Page 106 - Carbonate Facies in Geologic History

Porosity and Permeability 93

bentonitic marl, and pelagic fauna indicate deposition in a rather deep water,
offshore environment over which westerly winds distributed ash. The same events
were taking place along the whole Cretaceous Rocky Mountain orogenic belt.
Bentonites of the same age are widely traceable east from this front in terrigenous
strata in Wyoming and Montana.
Clay seams are of consequence in the petrography of carbonates, although
clay mineralogy itself is of debatable use in environmental interpretation because
of the detrital sources and the existence of diagenetic alteration in given environ-
ments (Grim, 1958, Weaver, 1958). Krumbein and Sloss (1963) furnish a table
giving, in abridged form, possible environmental interpretation from major clay
minerals (kaolinite, montmorillonite, illite, chlorite).
In addition, the mere presence of clay "contaminants" in dominantly carbon-
ate rock is significant, commonly indicating quiet-water deposition in deeper
water conditions. In many places basinal, dark limestones thin to dark argilla-
ceous units in the basin center. In such strata, clay layers may be well-segregated
from the carbonates, both rock types being rather pure, although regularly inter-
stratified. In shallower water deposits, currents, wave action, and burrowing tend
to intermix carbonate and argillaceous material, part of which was originally
deposited in segregated layers. Burial of such partially mixed, partially segregated
material results in compacted clay seams and noncompacted limestone nodules
or lenses-ball and flow structure or sedimentary boudinage. Seams of argilla-
ceous carbonate are subjected to stylolitization and contacts between boudins, or
for that matter, between bioclastic grains (e.g., crinoid pieces), are often sharply
defined by stylolites which concentrate organic matter, clay, or dolomite rhombs.
There is some reason to believe that clay seams in carbonates control position of
stylolites which are commonly parallel to bedding. It is of interest that clay
content of limestone generally is small, only from 5 to 10% being sufficient on
outcrop to cause weathering profiles generally characteristic of those of pure clay
and silt. As pointed out earlier, the general presence of argillaceous particles in the
sea inhibits organic production of CaC0 3 and carbonate is easily swamped by
clay content. Many marls (carbonate rich clay and silt) contains more than 50%
CaC0 3 but are in no sense limestones.

Porosity and Permeability

Pore space is an important attribute of some carbonates and has naturally been
much studied by petrographers to ascertain its origin and relationship to perme-
ability and fluid transmissibility. A discussion by Harbaugh (1967, in Chilingar et
aI., 1967) concerns techniques for porosity and permeability determination. These
include bulk density-grain density determinations, mercury capillary pressure
injection, plastic impregnation of porosity moulds, buffing of highly polished
surfaces of rock with jeweler's rouge or chrome oxide to outline pores and thin
section study under polarized light.
Porosity is notoriously erratic in carbonates compared to that of sandstones
and is generally slight. Producing reservoirs with 5 to 10% pore space are not