Dolomites heterogeneities

A few studies reported considerable intracrystalline isotopic variations within millimetre-sized saddle dolomite crystals. For example, a 1.4%o variation in 6 0 and a 0.7%o variation in 5 C were found in isotopically zoned saddle dolomite crystals from Cretaceous carbonates of south Texas (Woronick Land, 1985). In another study, a 5.2%o decrease in 6 0 (-6 to -11.2%o) and a systematic increase in fluid inclusion homogenization temperature (127-146°C) were reported for a single saddle dolomite crystal in Devonian rocks of Canada (Kaufman et al., 1990). Further, Spangen-berg et al. (1995) reported preliminary stable isotope results for intracrystalline heterogeneities in white sparry dolomite (largely equivalent to saddle dolomite) in samples from a Peruvian MVT... 

These SEM, then, show clearly the typical geometric and mineraological heterogeneity of reservoir rocks. They also give one an idea of the shapes and sizes of clay and mineral crystals, and they suggest intimacy of contact between micellar fluid and clay crystals. Finally, in some cases they suggest the possibility of surfactant precipitation, e.g., dolomite in the San Andres core sample. 

Surface conversion due to reactions of the dissolved species with the mineral surface can be predicted using thermodynamic stability diagrams for heterogeneous mineral systems based on relevant mineral dissociation equilibria. This is illustrated in Fig. 3.10 for the calcite/apatite/dolomite system. The activities of Ca + species in equilibrium with various solid phases show that the singular point for calcite and apatite is 9.3. Above this pH, apatite is less stable than calcite and hence conversion of apatite surface to that of calcite can be expected in calcite-apatite system. Similarly, the calcite-dolomite and apatite-dolomite singular points occur at pH 8.2 and 8.8, respectively. 

Chemical equilibria for sulfide minerals have also been developed similar to the heterogeneous systems made up of calcite, magnesite, dolomite, and gypsum. These include various sulfide minerals such as covellite, millerite and pyrrhotite (Acar and Somasundaran, 1990). For example, covellite and pyrrhotite yield chemical species shown in Figs. 3.11 and 3.12. It can be seen from Fig. 3.12 that when pyrrhotite is conditioned in the covellite supernatant, surface conversion of pyrrhotite can take place. Solubility and ESCA results showed such conversion to actually take place in the mixed covellite-pyrrhotite systems. 

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... 

Under these conditions the reservoirs exhibit a great petrophysical heterogeneity. Our research has shown that the factor controlling the petrophysical properties of the reservoirs, like in the two other zones and especially in the central zone, is a superimposed diagenetic dolomitization, in particular in the upper portions of the reservoirs. 

Thickness of such covers and layers vary from several centimeters to 10-15 meters. So compound conditions caused by 1) irregular alternation of carbonate and terrigenous units, 2) initial and secondary mineral heterogeneity of carbonate rocks (silicification/dolomitization prevent hmestones... 

Figure 5. Heterogeneous composition of carbonate eluvium (borehole NAF-06, sp. 06-62, 62 M depth) a) sandy-gravel material, within it b) silty-sandy grains, c) SEM micrograph of corroded dolomite crystal on dolomite gravel surface.

Johnson, E.R., Sciegienka, J., Carlos-Cuellar, S., Grassian, V.H. Heterogeneous uptake of gaseous nitric acid on Dolomite (CaMg(C03)2) and Calcite (Ca CO3) particles a Knudsen cell study using multiple, single, and fractional particle layers. J. Phys. Chem. A 109, 6901-6911 (2005)... 

Viscous acid fracturing uses viscous acid systems such as gelled, emulsified, and foamed acid, or chemically retarded adds, to both create the fracture and differentially etch the fracture face. Treatments with viscous acid are applicable in heterogeneous carbonates such as dolomites or impure limestones. 

Waste material is another promising avenue in solid heterogeneous catalyst. It has been claimed to be effective in transesterification reaction, especially involving egg shells (Joshi et al., 2015 Boro et al., 2014), clams (Girish et al., 2013), and cockle (Boey et al., 2011), while natural dolomite also has been adapted by in biodiesel reaction (Wilson et al., 2008). This will be discussed in detail in Section 6.3.2. 

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