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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Studies have shown that the composition of ocean water may be controlled by reactions with clay minerals which act as solid phase buffers. The compositions of interstitial brines are subject to similar controls, but at least two boundary conditions can be established. In permeable sandstones (open system) it has been proposed that reactions of the form (1), clay mineral + dolomite + H2O ^rarr chlorite + calcite + CO2, produce large quantities of CO2 as a vapor which may migrate, causing production of secondary porosity during later diagenesis. In this case the mineralogy is clearly controlled by the composition of interstitial waters.
In a closed system, represented by over-pressured shales from the Gulf Coast, published water compositions can be correlated with mineral reactions inferred from X-ray diffraction. Equilibria of the form (2), kaolinite + K+ ^rlhar2 clay mineral or feldspar + H+, can be used to document the path of fluid phase compositional buffering with increasing depth. Theoretical phase relations
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predict a decrease in the concentrations of K+, Na+, and CA++ such that K+ < Na+ < Ca++ in buffering capacity. This phenomenon is actually observed. Reactions such as from (2) also provide a mechanism to produce H+, lowering pH, and perhaps also forming secondary porosity during later diagenesis of clastic rocks.
Both examples demonstate that authigenic mineral assemblages, not the appearance or disappearance of single minerals, must be documented to relate diagenetic changes to burial depth. Also, diagenetic mineral assemblages are strongly controlled by water compositions in open systems and these are not a simple function of burial depth. This also implies that any disturbance of fluid compositions during drilling or well completion may profoundly affect mineralogy.
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