, these trends can be complicated by the strong facies control over the distribution of reactions. The flux of diagenetic fluids is influenced by the permeability architecture, as well as the distribution of brine density on the ramp top, while reaction rates are controlled by the effective surface area, brine composition and temperature. Because of the complex interaction of these hydrological and geochemical controls, prediction of the pattern of dolomitization and its effect on reservoir quality is challenging. Here we use reactive transport modelling to investigate interactions between these controls during sequential episodes of brine reflux in the Mississippian Madison, to explain why dolomitization is pervasive in the more proximal part of the ramp, but downdip undolomitised grainstones are interbedded with dolomitised mudstones. We simulate flow and reactions within three third order sequences (I to III) for a 2D section of the ramp extending 700 km from the transcontinental arch in SE Wyoming into the Central Montana Trough. Periodic reflux of evaporated Mississippian seawater is simulated from a brine pool that expands in width as the ramp progrades. Spatial variations in depositional texture give rise to a mosaic of sediments of differing permeability and effective reactive surface area. Beneath the brine pool, convection cells develop within the more permeable lower shoreface layers, with slow entrainment of overlying brines supplying Mg2+. Here reaction rates are flux limited and thus replacement dolomitization is initially focussed in the coarser sediments, before extending to more reactive but less permeable facies. Porosity is partially occluded by precipitation of diagenetic gypsum. In contrast, strong lateral fluid flow develops at the distal edge of the brine pool. Here dolomitization is reaction rate limited and thus preferentially affects the backshoal lagoon facies because of their higher effective reactive surface area. The paleo-distribution of salinity on the ramp-top thus determines how depositional texture affects reflux diagenesis.