Carbonate cements matter

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. 

Mitterer R.M. (1971) Influences of natural organic matter on CaCC>3 precipitation. In Carbonate Cements (ed. O.P. Bricker), pp. 254-258. The Johns Hopkins Press, Baltimore. 

Mitterer R.M. and Cunningham R Jr. (1985) The interaction of natural organic matter with grains surfaces Implications for calcium carbonate precipitation. In Carbonate Cements (eds. N. Schneidermann and P.M. Harris), pp. 17-31. Soc. Econom. Paleontoligists and Mineralogists, Tulsa, OK. 

Burns, S.J. Matter, A. (1995) Geochemistry of carbonate cements in surficial alluvial conglomerates and their paleoclimatic implications. Sultanate of Oman. J. sedi ment. Res., A65. 170-177. 

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. 

The iron-rich nature of the late carbonates is controlled by their mode of origin as the latter is derived from the bacterial fermentation or the thermal decarboxylation of the organic matter. The occurrence of early carbonate cement which has a low iron content is related to the presence of pyrite and the reduction of sulfate. 

The carbon dioxide produced during decarboxylation of the organic matter is not the only factor responsible for the formation of secondary porosity, although on a regional scale in the typical sedimentary basins it may explain a certain portion of the secondary porosity volume. The representative petrographic data, e.g. in the Cambro-Ordo-vician sandstones of the Hassi Messaoud field, show that the dissolved feldspars here account at least for 5% in volume. The amount of acid necessary to dissolve the feldspars depends on their mineralogical composition and on the type of dissolved aluminium. For this purpose each mole of mineral dissolved requires at least 1 mole of protons. In the sandstones, carbonate cementation took place probably because of the low solubility of CO in the formation waters migrating upwards. 

Transformation and dissolution of carbonate cement in the Paleozoic, i.e. Silurian, Devonian and Carboniferous, reservoirs in all basins are similar in nature to those in the Triassic reservoirs of the Ghadames Basin and certain parts of the Oued el-Mya Basin as these Paleozoic reservoirs alternate with numerous thick rather mature shales rich in organic matter. These beds are able to generate large volumes of H COj as weU as of organic acids which have immediate access to adjacent reservoirs. 

Taking into account solubility and stability boundaries of organic acids and of carbonic acid we may conclude that the organic acids are more efficient on a local scale for the dissolution of carbonate and silica cements. Where the source of the latter, i.e. the carbonic acid, is deep and further removed they may not increase the porosity by solution and removal of cement. The action of carbonic acid formed by decarboxylation on carbonate cement leads to the enrichment, in the dissolved carbonate, of the light carbon isotope inherited from the organic matter whereas the reaction of organic acids with the carbonate cement takes place under dissociation with the formation of secondary carbonate enriched with the heavy carbon isotope inherited from carbonates of mineral origin. 

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