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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
1Manuscript received August 19, 1998; revised manuscript received April 26,
1999; final acceptance June 25, 1999.
2Mississippi Office of Geology, P.O. Box 20307, Jackson, Mississippi 39289.
ABSTRACT
Ooid grainstones of the Upper Jurassic Smack over Formation are buried to a depth of
over 6 km and exposed to temperatures in excess of 200°C at Black Creek field,
Mississippi. Combined effects of mechanical compaction, intergranular pressure solution
,
and cementation have reduced intergranular porosity of these ooid grainstones to 0%,
indicating that porosity reduction has gone to completion. Modal analysis of 24 samples
lacking preburial cements indicates that from the original 40% porosity, 13 porosity units
(range: 4 to 21) were lost by mechanical compaction, 15 porosity units (range: 8 to 23)
were reduced by intergranular pressure
solution
, and 12 porosity units (range: <1 to
26) were destroyed by cementation.
Intergranular pressure solution
caused an average of 28% (range: 15 to 51%)
vertical
shortening in Smackover ooid grainstones. Under ideal conditions, the 28%
vertical
shortening will generate enough calcium carbonate to precipitate 10% calcite cement. This
is close to the measured volume of cements in Smackover grainstones (12%), suggesting that
intergranular pressure
solution
provided most of the calcite cement present. No external
sources of calcium carbonate are required.
Fine-grained samples that experienced high degrees of intergranular pressure solution
and contain only small amounts of cement occur at the top of the reservoir, whereas
coarse-grained samples with abundant cement and low degrees of pressure
solution
occur in
the middle and basal parts of the reservoir, suggesting that the fine-grained intervals
acted as sources and the coarse-grained intervals as sinks for calcium carbonate. Mass
transfer of pressure
solution
-generated calcium carbonate from the top of the unit to
precipitation sites in the middle and basal parts of the reservoir could have occurred by
a non-Rayleigh-type convection cell. Due to calcite's reverse solubility with respect to
temperature, the cooling, upward-moving limb of the convection cell would become
progressively more undersaturated, and hence able to transport more dissolved calcium
carbonate released by intergranular pressure
solution
in the upper portion of the
reservoir. Pore fluids descending in the downward-moving limb of the cell would become
progressively more supersaturated, and calcium carbonate would tend to precipitate as
cement in the middle and basal parts of the reservoir as fluids become progressively
hotter.
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