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In recent years a number of models have been developed relating porosity to depth of burial in carbonates. These models illustrate a progressive decrease in porosity as burial depth increases, with porosity values decreasing to less than 10% at depths below 10,000-12,000 ft (3,000-3,700 m).
To test the applicability of these general models to carbonates in southwest Alabama, porosity values of the Jurassic Smackover Formation were tabulated from 40 wells. These wells included both productive and nonproductive wells at depths of 10,500-19,750 ft (3,200-6,020 m). Although the data show trends similar to those predicted by the general models, the Smackover in southwest Alabama possesses significantly more porosity at depth.
Limestones show a very apparent decrease in porosity with increased depth of burial. Below 13,000 ft (3,900 m), limestones typically have porosities less than 10%. Dolomites, on the other hand, do not show any significant trends between porosity and burial depth, possessing porosities of more than 20% at depths in excess of 18,500 ft.
Petrographic analysis indicates that abnormally high porosities at depth in the Smackover of southwest Alabama can be related to favorable shallow diagenesis and the existence of significant amounts of mesogenetic secondary porosity which is related to migration of basinal brines and hydrocarbon maturation and migration.
These data indicate that significant amounts of porosity can exist at depths in excess of 15.000 ft (4,500 m) and that care must be exercised when applying general porosity/depth curves to specific areas.
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