Dolomite cement composition

Since we have no direct information about the chemistry of the Fountain fluid, we assume that its composition reflects reaction with minerals in the evaporite strata that lie beneath the Lyons. We take this fluid to be a three molal NaCl solution that has equilibrated with dolomite, anhydrite, magnesite (MgCC>3), and quartz. The choice of NaCl concentration reflects the upper correlation limit of the B-dot (modified Debye-Hiickel) equations (see Chapter 8). To set pH, we assume a CO2 fugacity of 50, which we will show leads to a reasonable interpretation of the isotopic composition of the dolomite cement. 

We can predict the oxygen and carbon isotopic compositions of the dolomite produced by this reaction path, using the techniques described in Chapter 19. Figure 25.4 shows the compositions of calcite and dolomite cements in the Lyons, as determined by Levandowski et al. (1973). The calcite and dolomite show broad ranges in oxygen isotopic content. The dolomite, however, spans a much narrower range in carbon isotopic composition than does the calcite. 

Fig. 25.4. Oxygen and carbon stable isotopic compositions of calcite ( ) and dolomite ( ) cements from Lyons sandstone (Levandowski et al., 1973), and isotopic trends (bold arrows) predicted for dolomite cements produced by the mixing reaction shown in Figure 25.3, assuming differing CO2 fugacities (25, 50, and 100) for the Fountain brine. Fine arrows, for comparison, show isotopic trends predicted in calculations which assume (improperly) that fluid and minerals maintain isotopic equilibrium over the course of the simulation. Figure after Lee and Bethke (1996).

The timing of the dolomitization of carbonate rock bodies and emplacement of dolomite cements has been one of the more controversial aspects of the "dolomite problem." Most of the basic factors controlling dolomite formation, where were discussed in Chapters 6 and 7, also apply to dolomite formation during the later stages of diagenesis. However, the extended periods of time, the solution compositions likely to be encountered, and the elevated temperature and pressure that occur during deep burial provide highly favorable conditions for dolomite formation. 

Our results indicate that kaolinite distribution in the Lunde Formation is not strictly controlled by the Kimmerian uplift and erosion. This is due partly to the formation of kaolinite during eodiagenesis and partly to the strong relationship between kaolinite abundance and detrital composition of the sandstones, particularly the original amounts of feldspars and mud intraclasts. Pervasive kaolinite formation, coupled with dissolution of calcite and dolomite cements, has been substantial in well 34/4-1. In well 34/7-A-3H sandstones, the top of which was buried deeper below the unconformity than that of well... 

The aims this study are to elucidate the factors controlling the distribution pattern, mineralogy and geochemical composition of calcite and dolomite cements, and of grain dissolution and kaolin-ization in the fluvial sandstones of the Serraria Formation, northeastern Brazil (Fig. 1). This unit provides an excellent opportunity to examine the influence of variations in the palaeogeographical setting, palaeoclimate and burial history on diagenetic processes. 

Fig. 5. Oxygen and carbon isotopic composition of early-formed dolomite cements in sandstones of the central San Joaquin basin.

Fig. 6. Isotopic composition of calcite cements from North Coles Levee, South Coles Levee and Canal fields. North Coles Levee dolomite cements shown for comparison. North Coles Levee data from Schultz et al. (1989) South Coles Levee and Canal data from Table 2. Increasingly negative 5 OpDB and lower Sr ratios are correlated with higher temperatures of crystallization. Strontium ratio decrease is attributed to Sr from plagioclase alteration. Shaded box shows calculated composition of calcite in equilibrium with present pore water at Coles Levee fields based on fluid temperature and composition in Fisher Boles (1990) and Sr isotopic data in Feldman et al.( 993).

Quantitative determination of elemental dolomite composition was carried out on five polished thin sections covered with a thin layer of carbon and using a CAMECA SX 51 electron microprobe at 15 kV, a 20 nA beam current and a 0.2 pm beam diameter. The BSE imaging system linked to the electron microprobe was used to detect zonation in the dolomite cement, and compositional analyses were carried out for each zone. Results were normalized to 100 mol% Fe, Mg and Ca. The precision of the analyses was 100% 2. 

