ABSTRACT

Nearly all porosity in lower Wilcox sandstones from Karnes County, Texas, has been developed secondarily at or near maximum burial at depths ranging from -5,100 to -7,500 ft. The sandstones were deposited in strike system environments designated as "shoreface" (which probably also includes tidal channel, tidal delta, barrier bar, and beach), tidal flat, and bay-lagoon. Framework grain mineralogy is similar for samples regardless of environment of deposition. The sandstones have an average composition of quartz 59 percent, feldspar 16 percent, and rock fragments 25 percent; they are generally fine to very fine, and well sorted. Matrix content ranges from 0 to 46 percent, and cement content ranges from 8 to 34 percent. Core analysis porosity values range from 6 to 36 percent with the respective thin section values ranging from 0 to 21 percent.

Overall, porosity was affected sequentially (with overlapping of some events probable) by bioturbation penecontemporaneous with deposition, compaction and mechanical rotation of framework grains, precipitation of quartz cement as overgrowths, precipitation of calcite and replacement of grains by calcite, dissolution of calcite cement and replaced grains and precipitation of kaolinite, precipitation of small amounts of ferroan calcite, and precipitation of minor amounts of dolomite.

Secondary porosity is the result of dissolution of early precipitated pore-filling calcite cement and dissolution of the calcite that had partially or completely replaced framework grains (mostly plagioclase). Secondary porosity is restricted to the shoreface sandstones. Considerable porosity was formed in these sandstones because precipitation of calcite occurred early in the burial history and prevented further compaction of the sediments. Later as the calcite cement and the calcite-replaced grains were dissolved abundant pore space was created.

The shoreface sands were rapidly cemented with calcite, arresting further compaction, while the sands from the tidal flat and bay-lagoon facies continued to compact as they were more slowly cemented with calcite. Initial porosity and permeability differences between facies and/or facies differences in amounts of available carbonate are suggested as likely factors causing different cementation rates. Lithification continued until practically all pore space was filled with cement. Later, acid formation water, of unknown origin, came in contact with these sandstones. The shoreface sandstones were then selectively leached of most or all of the earlier precipitated calcite cement and calcite-replaced grains.