Page 172 - Geology of Carbonate Reservoirs

DIAGENETIC ENVIRONMENTS AND FACIES 153

in general more permeable reservoir rocks than limestones with the same porosity.
There are no unique or universally applicable answers to the questions of how, at
what rate, and under what controlling circumstances dolomitization occurred, but

the job of finding and developing dolostone reservoirs is very real. As one is chal-
lenged to predict the spatial distribution of porosity and permeability in dolomitized
reservoirs, the questions listed above must be answered. The answers will be found

only by studying the rocks first - hand. Seismology and wireline logs will offer little
or no help.
Moldic, vuggy, and cavernous pores are formed when carbonate rock matrix and
pore walls are dissolved when fluids passing through the pore system are undersatu-

rated with respect to the reservoir rock. These pore types have a common origin
but they differ greatly in size and shape. The dissolution that affected them may
occur in shallow - or deep - burial environments. Molds represent dissolved former
grains or crystals, vugs represent dissolved spaces larger than the surrounding grains
or crystals, and caverns include very large dissolution pores. Carlsbad Caverns in
New Mexico can be thought of as a gigantic pore system. Moldic and vuggy porosity
may be related to depositional fabric or depositional facies in some cases. Moldic
pores typically indicate differential solubility between more and less soluble carbon-
ate constituents in the rock fabric. In some cases, the more soluble constituent may
be a metastable mineral such as Mg - calcite or aragonite. Preferential dissolution
may be due to a difference in grain surface area/volume ratio (Walter, 1985 ). Smaller
grains with large surface area/volume ratio can be dissolved more easily than larger
ones. Some moldic pores owe their existence to dissolution of large grains within a
mud matrix after the mud, but not the grains, has undergone replacement or neo-
morphism to become less soluble. Subsequent exposure to leaching removed the
soluble grains but the transformed matrix remained intact. Some vugs are simply
moldic pores that were dissolved to sizes greater than the preexisting grain or crystal
boundaries.

6.3 DIAGENETIC ENVIRONMENTS AND FACIES

The principal diagenetic environments are the vadose, meteoric phreatic, mixing -
zone, marine phreatic, and subsurface or burial environments (Figure 6.4 ). The
vadose environment lies above the water table and all pores in this domain are fi lled
with both air and water. Water resides only temporarily in vadose pores, depending
on the quantity and frequency of precipitation. Water moves through the vadose
zone leaving only surface - tension films on grain surfaces and meniscus fi lms across

pore throats. The depth where all pores are filled with water is called the water table.
That surface represents the top of the phreatic zone. Below that surface, where all

pores are filled with fresh water, is the meteoric or freshwater phreatic zone. The
depth to the water table varies depending on subsurface geology, topography, capil-
larity, and climate. Near the marine environment, fresh water mixes with seawater
in the mixed phreatic or mixing zone and aquifers saturated with seawater make up
the marine phreatic environment. Subsurface or burial diagenetic environments are
those in which water chemistry is unlike either the meteoric or marine phreatic
zones, and temperature and pressure become increasingly important. Subsurface
water chemistry is different because it reflects rock – water interactions and water