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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 27 (1977), Pages 99-107

Secondary Porosity Development and Subsequent Reduction, Overpressured Frio Formation Sandstone (Oligocene), South Texas

Sandra J. Lindquist (1)

ABSTRACT

Subsurface Frio Formation sandstone at depths of 9,000 to 14,000 feet in Nueces, Willacy, and Hidalgo Counties was deposited as deltaic and nearshore coastal complex sediment. Sandstone nearest the main depocenters is mineralogically immature, fine grained, and moderately well sorted. With increased distance from the main depocenters, the sandstone is finer grained, better sorted, and generally more mineralogically mature. This distal sandstone facies has the best porosity and permeability and has undergone the least total mechanical compaction.

At shallow to moderate depths of burial within normal fluid pressures, the sediment was cemented by micrite or feldspar overgrowths, quartz overgrowths, and sparry calcite. Primary porosity was probably reduced to several percent and permeability to tens of millidarcys or less. At a greater depth of burial and within transitional to abnormally high fluid pressures, secondary porosity and permeability were created by calcite dissolution. Porosities as high as 33 percent and permeability of up to several hundred millidarcys were preserved at depths of 10,000 feet because the overpressured fluids prevented subsequent compaction after dissolution. Post-dissolution recementation by kaolinite, ferroan calcite, ferroan dolomite, and analcime probably resulted from natural or induced reservoir pressure changes and fluctuating reservoir fluid chemistry. Recementation was locally extensive and able to substantially reduce secondary porosity and permeability.

In mineralogically immature, overpressured sandstone, porosity and permeability depend on early cementation and diagenesis, total mechanical compaction, maximum temperature and pressure, and changes in reservoir hydrodynamics and fluid chemistry in overpressured zones. Secondary porosity and permeability are likely to be developed in overpressured zones by dissolution of grains and cement, but they are also likely to be subsequently reduced or destroyed by recementation.


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