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Burial diagenesis, those changes in rock composition, mineralogy, and texture which occur below the zone of near-surface water circulation, generally becomes the dominant control on rock porosity at depths below a few hundred meters. Experimental, observational, and geochemical data show that porosity loss through burial diagenesis results from both physical and chemical compaction. In near-surface sections, dewatering, grain reorientation, grain breakage, and other mechanical processes lead to sediment/rock porosities as low as 30%. Continued porosity loss requires a combination of mechanical compaction, chemical dissolution at grain contacts and along solution seams, and reprecipitation as intergranular cement. Through these mechanisms carbonate rock porosity may be red ced to values near zero in "semi-closed" systems without significant introduction of allochthonous cementing material. Therefore, cementation may occur in systems where the only fluid movement is water expulsion.
Significant rates of noncompactional fluid flow increase the likelihood of cementation and make the identification of cements more complex. Such cements may be deposited by hydrothermal, artesian, or thermally convective fluids. Current research suggests that a combination of geochemical and petrographic criteria may eventually serve to distinguish cements of various origins.
Empirical observations in various basins indicate that patterns of porosity loss with depth are predictable (Fig. 1). These relationships provide general standards against which individual case studies of diagenesis may be compared. In many regions, these standards provide predictive tools for estimating porosity prior to drilling. In other areas, the standards allow identification of anomalously high porosity and focus attention on mechanisms which preserve early porosity or generate porosity at depth. Factors already shown to be important include geopressuring, early oil migration, hydrothermal alteration, diagenesis of organic matter, and dolomitization. Comparisions of oil field porosities with standard curves allow us to refine our basic understanding of diagenesis as well as our ability to predict reservoir quality.
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