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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 65 (1981)

Issue: 11. (November)

First Page: 2466

Last Page: 2467

Title: Evolution of Carbonate Previous HitPorosityNext Hit During Burial--Bahamas, Florida, and Gulf Coast: Holocene to Jurassic: ABSTRACT

Author(s): Robert B. Halley

Article Type: Meeting abstract

Abstract:

Modern carbonate sediments are deposited with large amounts of Previous HitporosityNext Hit; commonly they contain more pore space than grain volume. In contrast, ancient carbonate rocks usually retain only a few percent Previous HitporosityNext Hit. Although the details of Previous HitporosityNext Hit loss may be complex, estimates of Previous HitporosityNext Hit in large samples on the scale of aquifers and reservoirs reveal several relations that may be obscured by the detail of petrographic and geochemical studies.

Early diagenesis in carbonate sediments, with the exception of marine cementation, does not significantly reduce Previous HitporosityNext Hit. Examples from the Quaternary and Tertiary of the Bahamas and Florida demonstrate that Previous HitporosityNext Hit reduction by early freshwater diagenesis at shallow depths, less than 1,500 ft (500 m) for example, is quite inefficient. Although freshwater alteration efficiently stabilizes carbonate mineralogy and drastically alters permeability patterns, it leaves total Previous HitporosityNext Hit relatively unaffected. Thus large volumes of carbonate pore space are carried deeper into the subsurface during continued burial.

In the South Florida basin, carbonate Previous HitporosityNext Hit decreases persistently with depth from 0 to 18,000 ft (0 to 5,500 m) in rocks ranging in age from Pleistocene to Jurassic(?). Although this decrease is irregular in detail, on a broad scale both the average Previous HitporosityNext Hit and the range of measured Previous HitporosityNext Hit at any interval decrease with depth. A Previous HitporosityNext Hit basement (below which Previous HitporosityNext Hit is 5% or less) is encountered at about 14,000 ft (4,300 m). A south Florida standard curve defining Previous HitporosityNext Hit decrease with depth helps define an optimal exploration window bounded by thermal maturation criteria at the top and Previous HitporosityNext Hit criteria at the bottom.

Upper Jurassic hydrocarbon reservoirs in the Smackover Formation of southern Arkansas and northern Louisiana occur at depths ranging from 4,500 to 11,000 ft (1,400 to 3,350 m), and average Previous HitporosityNext Hit values for these reservoirs closely approximate the south Florida standard curve for the same depth range. In contrast, deeper Smackover reservoirs in Mississippi, Alabama, and Florida, which occur between 11,000 ft (3,350 m) and 22,000 ft (6,700 m), have considerably higher Previous HitporosityNext Hit than would be expected for their depth. Although these occurrences of Previous HitporosityNext Hit at considerable depth are poorly understood, this Previous HitporosityNext Hit development is in part the result of interactions of reservoir rocks with acidic brines, the occurrence

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of late secondary Previous HitporosityNext Hit, and the presence of fluid pressures in excess of normal hydrostatic pressure. Physical, geochemical, and lithologic parameters associated with these deep, highly porous reservoirs should be monitored during deep exploratory drilling in other areas. Overpressuring, abundant CO2 and H2S, and the development of late secondary Previous HitporosityTop may indicate the presence of unexpected carbonate reservoir rocks deep in sedimentary basins.

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