Geothermal Convection: A Viable Mechanism for Early Burial Dolomitisation of Platform Carbonates

Whitaker, Fiona F.¹, Yitian Xiao², Corinne Haynes¹ (1) University of Bristol, Bristol, England (2) ExxonMobil Upstream Research Company, Houston, TX

Reaction transport models (RTMs) provide a new means to quantitatively investigate carbonate diagenesis and its effects on reservoir quality. RTM studies have demonstrated that geothermal (‘Kohout') convection could drive replacement dolomitisation and also anhydritisation during shallow burial, controlled by the balance between fluid flux and temperature. However, extrapolation from earlier short-term (100ky) simulations suggests complete dolomitisation requires at least 30-60 My. Our more extended RTM simulations, using the same hydrogeological and geochemical parameterization, indicate that dolomitisation is likely considerably more rapid.

There are strong non-linearities in dolomitisation rate over time as a function of the reactive surface area. Although initial rates are indeed slow (~1 %/My), once more than 5-10% of the rock is dolomitised, the process proceeds much more rapidly. Beyond an undolomitised zone within 4 km of the margin where temperatures are <18 ^oC, dolomite abundance increases both with depth and towards the platform interior, forming with a broad-based, flat-topped mushroom-shaped plume. In the interior complete dolomitisation can occur within as little as 10-20 My. Across the platform top a thin (<10 m) cap of up to 1~2% dolomite forms within 1 My, with magnesium sourced from overlying seawater. Assuming a subsidence rate of 1 m/My, this process could generate a “seed” dolomite and effectively half the time to complete dolomitisation by geothermal convection. Syn-sedimentary fractures developed at the platform margin short-circuit geothermal convection. The lower total flux into the interior enables higher temperatures to be maintained inboard of the fractures and thus facilitates more rapid dolomitisation.