Dolomite

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Dolomite: Perspectives on a
Perplexing Mineral

Dolomite is a metastable carbonate. It forms in a variety of distinctly different settings
and can change as conditions change. The mode of formation influences dolomite
morphology and thus impacts exploration and production strategies. New approaches
to carbonate evaluation are helping geoscientists unearth reservoir-quality dolomite,despite its heterogeneous and often enigmatic nature.

Mishari Al-Awadi
Kuwait Oil Company
East Ahmadi, Kuwait
William J. Clark
William Ray Moore
Denver, Colorado, USA
Michael Herron
Tuanfeng Zhang
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
OilfieldReview 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.

32Oilfield Review

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 precursor carbonate rock or sediment—primarily limestone or calcareous muds. These carbonates tend
to be unstable, composed chiefly of calcite orits
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 mineral, dolomite is not merely an assemblage of
magnesium, calcium and carbonate (right).
Rather, it is a metastable mineral of dubious lineage with a variablechemical 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 disturbed—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 berepeated numerous times
during burial and diagenesis, with each new
phase forming through partial or complete dissolution 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 crystal growth that forms largeinterlocking crystals.
Because the morphology of a dolomite body is
controlled by processes that created it, geoscientists 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 environmentof recrystallization.2
By masking its mode of origin, dolomite recrystallization 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...
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