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Carbonates are prone to chemical alteration and solution (rather than me-
chanical compaction) and the effects of these diagenetic changes on bulk
density trends with depth are not clear. Many carbonates appear to suffer no
appreciable mechanical compaction because well-preserved, delicate fossils
and original structures are commonly found. On the other hand, some marls
apparently compact much as mudstones. McCrossan (1961) found that the
bulk density of some Devonian marls in Canada increased with CaCO, con-
tent as well as with depth; and the higher the CaC03 content, the smaller the
relative compaction.
One process is evidently solution at grain contacts and recrystallization in
pores and voids. This has the same effect as mechanical compaction - reduc-
tion of porosity and increase of bulk density -through chemical rather than
mechanical transport. Reduction of porosity can only be effected if the pore
fluids are commensurately compressed or expelled; and the expulsion of pore
fluids from compacting carbonates involves the removal of soluble compo-
nents. Not only are there no obvious new difficulties in primary migration
from carbonate source rocks, but the recrystallization may actually make the
process more efficient provided it does not take place before petroleum is
generated. The common occurrence of bituminous limestones may indicate
early recrystallization that traps the products of organic diagenesis in the
rock.
The matter of porosity and permeability in carbonates is much more varied
and complex than in sandstones, for there may be two generations of porosity,
primary and secondary, or more, and open fractures and joints are much more
common than in sandstones. Secondary migration, therefore, may take place
in exactly the same way in calcarenites as in sandstones, and accumulation
and production from calcarenite reservoirs will also be the same. But fracture
porosity in carbonates is more important than it is in sandstones.
Primary porosity in carbonates may be completely destroyed during dia-
genesis, the recrystallization leading to porosity of a different sort. Vugs may
be formed by the solution of fossils, for example, and considerable perme-
ability may be retained in a bed that consists largely of material of very small
porosity and negligible permeability. Recrystallization also increases the
strength of the material, in particular, its tensile strength, so that fractured
carbonate carrier beds and reservoirs are important in some areas. The prolific
Asmari Limestone in Iran and Iraq has a matrix porosity of 9-14% on average
in Iran, and an intrinsic permeability averaging about 10 md (Hull and War-
man, 1970, p. 431) but it is extensively fractured. These fractures are de-
monstrably open because some wells have very high production rates on very
small penetrations of the reservoir, and production changes in one well are
relatively rapidly detected in other wells at a considerable distance away in
the same reservoir. Hull and Warman note that where sandstones are present,
they appear to be unfractured. They also note that there is a regional hydraulic
gradient within the Asmari Limestone from the mountains to the Gulf and