Dolomite Perspectives on a Perplexing Mineral

Mineralization and Reservoir Quality 
Exploration efforts specifically targeted at dolo-
mite reservoirs have paid off in the form of 
numerous oil and gas fields around the world. It 
is estimated that up to 50% of the world’s carbon-
ate reservoirs are in dolomite, and in North 
America that estimate ranges to 80%.
10
Dolomite 
reservoirs also host significant volumes of hydro-
carbons in Russia, northwestern and southern 
Europe, northern and western Africa, the Middle 
East and Far East 
(above right)
. 
Dolomite formation has a marked effect on 
reservoir quality, though not all dolomites make 
good reservoirs. In some reservoirs, it is a detri-
ment to production. The permeability, solubility 
and original depositional fabric of a carbonate 
rock or sediment, as well as the chemistry, tem-
perature and volume of dolomitizing fluids, all 
influence dolomite reservoir quality. Given these 
variables, dolomitization can enhance, preserve 
or destroy porosity.
11
There are at least two trains of thought con-
cerning dolomite porosity: Some geologists main-
tain that dolostone porosity is inherited from its 
limestone precursor.
12
Others cite a long-held 
claim that the chemical conversion of limestone 
to dolostone results in a 12% porosity increase 
because the molar volume of dolomite is smaller 
than that of calcite.
13
Modern-day studies from different parts of 
the globe show interesting changes in porosity 
and permeability with increasing dolomite vol-
ume. For example, an evaluation of Jurassic 
Arab-D carbonates in Ghawar field, Saudi Arabia, 
indicated a steady decrease in porosity and per-
meability as dolomite volume increased from 10% 
to 80%.
14
However, as 80% to 90% of the rock is 
replaced by dolomite, both intercrystalline poros-
ity and permeability increased. Beyond 90%, 
porosity and permeability decreased again as 
more dolomite was added to the rock. A
similar result from Mississippian carbonates of 
Saskatchewan, Canada, showed that maximum 
porosity was developed in carbonates that con-
tained 80% to 90% dolomite.
15
These examples 
highlight important processes that take place as 
dolomite is forming—processes that can affect 
reservoir quality. 
Most dolomites are thought to form through 
the replacement of preexisting calcite or aragonite 
sediments. Dolomitization occurs more readily in 
lime muds than in coarser carbonate sands 
because muds have greater numbers of nucleation 
sites on which dolomite crystals can form.
16
During 
early stages of dolomitization in mud-dominated 
carbonates, porosity decreases slightly as dolomite 
crystals encroach upon space previously occupied 
by mud. As dolomite is buried, mechanical com-
paction caused by the steadily increasing weight of 
overburden will further reduce porosity.
However, as dolomitization continues, the 
dolomite crystals begin to develop a supporting 
framework. By the time a carbonate reaches 80% 
bulk-volume dolomite, it has acquired a grain-
dominated fabric in which dolomite crystals 
essentially support the overburden, thereby sub-
stantially inhibiting compaction.
17
This is one 
reason ancient or deeply buried dolomite is often 
much more porous than associated limestone 
(below)
. Higher porosity and permeability are 
>
Distribution of basins (blue dots) that host production from dolomite reservoirs. 
Most of these basins occupy a position along a broad belt between 60° north and 
south of the equator. (From Sun, reference 11.)
MattV_ORAUT09_Fig_3
60°N
30°N
30°S
60°S
0°
>
Progressive loss of porosity with depth. As 
expected, limestone and dolomite units from
the South Florida basin both show decreases in 
porosity with depth. The limestones tend to be 
more porous at shallower depths. Below 5,600 ft 
(1,700 m), however, the rate of porosity decline 
actually slows for dolomites (blue) as they 
become less susceptible to diagenesis and 
recrystallization than the more reactive 
limestones (green). (From Allan and Wiggins, 
reference 19.)
MattV_ORAUT09_Fig5_2
0
0
1,000
2,000
3,000
4,000
5,000
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
0
10
Depth, ft
Depth, m 
20
30
40
50
60
Porosity, %
75% to 100%
dolomite
75% to 100%
limestone
26678schD5R1.indd 4
11/5/09 3:53 PM

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Oilfield Review
more likely to be preserved in dolostone than in 
limestone because the supporting framework of 
dolomite crystals provides greater compressive 
strength, thus the limestone is more susceptible 
to compaction.
Beyond 90% dolomitization, the loss of poros-
ity can be attributed to the addition of carbonate 
and magnesium, through a process known as