Dolomite Perspectives on a Perplexing Mineral

Weishu Zhao
Cambridge, Massachusetts, USA
Neil Hurley 
Dhahran, Saudi Arabia
Djisan Kho
East Ahmadi, Kuwait
Bernard Montaron
Dubai, UAE
Fadhil Sadooni
Qatar University 
Doha, Qatar
Oilfield Review Autumn 2009: 21, no. 3. 
Copyright © 2009 Schlumberger.
For help in preparation of this article, thanks to
Tony Smithson, Northport, Alabama, USA.
Carbonate Advisor, CMR, EcoScope, ECS, ELAN,
Litho-Density and MDT are marks of Schlumberger.
“I think you should be more explicit
here in step two.”
Modified with permission from Sidney Harris, copyright ScienceCartoonsPlus.com.
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11/5/09 3:53 PM

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Autumn 2009
33
Dolomite is a complex mineral. It can precipitate 
directly from solutions containing magnesium, 
calcium and carbonate ions to form cement or 
unlithified sediment. However, most dolomite 
forms through the chemical alteration of precur-
sor carbonate rock or sediment—primarily lime-
stone or calcareous muds. These carbonates tend 
to be unstable, composed chiefly of calcite or its 
more thermodynamically unstable polymorph, 
aragonite. When these precursor materials are 
exposed to magnesium-rich fluids, a portion of 
the calcium ions may be replaced by magnesium 
ions to form a more stable magnesium calcium 
carbonate known as dolomite.
Dolomite is found in a wide range of settings 
including hydrothermal veins, lakes, shallow 
oceans, lagoons and evaporative basins. Theories 
surrounding the origins of dolomite continue to 
evolve. Amid controversy and speculation, many 
modes of origin have been proposed over the years, 
and nearly as many have been discarded.
1
A common sedimentary rock-forming min-
eral, dolomite is not merely an assemblage of 
magnesium, calcium and carbonate 
(right)
. 
Rather, it is a metastable mineral of dubious lin-
eage with a variable chemical composition and 
atomic structure. For a given span of geologic 
time, it may reside in one form, only to pass to a 
more stable state when its equilibrium is dis-
turbed—primarily through changes in pressure, 
temperature or chemistry. The crystals may 
even grow in size. Thus, early generations of 
crystals may subsequently be recast into ever 
more stable forms.
This process can be repeated numerous times 
during burial and diagenesis, with each new 
phase forming through partial or complete disso-
lution of an earlier dolomite. Recrystallization 
can be beneficial to reservoir formation when it 
generates intercrystalline porosity, but porosity 
gains can later be negated by the precipitation of 
pore-filling dolomite cement or by dolomite crys-
tal growth that forms large interlocking crystals.
Because the morphology of a dolomite body is 
controlled by processes that created it, geoscien-
tists usually try to integrate the mode of origin into 
their exploration strategies. Over time, however, 
the recrystallization of metastable dolomite can 
obliterate all traces of the mineral’s earliest mode 
of origin, with subsequent generations reflecting 
only the latest environment of recrystallization.
2
By masking its mode of origin, dolomite recrystal-
lization can hamper exploration efforts. 
Some dolomites host exceptional reservoirs 
characterized by high porosity and permeability. 
E&P companies therefore endeavor to predict 
where their drill bit will stand the best chance of 
encountering reservoir-quality dolomite—despite 
its chemical complexities and hidden modes of 
origin. This article describes various modes and 
settings in which dolomite is formed, as well as 
processes that are responsible for enhancing or 
destroying its porosity. It also reviews problems 
encountered when interpreting data from con-
ventional logging suites and provides a glimpse 
into advanced tools and methodologies used for 
evaluating reservoirs in this enigmatic rock.