Carbonate cement

Limestone. This is a sedimentary rock that is formed by the accumulation of organic marine life remains (shells or coral). Its main component is calcium carbonate. Cement rock. This is a sedimentary rock that has a similar composition as the industrially produced cement. 

Fig. 2.4. Example of a sliding-fugacity path. Deep groundwaters of a geopressured zone in a sedimentary basin migrate upward to lower pressures. During migration, CO2 exsolves from the water so that its fugacity follows the variation in total pressure. The loss of CO2 causes carbonate cements to form.

The reservoir rock in our model is composed of quartz grains, carbonate cement, and clay minerals in the following proportions, by volume ... 

Meyers, W. J. and K. C. Lohmann, 1985, Isotope geochemistry of regional extensive calcite cement zones and marine components in Mississippian limestones, New Mexico. In N. Schneidermann and P. M. Harris (eds.), Carbonate Cements. SEPM Special Publication 36,223-239. 

The sandstones studied contain different types of cements, such as clay, quartz and carbonate cements. Clay cements are often a mixture of alio- and authigenic minerals. Kaolinite is the main component... 

Carbonate cement content in the studied rocks varies from 0 to 45 vol. %, mostly forming the pore filling. The following carbonates were observed (Koztowska 2004) siderite, Fe-dolomite, ankerite and Fe-calcite. The term siderite corresponds to minerals from the isomorphic group FeCOs-MgCOs with 60-100 mol percent FeCOs. Most siderites fall into the interval siderite - sideroplesite (Fig. 3). 

Morad, S. 1998. Carbonate cementation in sandstones distribution patterns and... 

Carbonate sediments deposited in shallow marine environments are often exposed to the influence of meteoric waters during their diagenetic history. Meteoric diagenesis lowers 8 0- and 8 C-values, because meteoric waters have lower 8 0-values than sea water. For example. Hays and Grossman (1991) demonstrated that oxygen isotope compositions of carbonate cements depend on the magnitude of depletion of respective meteoric waters. 5 C-values are lowered because soil bicarbonate is C-depleted relative to ocean water bicarbonate. 

Ca, Mg carbonates, cementation, high bulk density Sometimes differentiate it from B... 

A major problem in the application of Mg2+ partition coefficients in calcite to natural carbonates has been the failure to predict commonly observed compositions of carbonate cements. The Mg content of shallow water Holocene marine cements is typically 1.5 to 3 times that predicted using experimentally measured partition coefficients, and assuming that the solution from which the cements formed had the same Mg to Ca ratio as normal seawater. This problem has been the basis for arguments against the use of experimentally determined partition coefficients (e.g., Given and Wilkinson, 1985a,b). Other observations of natural... 

Submarine lithification and precipitation of cements in deep sea carbonate sediments are relatively rare processes in typical major ocean basin sediments. Milliman and his associates have summarized much of the information on these processes (Milliman, 1974 Milliman and Muller, 1973,1977). The cements are of both aragonitic and magnesian calcite mineralogies, and are largely restricted to shallow seas such as the Mediterranean and Red seas, and sediments in the shallower parts of major ocean basins in which biogenic aragonite is also present. The formation of carbonate cements will be discussed in detail in subsequent chapters. 

Their studies are remarkable in indicating that organic matter may be important for the formation of magnesian calcite radial ooids, but not aragonitic tangential ooids. This observation is contrary to general concensus on the formation of these two types of ooids. It may also offer a major clue to the formation of aragonitic and calcitic carbonate cements. 

An extensive literature exists on the occurrence of early carbonate precipitates in marine sediments, where they are generally termed cements. Included in this literature are books devoted solely to carbonate cements (e.g., Bricker, 1971 Schneidermann and Harris, 1985) and numerous reviews (e.g., Milliman, 1974 Bathurst, 1974, 1975 Harris et al 1985). Many investigations have been largely descriptive in nature, focusing primarily on the distribution, mineralogy, and morphology of the cements. Here we will briefly summarize the major aspects of these observations, and we will concentrate on the chemical aspects of the formation of these precipitates. 

Factors controlling the mineralogy and chemical composition of carbonate cements in the marine environment have also been extensively investigated. Bathurst (e.g., 1975 1987) summarized many of the observations (e.g., Glover and... 

The formation of beachrock will be examined as an example of carbonate cement formation, because it has been extensively investigated and because it represents a chance to study carbonate cement emplacement under conditions where the rate of cement precipitation is relatively rapid and the associated solutions can be analyzed directly. It also differs from the cementation process in our model in that carbon dioxide can be degassed to the atmosphere, resulting in major changes in the saturation state of the cementing solution. 

The reasons for the equant sparry habit of cements in the meteoric realm are probably more debatable than the fact that these cements should be calcite. There are two principal models in the literature used to explain carbonate cement morphology these models are shown schematically in Figure 7.22. In the Folk (1974) model of Mg-poisoning, the Mg2+/Ca2+ ratio of the water plays an... 

Figure 7.42. Comparison between (A) an idealized plot of variation in 8180 and 813C for carbonates subjected to vadose and phreatic meteoric diagenesis (after Lohmann, 1988) with (B) the meteoric alteration trend observed for the Key Largo Limestone, Florida, U.S.A. (after Martin et al., 1986). The critical trend in isotopic composition is termed the meteoric calcite line. This trend may be modified at the water recharge surface where evaporation is an important process, caliche is formed and the diagenetic phases are depleted in 13C derived from soil-gas CO2. Another modification can occur distally to the recharge area where precipitating carbonate cements may have isotopic ratios nearly equivalent to dissolving phases.

Secondary porosity results from the dissolution of carbonates in the subsurface environment. It can occur both in limestones and in sandstones where carbonate cements of original labile detrital minerals are dissolved. Because the formation of secondary porosity can substantially enhance the reservoir properties of sediments, it has received considerable attention from the petroleum industry. 

Some of the basic processes in the formation of secondary porosity are similar to those for formation of carbonate cements. A solution of proper composition must be generated by subsurface processes, and this solution must also flow through the formation in which the dissolution reaction takes place in sufficient quantities to transport the dissolved carbonate. The primary differences between cement and secondary porosity formation are that an undersaturated solution must be generated rather than a supersaturated solution, and that while cement formation reduces porosity and can inhibit flow, formation of secondary porosity increases porosity and can result in enhanced flow of subsurface fluids. 

Franks and Forester (1984) have discussed this mechanism in detail for Gulf Coast sediments, with particular emphasis on pre- and post-secondary porosity mineral assemblages. For many localities they found strikingly similar mineral assemblages (Table 8.1). Early carbonate cements had precipitation temperatures in the range of 40° to 75°C. Quartz overgrowths were observed to precipitate... 

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.)...

Badiozamani K Mackenzie F.T. and Thorstenson D.C. (1977) Experimental carbonate cementation Salinity, temperature, and vadose-phreatic effects. J. Sediment. Petrol. 47, 529-542. 

Bricker O.P. (1971) Carbonate Cements. John Hopkins Univ. Studies in Geology, 19, 376 pp. 

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