Calcite cement-rock reactions

The rock in question might contain a large amount of calcite cement, but the reaction path predicts that only a trace of calcite forms during burial. Considering this contradiction, the modeler realizes that this model could not have been successful in the first place there is not enough calcium or carbonate in seawater to have formed that amount of cement. The model in this case was improperly conceptualized as a closed rather than open system. 

Let us now consider the problem from the standpoint of calcite precipitation kinetics. At saturation states encountered in most natural waters, the calcite reaction rate is controlled by surface reaction kinetics, not diffusion. In a relatively chemically pure system the rate of precipitation can be approximated by a third order reaction with respect to disequilibrium [( 2-l)3, see Chapter 2]. This high order means that the change in reaction rate is not simply proportional to the extent of disequilibrium. For example, if a water is initially in equilibrium with aragonite ( 2c=1.5) when it enters a rock body, and is close to equilibrium with respect to calcite ( 2C = 1.01), when it exits, the difference in precipitation rates between the two points will be over a factor of 100,000 The extent of cement or porosity formation across the length of the carbonate rock body will directly reflect these... 

The stability relationships between calcite, dolomite and magnesite depend on the temperature and activity ratio of Mg " /Ca " (Fig. 5d). Lower Mg/Ca activity ratios are required to induce the dolomitization of calcite and to stabilize magnesite at the expense of dolomite (Fig. 5d) (Usdowski, 1994). Formation waters from the Norwegian North Sea reservoirs have an average log(an g -/ cz- ) - TO to 0.0 and thus fall within the stability field of dolomite. Nevertheless, both calcite and dolomite are common cements in these rocks, indicating that dolomitization is a kinetically controlled reaction. Further evidence of this is revealed from Recent sediments, such as the Fraser River delta in Canada (Simpson Hutcheon, 1995) (log (aMg2+/aca=+) -2.2 to h-1.0), where the pore waters are saturated with respect to dolomite, but it is calcite rather than dolomite that precipitates. Calcite rather than dolomite forms below the deep>-sea floor, yet the pore waters plot at shallow, near sea bottom temperatures in the stability field of dolomite and shift with an increase in depth towards the stability field of calcite (Fig. 5d). This shift is due to a diffusion-controlled, downhole decrease in Mg/Ca activity ratio caused by the incorporation of Mg in Mg-silicate that results from the alteration of volcanic material, a process which is coupled with the release of calcium (McDuff Gieskes, 1976). 

The lA Pliocene, Ranzano and Antognola formations were cemented by meteoric water the Bismantova Formation was cemented in part by water with a meteoric component the Loiano and the Borello formations were cemented by slightly modified marine pore water and all the foreland basin units (except the Borello) were cemented by water variably enriched in 0 (8 0 = -2 to +8) generated from silicate reactions. The most 0-enriched values for S 0, a,er are compatible with depths and temperatures of cementation of the three deepest formations, but not for the less deeply buried Loiano and upper part of the Mamoso-arenacea formations. 0-enriched fluids in these latter formations were more probably derived from underlying, more deeply buried rocks apd expelled by compaction. Possibly, the calcite in the deepest buried formations re-equilibrated with hot water after precipitation. 

The problem considered here is a geometrically simple one in two dimensions designed to simulate isothermal flow and reaction in a medium in which some percentage of the rock is reactive (e.g, a carbonate cement) while the remainder is treated as inert (e.g, a quartz sandstone at low temperature). The analysis presented here should apply in most respects to the case where the entire rock is reactive (e.g., a pure limestone), although in this instance the flow can no longer be treated as Darcian. The problem as formulated here is essentially the same as that considered by Ortoleva et al. (2). Although our formulation is based on a one-component system, the results should be broadly applicable to relatively simple multi-component reactions (e.g., calcite dissolution). 

Metasomatic dolomitization is a common process, which actively affects the composition of both rocks and ground water. Three major types of dolomite are distinguished primary, precipitated together with the rock, secondary in the form of cement and the third one at dolomitization of limestones. In the latter case it forms due to the substitution of Mg instead of Ca. Because of this the very substitution process is considered as a number of sequential reactions of dissolution and precipitation. However, total substitution of dolomite instead of calcite is rare. Usually form limestones at various dolomitization stages (magnesian calcite, protodolomite). In its substance this is isomorphism of cations with the same valence, which may be presented by equation... 

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