The Natural History of Crystalline Calcium Carbonate: Effect of Magnesium Content and Salinity1
Robert L. Folk, 1979. "The Natural History of Crystalline Calcium Carbonate: Effect of Magnesium Content and Salinity1", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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Morphology of calcium carbonate crystals is controlled mainly by rate of crystallization, and Mg and Na content of the precipitating waters. Together, these factors integrate to provide important indicators of environment. Magnesium selectively poisons sideward growth of calcite; thus CaCO3 prefers to crystallize as aragonite, or as minute fibers or steep rhombs of magnesian-calcite, whose sidewise growth is generally stopped at widths of a few microns. Thus in Mg-rich environments, such as beaches or marine bottoms, micritic or fibrous aragonite and magnesian calcite cements form. As seawater is buried, Na usually remains high but Mg is selectively lost so that the Mg/Ca ratio drops from 3:1 to about 1:3. Thus, in the absence of Mg-poisoning, coarse sparry calcite cement can form in the subsurface, and crystallizes as irregular polyhedra. In meteoric waters, both Mg and Na are very low. If precipitation is very rapid, calcite micrite may form (caliche). Fresh-water calcite can also occur as euhedral rhombs in very dilute solutions. In the phreatic-meteoric zone, sparry calcite develops.
Carbonate ooze initially contains much Mg. Upon lithification, it is proposed that much of the Mg is retained as a sort of “cage” around each polyhedron of calcite, preventing growth beyond a few microns. Fresh-water flushing removes this Mg-cage, and allows recrystallization to coarser microspar.
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Geology of Carbonate Porosity
In clastic situations, primary porositv is a direct function of texture and fabric, including size, sorting and shape (Fig. 1). Grain size, sorting, fabric, as well as sedimentary structures are related directly to sedimentary processes acting at the time of deposition (Fig. 1). Each depositional environment is characterized by a distinct suite of processes distributed across the active sediment water interface in a pattern unique for that environment (Fig.2). This suite of processes gives rise to a group of products, including sediment texture, fabric, and structures distributed across the active sediment water interface in a pattern unique for each depositional environment (Figs. 1 and 2). In a prograding or regressive situation, when sedimentation is taking place at the active sediment-water interface, a vertical sequence of sediments is formed which reflects, in an orderly fashion, from deepest at the base, to shallowest at the top, the progressive changes in texture, fabric and sedimentary structures resulting from the progressive changes in processes found along this interface from shallow to deep water (Fig. 3). Each sedimentary environment then, can be characterized by a unique vertical sequence of sediment textures, fabrics and sedimentary structures. It is this unique suite of characteristics that is commonly used for the identification of depositional environments in ancient rock sequences, and most importantly, is used to predict the presence and detailed distribution of the most porous (best sorted, coarsest) potential reservoir facies (Fig. 3).
In a regional setting, the recognition of distinct sedimentary environments and knowledge of logical lateral relationships is the keystone for prediction of the lateral extension or even presence of potential reservoir facies.