Table 4. Bulk-rock carbon and oxygen isotope results for dolomite cement in the Angel Formation, Angel Field. Precipitation temperatures for dolomite cement were calculated using the fractionation factor of Northrop Clayton (1966) and assuming a marine composition for the original 8 0 pore water (6 0 = 0%o). When integrating the calculated dolomite precipitation temperatures (93-97 °C) with the geohistory plot for Angel-2 (Fig. 4) an Eocene to Late Miocene age for the dolomite cement is suggested, closely matching the seismic evidence (Fig. 22c)...

Fig. 12. Ternary diagram showing the elemental composition of CaO, MgO and FeO (mol%) for the dolomite cement in Angel Formation reservoirs, Angel Field.

The overall width of the cemented fractures (defined as the extent of complete dolomite cementation) ranges from 0.5 cm to 1 m (Fig. 6D), and shows no relationship to position on the transect (Fig. 8A). The thickest examples are termed composite, consisting of two or more closely spaced. 

Yang, W., Spencer, R.J. Krouse, H.R. (1995) Stable isotope and major element compositions of fluid inclusions in Devonian and Cambrian dolomite cements, western Canada. Geochim. Cosmochim. Acta, 59, 3159-3172. 

Lateral variations in the eogenetic association may be related to changes in sedimentary conditions or to the starting mineral composition. The development of nitrates was probably controlled in part by sedimentary rhythms. In the border zones of the Triassic Basin of the Sahara where the source of supply (Hoggar Massif) was probably closer by, calcretes were formed and the deposits were cemented by calcite with a heterogeneous structure. In the case of farther removed source areas and a more stable sedimentation the sandstones are characterized by a poikilitic nodular dolomite cement. Variations in the composition of the sandstone cements between calcite and dolomite may be explained by the ionic composition of the solutions in the... 

Chemical, carbon-isotopic, and oxygen-isotopic compositions from selected samples of the Latrobe dolomite cement are presented in Table 5(A and B). All of the values in Table 5B fall between —10 and —22%o (PDB, Peedee Formation Belemnite). values of the dolomite cement range from 22.7 to 25.2%o (SMOW, Standard Mean Ocean Water) with an... 

Table 5. Chemical and isotopic composition of dolomite cement, Latrobe Group, Gippsland Basin...

Fig. 12. Range of formation temperatures for Latrobe dolomite cement, calculated from its isotopic composition...

It is known that calcites formed 1n the presence of Mg + ions turn out to be magnesian calcites with 0.70 < x < 1 (1, 6 ). The calcites may be bulk precipitates as, for example, in marine cements or, in the case of seeded runs, may form coatings of a different composition from that of the bulk phase. Under special circumstances dolomite may result [ ). 

The Rose Run Sandstone is the best characterized of the Cambrian sandstones because it is also an oil and gas reservoir (Fig. 3). It is also the only one of the Cambrian sandstones that is known to retain its sandstone composition in the eastern part of the state rather than passing laterally into carbonate. The Rose Run Sandstone is a sandy layer in the middle of the Knox Dolomite (Fig. 4), which across much of eastern Ohio lies at depths suitable for injection of supercritical C02 (Fig. 3). The Rose Run Sandstone was deposited in a passive margin phase of the Appalachian Basin and consists of interbedded layers of carbonate, primarily dolos-tone, and sandstone (Fig. 5). The sandstone is compositionally mature, consisting largely of quartz. Subordinate reactive minerals are the alkali feldspars and locally abundant glauconite (Fig. 5). Dolomite and quartz are the dominant cements (Janssens 1973 Riley et al. 1993). 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Figure 8.16. A hypothetical trend of changes in the stable isotope composition of carbonate cements in different diagenetic environments. A- marine realm B-meteoric realm C- mixing zone D- successively deeper burial for calcite spar E-successively deeper burial for saddle dolomite. B through E are precipitated in progressively hotter waters. (After Choquette and James, 1987.)...

Composite cements may contain mineral additions other than, or as well as, ones with pozzolanic or latent hydraulic properties. Regourd (R34) reviewed the use of ground limestone, which is widely used in France in proportions of up to 27%. The limestones used consist substantially of calcite, with smaller proportions of quartz or amorphous silica and sometimes of dolomite. They must be low in clay minerals and organic matter because of the effects these have on water demand and setting, respectively. The XRD peaks of the calcite are somewhat broadened, indicating either small crystallite size or disorder or both IR spectra confirm the occurrence of disorder. 

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