Dolomite cement phase

Fig. 10. Theoretical dolomite distributions from four potential controlling processes. The model represents a sandbody sandwiched between pedogenic dolocrete layers. (A) Pedogenic dolomite cement there would be most dolomite at the top of each sandbody. (B) Dolomite cement sourced from the dolocrete during burial, transported by diffusion cement should be equally abundant at the tops and bases of sandbodies, with a minimum at the centre. (C) Dolomite distribution controlled by high-permeability streaks allowing input from external sources fluvial sandstones usually fine upwards, leading to high-permeability bases and thus most dolomite at sandbody bases. (D) Dolomite distribution controlled by the relative buoyancy of oil (which may have carried dissolved CO2), or a separate CO2 gas phase caused dolomite cementation and thus led to most dolomite at the tops of sandbodies.

Dolomite cement may be localized at the tops of the sandbodies because of the proximity of overlying fine-grained units when they were undergoing pedogenesis, and because the tops were the first part of each sandbody to receive a charge of COj. The CO2 influx may have occurred as a separate buoyant gas phase or as a gas dissolved in oil. 

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). 

The intergranular carbonate cement is always iron-rich dolomite (or ankerite). Electron probe microanalysis gives between 7 and 18 mol% FeCOj (and 0.2-0.8 mol% MnCOj) (Fig. 11). The calcium content of the dolomites is relatively stoichiometric, with an average of 51 mol% CaCOj (range 50.0-53.3 mol%). The cement is generally dull to very dull red-brown in CL, with subtle concentric zonation. It is relatively homogeneous when examined in BSEM, suggesting that intracrystalline chemistry is reasonably consistent within and between individual fractures. The dolomite crystals have a variable fluid inclusion density, with a vast preponderance of monophase aqueous inclusions. Any two-phase inclusions identified in thin section... 

Phase Diagram Calcium silicates are the most important constituents of hydraulic Portland cements (see Chapter 5), as well as of basic and acidic blast furnace slags and stabilized refractories based on dolomite they also occur as devitrification products of some technical CNS (calcium sodium silicate) glasses. The phase diagram is complicated, as shown in Figure 3.24. 

FIG. 10—This plane polarized photomicrograph illustrates a residual grain of Rosendale natural cement circa 1850. Rhombic-shaped crystals of calcined dolomite are surrounded by reddish rims of an ironbearing mineral phase (arrow). Note the similarity to the laboratory produced cement depicted in Fig. 9. This microtexture is diagnostic of natural cements produced from dolomitic raw materials and is never observed in portland cements or limes. (See color insert for color version of this figure). 

Figure 4.4 XRD scans of SAM selective dissolution treatment residues of a series of five different portland cements. The concentration of minor phases in the residues exposes significant differences between cements. The differences in minor phase composition result in variations in the early-age hydration of the cements (setting, early-age strength development). The main diffraction peaks of the phases are assigned as follows bassanite (Bas), aluminate (CjA), anhydrite (Anh), ferrite (C4AF), quartz (Qtz), calcite (Cc), periclase (Per), gypsum (Gyp), dolomite (Dol), goergeyite (Goe), syngenite (Syn) and portlandite (CH).

---