Page 160 - Well Logging and Formation Evaluation

150 Well Logging and Formation Evaluation

have fallen to a greater depth and accumulated. Below a certain depth
(4,000–6,000m), all carbonate is dissolved as a result of the high pressure.
The main difference between carbonate and clastic reservoirs is that
clastic deposition requires the transportation of grains to the sedimentary
basin, whereas carbonates originate within the basin of deposition. Since
the effect of clastic deposition is to typically cloud the water, making the
environment unsuitable for organisms relying on photosynthesis, it is
usually not possible to have carbonate and clastic reservoirs coexisting.
However, it is of course possible for one to be overlying another due to
changes in the environment of deposition over geological time.
Carbonate reservoirs comprise the following types:

• Shallow marine carbonates. The rate of skeletal production in shallow
marine water is generally high. These skeletons break down, due to
action by crustaceans and fish or by turbulence. The effect is to gener-
ate carbonate sediment that may be transported to the final place of dep-
osition. This sediment may be modified by burrowing organisms. Fecal
pellets so generated may form grains, and hence result in porosity.
• Deepwater carbonates. Deepwater carbonates are deposited at a depth
below that at which photosynthesis occurs. Typically the sediments are
formed from oozes consisting of skeletons of pelagic organisms.
• Reefs. Reefs are built by calcium carbonate–secreting organisms
growing on the remains of previous generations. The large skeletal
organisms (e.g., corals) generally remain in place after death, and this
may result in the formation of cavities partially filled with sediment.
Most reef sediment is produced by segmented (e.g., crinoids, algae) or
nonsegmented organisms (bivalves, brachiopods, foraminifera) that
grow in the spaces left by larger skeletal organisms.

Initially, the porosity in calcisands (i.e., matrices comprising carbonate
grains) is very high (45% porosity and 30D permeability). However, post-
depositional diagenetic processes have the effect of drastically reducing
this. Factors that reduce the porosity are:

• Cementation: precipitation of CaCO 3 from the pore waters into the
porespace
• Internal sedimentation: or filling of the porespace by sediment
• Compaction: grain repacking
• Pressure solution: dissolution of CaCO 3 in one part and precipitation in
the porespace of another part