Carbonate in rocks

With a few exceptions reservoir rocks are sediments. The two main categories are siliciclastic rocks, usually referred to as elastics or sandstones , and carbonate rocks. Most reservoirs in the Gulf of Mexico and the North Sea are contained in a clastic depositional environment many of the giant fields of the Middle East are contained in carbonate rocks. Before looking at the significance of depositional environments for the production process let us investigate some of the main characteristics of both categories. 

Carbon. Most of the Earth s supply of carbon is stored in carbonate rocks in the Hthosphere. Normally the circulation rate for Hthospheric carbon is slow compared with that of carbon between the atmosphere and biosphere. The carbon cycle has received much attention in recent years as a result of research into the possible relation between increased atmospheric carbon dioxide concentration, most of which is produced by combustion of fossil fuel, and the "greenhouse effect," or global warming. Extensive research has been done on the rate at which carbon dioxide might be converted to cellulose and other photosyntheticaHy produced organic compounds by various forms of natural and cultivated plants. Estimates also have been made of the rate at which carbon dioxide is released to soil under optimum conditions by various kinds of plant cover, such as temperature-zone deciduous forests, cultivated farm crops, prairie grassland, and desert vegetation. 

These deposits would result in carbonate rock (e.g., limestone). A third source rock possibility would be evaporite rocks (e.g., salt, gypsum, anhydrite), which often contain large organic concentrations when originally deposited [26-29]. 

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth s crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust the excess oxygen occurs mostly as the volatiles CO2 and H2O. The CO2 now resides primarily in carbonate rocks whereas the H2O is almost all in the oceans. 

E. J. Fordham, W. E. Kenyon, D. J. Wilkinson 1999, (Forward models for nuclear magnetic resonance in carbonate rocks), Log Analyst 40 (4), 260-270. 

Clark, J., Demonstration of presence and size of a C02-rich fluid phase after HCL injection in carbonate rock, in Underground Injection Science and Technology, Tsang, C.F. and Apps, J.A., Eds., Elsevier, New York, February 2007. 

So-called "wormholes" can be formed when the injected acid primarily enters the largest diameter flow channels in carbonate rock further widening them (107). Acid only invades the small flow channels a short distance greatly reducing treatment effectiveness. High fluid loss rates, low injection rates, and reduced rates of acid-rock reactions decrease the wormhole length. 

It has been shown that the development of wormholes in carbonate rocks is a consequence of diffusion-limited (mass-transfer-limited) kinetics of attack (6). Such kinetics prevail in most of these rocks, i.e. limestones and dolomites, providing that, for the latter, the temperature is larger than about 200°F (90°C) (7-8). 

Acid fracturing, friction reducers, 15 Acid hydrolysis, lignin, 173 Acid injection into carbonate reservoir, 610-611 Acid-rock reactions, rate, 15,16 Add wormholing in carbonate reservoirs, 608-620 in carbonate rocks, 610-611 Acidity-controlled redox reactions, 141-142 Addization... 

BlLLl, A., Salvini, F. Storti, F. 2003. The damage zone-fault core transition in carbonates rocks ... 

One should notice the possibility of producing single-phase illite materials by the same type of process. If, for reasons unknown at the moment, the path of chemical change leads to aluminous illite instead of iron glauconite, i.e., parallel to the K axis with low initial iron content, one could produce single phase illite or mixed layered mineral assemblage. These are apparently rare, but such an explanation could be used to explain the illite and mixed layered mono-mineral layers of "metabentonite" deposits which cannot be explained as recrystallization of an eruptive rock. Mono-mineral layers in carbonate rock the so called... 

KASTNER (M.), 1971. Authigenic feldspars in carbonate rocks. Amer. Min. 

Fricke HC, Wing SL (2004) Oxygen isotope and paleobotanical estimates of temperature and 5180-latitude gradients over North America during the early Eocene. Am J Sci 304 612-635 Friedman GM (1977) Identification by staining methods of minerals in carbonate rocks. In Subsurface Geology. 

Sperber et al. (1984) conducted an extensive examination of Paleozoic dolomites from North America that provides general insight into how different types of dolomitization processes may be reflected in carbonate rocks. Their data indicate that two separate processes may lead to two distinctive populations of sedimentary dolomite. The processes are divided into closed and open system... 

Many authors have noted the depletion of l3C and 180 in carbonate rocks with increasing extent of diagenesis (Gavish and Friedman, 1969 Allan and Matthews, 1977, 1982) and a correlation between 813c and 8 80 for Holocene and Pleistocene carbonates (Figure 7.29). For the limestones of Bermuda, the depletion... 

Kinetic and thermodynamic considerations show that the mass transfer leading to extensive alteration of carbonates in the meteoric realm is difficult to account for by transport of chemical components over long distances. This conclusion has important implications for the formation of regionally-distributed cements in carbonate rocks. These implications and further problems of mass transfer are subjects of the next chapter. 

Pressure solution can cause major alterations in carbonate rock structures on megascopic to microscopic scales. Numerous papers and reviews deal with this topic (e.g., Bathurst, 1975 Choquette and James, 1987). We feel that one of the best attempts to bring an orderly picture out of the many complex features that are observed was that by Wanless (1979), who also emphasized the importance of pressure solution for subsurface dolomitization (see next section). Figure 8.12 presents his general model for the characteristics and controls on pressure solution types in limestones. The primary variables that Wanless considered were the clay content of the limestone, the concentration of structurally resistant elements, and variations between different units or beds. Temperature, pressure and fluid composition are also likely to play an important role in determining the timing and extent of pressure solution. 

Dunham R.J. (1969) Early vadose silt in Townsend mound (reef). New Mexico. In Depositional Environments in Carbonate Rocks A Symposium (ed. G.M. Friedman), pp. 139-181. Soc. Econ. Paleontologists Mineralogists Spec. Publ. 14, Tulsa, OK. 

Solidification of minerals in carbonate rocks or in sea shells, or that of silica and alumina in lateritic or desert soils, is a very slow process. It takes years and centuries and even geological time to consolidate some minerals. Unfortunately, little is known on the exact chemical reactions and the resulting hardening process. As a result, the chemical hardening in nature cannot be translated into technological applications where accelerated hardening and solidification are desired. 

Schneider, F.N. and Owens, W.W. "Relative Permeability Studies of Gas-Water Flow Following Solvent Injection in Carbonate Rocks," Soc. Pet. Eng. J. (Feb. 1976), 23-30. 

Fig. 19. Relationship between volumes of submarine and terrestrial volcanic effusions (/) and volumes of COj (2) buried in carbonate rocks, in the area of the present continents (after Ronov).

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