Geology Reference
In-Depth Information
7.3.3 Porosity in Limestones and Dolomites
Box 7.5.
Porosity and permeability of modern carbon-
ate sediments
based on samples from Florida and the
Bahamas (Enos and Sawatsky 1981). Interrelationships
between porosity and permeability in recent carbonates
is largely controlled by depositional texture, particularly
the amount of fines <62
m.
Primary and secondary porosity of carbonate rocks are
strongly facies-controlled and depend on the deposi-
tional patterns and the diagenetic history (e.g. Sun et
al. 1992). Porosity relies upon the composition and fab-
ric of rock constituents, the distribution and quantity
of cement as well as the abundance and distribution of
fractures and stylolites.
Grainstones
: Porosity: range 40-53%, mean 44.5%. Per-
meability: range 15 800-56 600 md, mean 30 800 md.
Mud-free skeletal sands near the shelf break consist-
ing of
Halimeda
, foraminifera and mollusk grains.
Packstones:
Porosity: range 45-67%, mean 54.7%. Per-
meability: range 31.5-9,300 md, mean 1,840 md.
Grain-supported sediments containing some mud.
Wackestones:
Porosity: range 64-78%, mean 68%. Per-
meability: range 37.6-6,570 md, mean 228 md. Mix-
ture of calcareous mud and coarse-shell grains, low
energy areas.
Very fine wackestones:
Porosity: range 67-73%, mean
70.5%. Permeability: range 0.63-1.37 md, mean
0.87 md. Loosely pelleted muddy sediment of sea
grass-free low-energy shelf areas.
Supratidal wackestones:
Porosity: range 61-66%, mean
63.5%. Permeability: range 617-24,100 md, mean
5,500 md. Large-scale pore network due to desicca-
tion and lamination.
Porosity in carbonate sediments:
The volume of
pore spaces
in Holocene unconsolidated carbonate sedi-
ments may exceed that of the sedimentary particles.
The average porosity of calcareous sands and muds lies
in the range between 40% and 80% (see Box 7.5). Po-
rosity is largely interparticle, with some intraparticle
contribution. Porosity and permeability data show a
marked inverse relationship. The correlation of high
porosities with the lowest permeabilities reverses the
relationships in ancient carbonate rocks, which despite
great variations in porosity-permeability interrelation-
ships show generally positive correlation of high po-
rosity with high permeability within the same pore type.
The main contrast between Holocene carbonate sedi-
ment and ancient limestones is the very high porosity
of modern muddy sediment, indicating an enormous
pore-volume reduction in the transition from carbon-
ate sediment to carbonate rock. Grain-supported sedi-
ments undergo relatively slightly early diagenetic
changes in porosity and permeability.
and the destruction of porosity by shallow burial ce-
mentation. A preservation of porosity in shallow burial
environments is a consequence of minimal burial, re-
duced burial stress, increased framework rigidity, ex-
clusion of pore water, Low-calcite mineralogy, perme-
ability barriers, and pore resurrection. Porosity in mud-
supported limestones affected by a prolonged burial di-
agenesis is commonly strongly reduced.
Porosity in limestones:
Porosities change with in-
creasing age and/or burial depth of the sediment
(Scholle and Halley 1985). The decrease in total po-
rosity versus depth is described by the normal curve
exhibiting a decrease from > 60% at the water/sedi-
ment interface of deep-water calcareous muds to about
35% at a depth of 1000 m, about 15% at 2000 m and
< 5% at 3000 m. This curve has many deviations, de-
pending on pore water pressure, as exhibited e.g. by
chalks (Scholle 1977; Feazel and Schatzinger 1985).
Mechanical compaction can be responsible for about
one third of porosity in micritic limestones. The total
porosity of many limestones is often less than 10%
whereby grain-supported limestones often exhibit
higher porosities than mud-supported limestones. Com-
mon porosities of wackestones formed in inner shelf
and lagoonal environments are < 3%.
The preservation of primary pores requires that post-
depositional diagenesis be limited in its pore-destroy-
ing effects, that compaction be kept at a minimum, and
that fluctuations between exposure and submergence
create a balance between the formation of solution pores
Porosity in dolomites:
Many dolomites have higher
average porosities and permeabilities than limestones,
because of differences in the size, shape and arrange-
ment of crystals. Porosity tends to increase slightly in
the initial stages of dolomitization of limestones, but
increases abruptly with higher amounts of dolomite.
At this stage, the dolomite is characterized by a sucrosic
texture composed of equally-sized rhombohedra with
intercrystalline porosity originating by dissolution of
associated calcite. Dolomitization is a selective pro-
cess and fine-grained constituents tend to be preferen-
tially replaced.
Common pore types in carbonate rocks:
Interpar-
ticle, vuggy, intercrystalline, and framework pores are
common in limestones. Porosity types in reef limestones
are represented by growth framework pores, meteoric
and vadose solution vugs, and fracture pores. Based on
an evaluation of 38 large oil and gas reservoirs located
