Secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems: Insights from reactive transport modelling

Dolomitisation 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 dolomitisation 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 dolomitisation and early porosity modification was evaluated using one-dimensional Reactive Transport Models (RTM) and fluids based on modern (aragonite sea), Mississippian and Aptian (calcite sea) seawaters. The sensitivity of dolomitisation 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 dolomitisation 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, whilst secular variations of pH and carbonate alkalinity associated with changes in pCO2 are only secondary controls. Above flow rates of 0.01 m/yr replacive dolomitisation 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, contrarily to replacement, supersaturation is a more important driver than Mg/Ca. Whilst uncertainties remain regarding low-temperature dolomitisation 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.