http://ciei.colorado.edu/szhong/papers/Li_Zhong_PEPI_2009.pdf

Supercontinent–superplume coupling, true polar wander and plume mobility: Plate dominance in whole-mantle tectonics

Geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations. We used reactive-transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick. In the simulations, porosity and permeability decrease with depth to account for sediment compaction. Dolomitization of a platform consisting of medium grained ( 0 . 0 5 mm) sediments occurred in a broad band ranging from 2.5 km depth near the margin to 1 . 5 km depth near the platform center. The area of dolomitization is deep enough that temperatures exceed 50°C but not so deep that low permeabilities restrict mass transport. Complete dolomitization in the center of this zone is estimated to require at least 60 my. Incorporation of permeability contrasts, permeable beds, and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent. Dolomitization created magnesium-depleted, calcium-rich fluids in less than 10 ky, and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available.

/pdf/spatial-patterns-of-diagenesis-during-geothermal-circulation-m7n1pbat6p.pdf

Spatial Patterns Of Diagenesis during Geothermal Circulation in Carbonate Platforms

The kinetics of dissolution and precipitation is of central importance to understanding the long-term evolution of fluid flows in crustal environments, with implications for problems as diverse as nuclear waste disposal and crustal evolution. The authors examine the dynamics of such evolution for several geologically relevant permeability distributions (models for en-echelon cracks, an isolated sloping fractured zone, and two sloping high-permeability zones that are close enough together to interact). Although the focus is on a simple quartz matrix system, generic features emerge from this study that can aid in the broader goal of understanding the long-term feedback between flow and chemistry, where dissolution and precipitation is under kinetic control. Examples of thermal convection in a porous medium with spatially variable permeability reveal features of central importance to water-rock interaction. After a transient phase, an accelerated rate of change of porosity may be used with care to decrease computational time, as an alternative to the quasi-stationary state approximation (Lichtner, 1988). Kinetic effects produce features not expected by traditional assumptions made on the basis of equilibrium, for example, that cooling fluids are oversaturated and heating fluids are undersaturated with respect to silicic acid equilibrium. Indeed, the authors observe regions of downwelling oversaturatedmore » fluid experiencing heating and regions of upwelling, yet cooling, undersaturated fluid. When oscillatory convection is present, the amplitudes of oscillation generally increase with time in near-surface environments, whereas amplitudes tend to decrease over long times near the heated lower boundary. The authors examine the scaling behavior of characteristic length scales, of terms in the solute equation, and of the typical deviation from equilibrium, each as a function of the kinetic rate parameters.« less

Long-term flow/chemistry feedback in a porous medium with heterogenous permeability: Kinetic control of dissolution and precipitation

Recently there has been a growing interest in modelling fluid flow in sedimentary basins (Parnell, 1994). The main reason is the increasing realization that the capacity of water to transport heat and solids in solution must be quantified and that the fluid pressure conditions in basins need to be constrained while the relevance of subsurface pressure both to drilling safety and to petroleum expulsion and migration has led to increasing interest in overpressure (Fertel, Chapman and Hotz, 1994).

Lithological Control on Fluid Flow in Sedimentary Basins

The juxtaposition of fault-bounded sedimentary basins, above crustal-scale detachments, with warmer exhumed footwalls can lead to thermal convection of the fluids in the sediments. The Devonian basins of western Norway are examples of supradetachment basins that formed in the hanging wall of the Nordfjord-Sogn Detachment Zone. In the central part of the Hornelen and Kvamshesten basins, the basin-fill is chiefly represented by fluvial sandstones and minor lacustrine siltstones, whereas the fault margins are dominated by fanglomerates along the detachment contact. Prominent alteration and low-greenschist facies metamorphic conditions are associated with the peak temperature estimates of the sediments close to the detachment shear zone. Fluid circulation may have been active during the burial of the sediments, and we quantify the potential role played by thermal convection in redistributing heat within the basins. Different models are tested with homogeneous and layered basin-fill and with material transport properties corresponding to sandstones and siltstones. We found that thermally driven fluid flow is expected in supradetachment basins as a transient process during the exhumation of warmer footwalls. We demonstrate that the fluid flow may have significantly affected the temperature distribution in the upper five kilometers of the Devonian basins of western Norway. The temperature anomaly induced by the flow may locally reach about 80°C. The sedimentary layering formed by sand- and siltstones strata does not inhibit fluid circulation at the scale of the basin. The presence of fluid pathways along the detachment has an important impact on the flow and allows an efficient drainage of the basin by channelizing fluids upward along the detachment.

/pdf/modeling-thermal-convection-in-supradetachment-basins-1slpqss34q.pdf

Modeling thermal convection in supradetachment basins: example from western Norway

Unlike fluid displacement due to regional hydraulic head, thermoconvective motions are generally slow. The thermal impacts of such movements are very weak, whereas their chemical impacts may be significant because of their cumulated effects over geologic time. For nonhorizontal thick sedimentary reservoirs, the fluid velocity due to thermal convection can be accurately approximated by an explicit function of the dip of the reservoir, the permeability and the difference in thermal conductivity between the aquifer and the confining beds. The latter parameter controls the rotation direction of the flow and, for clastic reservoirs bounded by impervious clayey media, fluid moves up the slope along the caprock layer. As the fluid velocity is small, the major rock-forming minerals control the fluid composition by thermodynamic equilibrium. Thus, whereas the volume of redistributed minerals depends on the volume of water circulated, the localization of porosity enhancement is strongly controlled by the reservoir mineralogy. With realistic values of permeability and layer thickness, several per cent of secondary porosity per million years can be created or lost at shallow depth (<2 km), depending on the chlorinity, the set of representative minerals and the temperature. In sandstone reservoirs and high-chlorinity calcarenite reservoirs, the porosity decreases under the caprock where hydrocarbons can accumulate. In chloride-depleted carbonate aquifers, the simultaneous control by carbonates, silica and aluminosilicates can produce a decrease of porosity above the bedrock and an enhancement of porosity under the caprock. However, computations show that the quality of the upper part of the reservoir is mainly reduced by the precipitation of silica and clays.

/pdf/computation-of-porosity-redistribution-resulting-from-1la88k9kl9.pdf

Anne Coudrain Ribstein

Papers

Computation of porosity redistribution resulting from thermal convection in slanted porous layers