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Abstract

AAPG Bulletin, V. 84, No. 1 (January 2000), P. 100-118.

Porosity Loss, Fluid Flow, and Mass Transfer in Limestone Reservoirs: Application to the Upper Jurassic Smackover Formation, Mississippi1

Ezat Heydari2

©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved.
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.
This research was supported by the Louisiana State University. Phillips Petroleum Company generously provided the cores used in the study. The manuscript benefited from editorial comments and discussion with W. J. Wade. I thank R. E. Ferrell and W. LeBlanc for x-ray diffraction analyses. Critical reviews by AAPG reviewers S. N. Ehrenberg, J. M. Gregg, and R. G. Loucks improved the clarity of the concepts presented.

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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