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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 12. (December)

First Page: 2561

Last Page: 2570

Title: Carbonate Previous HitPorosityNext Hit Previous HitVersusNext Hit Previous HitDepthNext Hit: A Predictable Relation for South Florida

Author(s): James W. Schmoker, Robert B. Halley (2)

Abstract:

This study examines the Previous HitporosityNext Hit of limestones and dolomites in the South Florida basin. Previous HitPorosityNext Hit data are derived from borehole-gravity measurements and from suites of acoustic, neutron, and density logs. Both types of wire-line measurements sample large volumes of rock relative to petrographic methods and can be examined at vertical scales approaching those of aquifers and hydrocarbon reservoirs. Investigation depths range from the surface to about 18,000 ft (5,500 m) and span the transition from high-Previous HitporosityNext Hit near-surface carbonate sediments of Pleistocene age to much denser Mesozoic carbonate rocks with porosities of only a few percent.

Carbonate Previous HitporosityNext Hit in the South Florida basin was affected by a variety of diagenetic processes. However, a number of factors that could complicate Previous HitporosityNext Hit-Previous HitdepthNext Hit relations are of limited importance in southern Florida. The basin contains little clastic material; present depths of burial are about equal to Previous HitmaximumNext Hit depths of burial; the influences of tectonism, geopressures, and hydrocarbon accumulations are minimal.

Curves of Previous HitporosityNext Hit Previous HitversusNext Hit Previous HitdepthNext Hit, reflecting large-scale Previous HitporosityNext Hit-loss processes in the subsurface, are derived for a composite carbonate section and for carbonate strata of different ages and compositions. The decrease of Previous HitporosityNext Hit with Previous HitdepthNext Hit for a composite carbonate section representing a wide range of depositional environments and subsequent diagenetic histories can be characterized by the exponential function ^phgr = 41.73e -z8197/ (ft) [^phgr = 41.73e-z2498/ (m)], where ^phgr is the Previous HitporosityNext Hit (%) and z is the Previous HitdepthNext Hit below ground level (feet or meters). Average Previous HitporosityNext Hit is reduced by a factor of two in a Previous HitdepthNext Hit interval of about 5,700 ft (1,740 m).

Carbonate strata of different ages that are buried to equal depths show no systematic Previous HitporosityNext Hit differences. This implies that the effect of time on Previous HitporosityNext Hit in these rocks is probably subordinate to that of burial Previous HitdepthNext Hit. The data also show a faster than expected rate of Previous HitporosityNext Hit decrease with Previous HitdepthNext Hit for rocks of Eocene age and younger. If it is assumed that the decrease in the volume of evaporites in these rocks indicates less saline pore fluids, Previous HitporosityNext Hit loss in shallow-water carbonates may be inversely related to the magnesium content of pore waters.

Dolomite Previous HitporosityNext Hit is lower than limestone Previous HitporosityNext Hit in the near surface, but does not decrease as rapidly with Previous HitdepthNext Hit. Below about 5,600 ft (1,700 m), dolomite is more porous than limestone. It is hypothesized that most dolomitization occurred relatively early and either reduced original Previous HitporosityNext Hit or selectively favored lower-Previous HitporosityNext Hit limestones. With continued burial, dolomite was more resistant than limestone to associated Previous HitporosityTop-reducing effects.

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