Many hydrocarbon-producing dolomite reservoirs are associated with reflux of fluids formed by solar-driven concentration of seawater. Although development of a sucrosic crystalline texture enhances porosity and permeability, precipitation of pore-filling dolomite cement or dolomite crystal growth (“overdolomitization”) may then destroy reservoir quality. Here, we use reactive transport modeling to explore the role of secular variations in seawater composition in controlling reflux dolomitization and implications for porosity.
1D simulations of reflux over a period of 500 ky, with a 1m/yr flux of mesosaline brines sourced from modern seawater (c.35‰ salinity and Mg/Ca molar ratio of ~5) at 30oC, resulted in a formation of a porous replacement dolomite from calcite, increasing porosity by up to 6% from an initial 40% based on mineral density differences. When replacement is complete, dolomite continues to form as primary cement. Increasing the concentration of the source brine enhances reaction rate independent of the increase in density-driven flux. Calcite is completely replaced by 90 ky with a 44‰ brine, while this requires 75 ky with brines of 89‰. Porosity is subsequently reduced by 10% and 16% by overdolomitization by the 44‰ and 89‰ brines respectively, after 500 ky of reflux.
Reflux of brines formed from Mississippian seawater (c.44‰ salinity and Mg/Ca molar ratio of ~2) leads to synchronous primary dolomite cementation and replacement of calcite, with net porosity occlusion at almost all stages. Replacement is much slower, requiring 344 ky for a 89‰ brine (only 11 ky faster than for the 44‰ seawater), at which point porosity has decreased by almost 6%. The rate of subsequent porosity occlusion is, however, slower than that for modern brines, with reduction by a further 5% for the 89‰ brine by 500 ky.
Secular variations in atmospheric pCO2 are reflected in differences in the pH and carbonate alkalinity of seawater. High alkalinity and low pH accelerate the rate of replacement dolomitization, but reduce overdolomitization leading to a higher final porosity. Whilst dolomite kinetics at near-surface conditions are poorly constrained experimentally, simulations suggest that the rate of replacement dolomitization by a Mississippian brine of 89‰ salinity at 35°C is more than twice that at 30°C, and also promotes dolomite cementation.
Phanerozoic oceans have undergone long-term ﬂuctuations in Mg/Ca in phase with 100–200 My oscillations in sea level, icehouse-greenhouse climates, and global volcanicity. This study suggests that mesohaline brines have the potential for dolomitization even during periods of low marine Mg/Ca, but that the evolution of reservoir quality in reflux dolomites may reflect secular variations in the composition of their parent fluids. However, numerous questions remain, including the extent to which reflux diagenesis may be impacted by changes in seawater SO4 and oxygen, as well as by global sea-level and thus the area of shallow shelf over which brine generation can occur.
|Publication status||Unpublished - 2013|
|Event||30th IAS Meeting of Sedimentology - Manchester, United Kingdom|
Duration: 2 Sep 2013 → 5 Sep 2013
|Conference||30th IAS Meeting of Sedimentology|
|Period||2/09/13 → 5/09/13|