Dolomite Perspectives on a Perplexing Mineral

overdolomitization. After an initial replacement 
phase during which calcite is replaced by dolo-
mite, a pore-filling phase may occur, whereby 
dolomite precipitates to form crystal overgrowths 
or pore-occluding cement. Thus, overdolomitiza-
tion causes young dolostones to have less porosity 
than associated limestones.
18
Dolomite crystal formation plays another role 
in reservoir quality. Dolomite frequently forms 
larger crystals than the calcite it replaces. 
Enlarged crystal size is associated with increases 
in pore-throat size and pore smoothness, which 
boost permeability in dolostones.
19
Because the quality of a dolomite reservoir is 
characterized by its texture, this interrelation-
ship of crystal shape and grain size, orientation 
and packing within a rock can also affect reser-
voir quality. Textural classification schemes help 
geologists infer processes that controlled crystal 
nucleation and growth.
20
One widely accepted 
dolomite classification scheme is based on crys-
tal boundary relationships and divides textures 
into two types: planar and nonplanar. The planar 
crystals are further divided into euhedral and sub-
hedral classes 
(above)
. 
Planar dolomite forms in both shallow and 
burial diagenetic environments. Texture develops 
when crystals undergo faceted growth with pla-
nar interfaces, characteristic of dolomite crystals 
formed during early diagenesis and, under cer-
tain conditions, at elevated temperatures in the 
subsurface. Two porosity-permeability popula-
tions exist for planar dolomite. 
• Planar-e (euhedral) dolomite: This texture,
often referred to as “sucrosic,” forms important 
reservoirs worldwide. Permeability varies strongly 
with porosity. Uniform pore-throat sizes and 
well-interconnected pore systems are found in 
planar-e dolomite, as seen in capillary pressure 
data and scanning electron microscope (SEM) 
pore-cast analysis. 
• Planar-s (subhedral) dolomite: Permeability
is lower than in planar-e dolomite and does
not increase as rapidly with increasing poros-
ity. Uniform throat sizes and well-connected 
pore systems are not seen in this dolomite, 
probably because of continued cementation 
during diagenesis.
Nonplanar dolomite occurs in the subsurface at 
temperatures greater than 50°C [122°F]. This dolo-
mite exhibits no significant correlation between 
permeability and porosity 
(below)
. Permeability in 
>
Dolomite textures. Dolomite can be divided into planar and nonplanar 
textures (
top). The planar texture is further subdivided into euhedral and 
subhedral classes. Euhedral (planar-e) dolomite is characterized by well-
developed crystal faces with sharp boundaries, with the area between 
crystals being either porous or filled by another mineral. Subhedral (planar-s) 
dolomite grains are still planar but less distinct than planar-e grains and show 
compromised boundaries between crystals. Nonplanar dolomite consists of 
anhedral grains that lack well-developed crystal faces. These anhedral grains 
are closely packed, with curved, lobate, serrated or otherwise irregular 
crystalline boundaries. (Adapted from Sibley and Gregg, reference 20.) Actual 
examples of these textures are captured in polished thin-section micrographs 
obtained through a petrographic microscope under polarized light. Euhedral 
dolomite (
bottom left) from a Cretaceous reservoir of the Middle East exhibits 
well-developed faces associated with intercrystalline porosity. Subhedral 
dolomite (
center bottom) was obtained from a Triassic reservoir of the 
northern Arabian Platform. Anhedral dolomite from a Jurassic reservoir of the 
Arabian basin (
bottom right) shows a lack of crystal faces and interlocked 
crystals that destroy porosity. (Photographs courtesy of Fadhil Sadooni.)
MattV_ORAUT09_Fig6_2 
Planar texture
Increase in temperature
Nonplanar texture
Euhedral
Subhedral
Anhedral
>
Porosity versus permeability. Quantitative 
analysis of different textural types indicates that 
permeability in dolomites is not directly related to 
total porosity or crystal size, but rather to the 
connectivity of pore throats. There is a strong 
relationship between increasing porosity and 
permeability in planar-e dolomites (
top, green), 
and an apparent strong relationship in planar-s 
(blue). The correlation coefficient (
r) between 
porosity and permeability in nonplanar dolomites 
(
bottom, yellow) is low, as permeability in this type 
of dolomite is a function of secondary features 
such as connected vugs and fractures. Points 
plotted at 0.5 mD represent measurements that fell 
below the lower determination limit of the 
permeameter and are not part of a statistical 
trend. (From Woody et al, reference 21.)
MattV_ORAUT09_Fig_7 
Total porosity, % by volume
10
5
10
4
10
3
10
2
10
1
r = 0.99
r = 0.99
10
0
10
–1
Permeability
, mD
10
5
10
4
10
3
10
2
10
1
10
0
10
–1
0
5
10
15
20
25
30
Total porosity, % by volume
0
5
10
15
20
25
30
Permeabilit
y,
mD
r = 0.15
Planar-e dolomite
Planar-s dolomite
Nonplanar dolomite
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Autumn 2009
37
nonplanar dolomite is often attributed to secondary 
porosity features such as fractures or intercon-
nected vugs, rather than intergranular porosity 
found between crystals.
21
Researchers continue to unravel the mysteries 
of dolomite mineralization. The discovery that 
dolomite is metastable was a revelation that 
helped geoscientists explain the variations in 
chemical proportions and structural order that are 
seen as the mineral evolves. Dolomitization is not 
a single event; it is a sequence of responses caused 
by changing geologic conditions.