--> Abstract: Secondary Porosity in Sandstones of Lower Wilcox (Eocene), Karnes County, Texas, by George Stanton; #90967 (1977).

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Abstract: Secondary Porosity in Sandstones of Lower Wilcox (Eocene), Karnes County, Texas

George Stanton

Nearly all porosity in lower Wilcox sandstones in Karnes County, Texas, has been developed secondarily at or near maximum burial at depths ranging from 5,100 to 7,500 ft (1,550 to 2,300 m). The sandstones were deposited in strike-system environments designated as shoreface (with probable tidal channel and/or small tidal delta), tidal flat, and bay-lagoon. Framework-grain mineralogy is similar for samples regardless of environment of deposition. The sandstones generally are fine to very fine grained and well sorted. Matrix content ranges from 0 to 46% and cement content ranges from 8 to 34%. Petrophysically determined porosity values range from 6 to 36% with the respective thin-section values ranging from 0 to 21%.

Porosity of the sands was reduced by the following sequence of partly overlapping events: bioturbation penecontemporaneous with deposition; compaction rotation of framework grains; compaction deformation of ductile framework grains; precipitation of authigenic quartz, kaolinite, calcite, ferroan calcite, and dolomite cements. The Wilcox sandstones with high porosity values have undergone stages of cementation, followed by dissolution during burial. High porosities resulted from the dissolution of calcite from the sandstones by acid formation water at or near maximum depth of burial. Dissolution was of both calcite cement and calcite-replaced framework grains (chiefly plagioclase) and is most pronounced in the shoreface sandstones.

The shoreface sands filled rapidly, relative to the other facies, with calcite cement which arrested further compaction. The sands from the tidal-flat and bay-lagoon facies continued to compact as they filled more slowly with calcite cement. Initial lower porosity and permeability values in the tidal-flat and bay-lagoon sandstones, interpreted to be due mainly to higher matrix content, account for the retarded cementation rate. Tidal-flat and bay-lagoon sands retained some porosity and permeability long enough for a change in fluid composition to occur, which resulted in the precipitation of the ferroan calcite cement. Lithification continued until practically all pore space was filled with cement. Later, acid formation water, possibly moving upward from deeper, downdip overpressured ones, came in contact with these sandstones. The shoreface sandstones, with the higher initial porosity and permeability and lower matrix content, were selectively leached of some or all of the earlier precipitated calcite cements and calcite-replaced grains as the formation waters flowed along these paths of least resistance.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas