Reactive transport models of structurally controlled hydrothermal dolomite: Implications for Middle East carbonate reservoirs. 9th Middle East Geosciences Conference, GEO 2010.

Hydrothermal dolomitization is present in several Middle East carbonate reservoirs including Ghawar, the North Field and South Pars. Structurally controlled hydrothermal dolomitization describes the replacement of limestone with dolomite and/or the precipitation of dolomite cement and associated MVT minerals (anhydrite, sulfides, quartz and fluorite) as a consequence of subsurface brines that ascend upwards through fault and fracture systems. This fluid rock interaction in the burial environment has the potential to both improve and/or degrade reservoir quality depending on the properties of the host rock, fluid composition, timing of fluid flow and spatial position relative to structure. A reactive transport model (TOUGHREACT) that couples fluid flow with chemical reactions, was used to simulate hydrothermal dolomitization. Specifically we investigated the sensitivity of hydrothermal dolomite to: fault permeability/flow rates of ascending fluids, reservoir heterogeneity (alternating high and low permeability strata), temperature of host rock and ascending fluids (including their relative temperature difference), fault spacing/multiple fault scenarios, fault vertical separation and strata juxtaposition, episodic versus continuous brine injection and subsurface brine composition (in particular, Na-Cl versus Ca-Cl brines). Results from 2-D and 3-D models suggest that diagenetic modification and evolution of petrophysical properties in response to hydrothermal dolomitization are a complex function of the hydrodynamics and fluid chemistry. Variations in fault and matrix permeability strongly control the spatial patterns of diagenesis. Brine chemistry of both the host rock and the ascending fluids affect the extent and distribution of dolomitization. Na-Cl brines produce more dolomite than Ca-Cl brines because of higher Mg/Ca ratios but this result is salinity and temperature dependent. Hanging wall fault blocks are preferentially dolomitized. Depending on their permeability, relay zones between faults may remain undolomitized. For the systems simulated, hydrothermal dolomitization enhances matrix porosity and permeability but fault zones begin to seal due to the precipitation of anhydrite and dolomite cement. Breccia, vugs and fracture pore types that are commonly observed in hydrothermal dolomites are beyond current simulation capability. Thus predictions of reservoir quality from reactive transport models may be less useful than predictions of hydrothermal dolomite geobody dimensions.