Dolomite Perspectives on a Perplexing Mineral

Dolomite Reservoirs: 
Geochemical Techniques for Evaluating Origin and 
Distribution. Tulsa: The American Association of 
Petroleum Geologists, AAPG Continuing Education 
Course Note Series 36 (1993).
20. Sibley DF and Gregg JM: “Classification of Dolomite 
Rock Textures,” 
Journal of Sedimentary Research 57, 
no. 6 (November 1987): 967–975.
21. Woody RE, Gregg JM and Koederitz LF: “Effect of 
Texture on Petrophysical Properties of Dolomite: 
Evidence from the Cambrian-Ordovician of
Southeastern Missouri,” 
AAPG Bulletin 80, no. 1 
(January 1996): 119–132.
22. Adams JE and Rhodes ML: “Dolomitization by Seepage 
Refluxion,” 
AAPG Bulletin 44, no. 12 
(December 1960): 1912–1920.
23. Land LS: “The Origin of Massive Dolomite,” 
Journal of 
Geological Education 33, no. 2 (1985): 112–125.
24. Kocurko MJ: “Dolomitization by Spray-Zone Brine-
Seepage, San Andrés, Colombia,” 
Journal of 
Sedimentary Research 49, no. 1 (March 1979): 209–213.
Müller G and Teitz G: “Dolomite Replacing “Cement A”
in Biocalcarenites from Fuerteventura, Canary Islands, 
Spain,” in Bricker OP (ed): 
Carbonate Cements. 
Baltimore, Maryland, USA: Johns Hopkins Press, 1971.
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of Recent and Plio-Pleistocene Sediments by Marine 
Evaporite Water on Bonaire, Netherlands Antilles,” 
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38
Oilfield Review
Perhaps an even better-known variation on 
lagoonal brine reflux is seen on a localized scale 
in the sabkha model. In this arid-climate sce-
nario, storm surges or high tides push seawater 
landward, over the peritidal sediment of a sabkha 
flat. As the surface water sinks into the sediment, 
some of the pore waters are lost to capillary evap-
oration, leaving a hypersaline brine. Here, hydro-
dynamic pressure provides the hydrologic pump 
for moving Mg through the system. As it becomes 
more concentrated, the brine precipitates arago-
nite and anhydrite or gypsum—minerals that 
sometimes form an updip seal in dolomite reser-
voirs. Precipitation of these minerals removes 
calcium from the solution but leaves the magne-
sium content unchanged, thus raising the Mg/Ca 
ratio and promoting dolomite precipitation or 
dolomitization.
25
The dense brine continues to 
percolate downward into underlying lime sedi-
ments and refluxes back to its source 
(above)
. 
Sabkha dolomite is commonly associated with 
supratidal sediments and features, such as algal 
stromatolites, nodular anhydrites and wind-
driven interbedded deposits. 
A prime example of the sabkha model is the 
Ordovician Red River dolomite in the Williston 
basin of the USA and Canada. Modern-day
sabkhas are undergoing extensive study in the 
Trucial Coast of the United Arab Emirates. There, 
dolomitization takes place only in the storm 
recharge zone, and the amount of dolomite cor-
relates with the frequency of recharge.
26
Researchers have, however, raised doubts as 
to whether reflux can operate on a regional scale, 
as originally proposed. The hydrologies of modern 
brine-reflux dolomites never approach the scale 
of processes that caused dolomitization of shelf 
carbonates adjacent to ancient evaporites. In 
modern settings, reflux dolomites have been 
found beneath evaporite crusts, but the areas of 
evaporite precipitation are both small in scale 
and localized.
27
Modern analogs to ancient dolo-
mite deposits are often hard to find. As with other 
modes, this popular conceptual construct must 
be applied judiciously, on a case-by-case basis.