Page 214 - Carbonate Facies in Geologic History

Types of Porosity and Permeability in Pennsylvanian-Wolfcampian Buildups 201

4. Komia, Tubiphytes, and tubular calcitornellid foraminifera are composed of
finely porous tests. Komia was probably composed of metastable aragonite on the
assumption that it was a stromatoporoid. Such microporous forms were suscepti-
ble to diagenesis. In addition, these supine. encrusting forms constructed cavities
during growth which offered good channe1ways for fluids and hence a permeable
reservoir. Thus boundstone tops of mounds and some flank beds may have good
porosity and permeability.
5. Subaerial exposure and the influence of meteoric water is important. The
considerable sea level fluctuation through the Late Paleozoic gave good opportu-
nity for leaching at vadose zones above old water tables. The reservoirs often
contain multiple oil-water contacts in relatively thin zones. There is also much
lithological evidence of sea level fluctuation. Even within the cores of many
bioherms are reddish, oxidized zones containing conglomerates. Shelf cycles may
contain further evidence of sea-level fluctuation.
6. Porosity developed through dolomitization is rare in the Pennsylvanian-
Wolfcampian beds of the southwestern U.S.A. except where evaporite basins
occur. Little stratigraphically controlled dolomite is present and tidal flat deposits
are rare except in the Earp Formation, Lower Permian of Arizona and New
Mexico, and in the Minnelusa Formation, Pennsylvanian of the northern High
Plains.