Reactive transport modeling of brine reflux: dolomitization, anhydrite precipitation, and porosity evolution

Abstract

Both geothermal convection and brine reflux drive circulation of sea-water-derived fluids through carbonate platforms during early burial, but dynamic interactions between heat and solute transport and resulting diagenesis are at present poorly understood. This paper describes high-resolution reactive transport model (RTM) simulations that suggest that reflux of 85 ppt brines rapidly restricts geothermal convection to the platform margin, with flow focused in the more permeable shallow carbonates. In a baseline simulation, involving an elongate, 25-km-wide grain-dominated packstone platform, brine reflux resulted in complete dolomitization beneath the 5-km-wide brine pool in 335 ky. The dolomite body then extends downward at c. 22 m/100 ky into an underlying broad area of partial dolomitization. This process enhances porosity at shallow depth, but beneath the dolomite body precipitation of anhydrite occludes porosity and limits the depth of reflux. In contrast, geothermal convection at the platform margin forms a smaller partially dolomitized body over a longer time (, 60% dolomite after 1 My), with very minor associated anhydrite cementation. Reflux diagenesis is sensitive to platform geometry, with higher rates of fluid flow increasing the depth of alteration beneath the brine pool for a circular platform compared to the linear baseline.Fluid flow across thermal gradients enhances reaction rates, and ignoring heat transport by 85 ppt brine reflux underestimates the extent of reflux dolomite by 25% and associated anhydrite by 90%. The depth and rate of anhydritization is sensitive to the geothermal heat flux, whereas platform-top temperatures affect dolomitization rate. Reflux diagenesis is also sensitive to brine density, which affects both fluid flow and reaction rates. Sediment permeability and reactive surface area (RSA) are key intrinsic controls on diagenesis. Where the permeability structure permits sufficient fluid flow, diagenesis preferentially affects more reactive fine-grained sediments. However, as flow rates decline, reactions become focused in the more permeable but less reactive sediments. Simulations thus shed light on why in some settings reflux preferentially dolomitizes muddy sediments, but elsewhere favors grainstones. Once active reflux ceases, brines continue to flow in the subsurface, but this ''latent reflux'' causes only minor dolomitization due to prior Mg 2+ consumption at shallow depth.

Polyphase dolomitization of the Wuchiapingian Zechstein Limestone (Ca1) isolated reefs (Wolsztyn Palaeo-Ridge, Fore-Sudetic Monocline, SW Poland)

“…The ge om e try of re place ment do lo mite bod ies can be com plex, as this is a direct im age of the ge om e try of dolomitising fluid flow paths (Wilson et al, 1990) that in turn is con trolled by the prox im ity to the brine source, zones of rel a tively higher po ros ity, and per me ability con trasts, as in di cated by Gar cia-Fresca et al (2012) for the Perm ian San Andres For ma tion (cf. Al-Helal et al, 2012). In ad di tion, com plex lime stone-do lo mite dis tri bu tion as ob served in the Zechstein Lime stone reefs re sulted from mul ti ple reflux events, both in time and space (cf.…”

Reactive transport modelling of hydrothermal dolomitisation using the CSMP++GEM coupled code: Effects of temperature and geological heterogeneity

“…Here the sharp fronts are again developed perpendicular to fluid flow, forming halos around fractures. Some previous RTM studies invoke a rather higher dolomite rate compared to that suggested by Arvidson and Mackenzie (1999) and used in most previous RTM simulations of dolomitisation (Jones and Xiao, 2005;Al-Helal et al, 2012;. The use of a faster kinetic rate results in sharp dolomite fronts (Consonni et al, 2010).…”

“…RTM has already been extensively applied to study the formation of dolomites in low-temperature shallow reflux systems (Jones and Xiao, 2005;Garcia-Fresca et al, 2009;Al-Helal et al, 2012;Xiao et al, 2013;Lu and Cantrell, 2016), as well as during burial diagenesis by fluids circulating due to geothermal convection (Wilson et al, 2001;Whitaker and Xiao, 2010) and sedimentary compaction (Consonni et al, 2010;Frazer et al, 2014).…”

Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China

“…These dense brines are potential dolomitizing fluids that can descend into underlying pore networks under the influence of gravity. However, the flow of diagenetic fluids critically depends on spatial variations in the permeability, generating complex dolomite bodies at a range of scales (Al-Helal et al 2012). Intervals of peritidal cycles dominated by dolomite are abundant in the areas of regional highs and on the inner platform during regressive periods, whereas limestone-rich cyclic intervals develop best near depocenters during transgression periods .…”