Carbonate rocks, marine

Carbonate rocks are not normally transported over long distances, and we find carbonate reservoir rocks mostly at the location of origin, in situ . They are usually the product of marine organisms. However, carbonates are often severely affected by diagenetic processes. A more detailed description of altered carbonates and their reservoir properties is given below in the description of diagenesis . 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Another aspect of marine science that broadly affects our daily lives is the origin of carbonate deposits. Limestone and dolomite are important industrial commodities, are host rocks for ore and oil sources, and are geochemical indices (7). The importance of carbonate rocks is unquestioned. The chemical origin is of scientific interest, and a considerable body of background information is available. (7, 10, 15, 16, 17,40). 

Chert is another organic marine sediment, less common than carbonate rocks, but found in huge deposits in some parts of the world. It initially consists of the skeletons of billions of tiny, single-celled animals called radiolaria. These skeletons are composed of microcrystalline quartz or chalcedony (Si02). Dense layers of this material accumulate on the ocean floor, where they are buried and compressed over time. The term chert is sometimes also applied to any compact, very fine-grained siliceous sediment that has resulted from precipitation or consolidation of silica gel. There may be chert lenses or very thin layers within other types of sediments, such as limestone. 

So far we have discussed the occurrence of 14C in hydrological systems. In a similar way one can follow 13C. Its abundance in rocks, organic material, and groundwater is expressed in permil deviation of the 13C 12C ratio in the sample from that in a standard (PDB—a belamnite carbonate from the Pee Dee formation of South Carolina). Most marine carbonate rocks have <513C = — 2 to 0%o, whereas frequent values for organic material and C02 in soil are —28%o to — 20%o. Most plants have values around —23 + 3%o, but certain plants have more positive values, around —12 + 2%o (Tables 11.3-11.5 and Figs. 11.6 and 11.7). 

Acid rain can do much damage over time to marine life and to marble structures (made of carbonate rocks). It can be formed from oxides of carbon, nitrogen, and/or sulfur. Acid rain, however, does not attack the ozone layer. CFCs are responsible for destroying the ozone layer. 

Occurrences of elemental sulfur in peat, coal, and petroleum are described in Chapter 6.4. The role of sulfate reducers in these environments is suggested by the fact that fossil fuels formed in marine environments, where sulfate is in abundant supply, have significantly more sulfide and native sulfur than those formed under freshwater conditions. In fact, a general geological feature of native sedimentary sulfur deposits is their location in sulfate-carbonate rocks and proximity to oil-gas-bearing strata and hydrologic zones where sulfate waters mix with chloride brines (Ivanov, 1964). 

A second important factor is the particle size of the FeS2. Caruccio et al. (1976) concluded that AMD production was a serious problem when exposed pyrite grains were about 0.25 fxm in diameter in framboidal pyrite. This corresponds to a surface area of 4.8 m /g. For crystal sizes in excess of 5 to 10 m (5 fxm s 0.24 m /g), AMD production was greatly diminished, and became unlikely when crystal sizes exceeded 400 fiva. Particle sizes were smallest in marine coals and became larger in lacustrine coals (Caruccio and Perm 1974). A third factor determining the risk of AMD is the proximity of carbonate rocks that can neutralize AMD acidity. 

Fig. 8. 6 CpDB versus 5 Opdb plot of representative diagenetic and detrital calcite in the various depositional sequences of the Bismantova-Termina succession. Arrow denotes the probable derivation of S4 cements from marine carbonate rock fragments.

Finally, carbonate cements reveal the sources of dissolved carbon in the evolving pore waters of the San Joaquin basin. The clastic-rich basin is free of carbonate rocks but contains a considerable amount of organic matter, both in fine-grained sediment and as relatively recent hydrocarbon accumulations. Potential carbon sources for the carbonate cements are marine shell tests, thermogenesis and, possibly, organic reactions related to the presence of the oil. 

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