Dolomite Perspectives on a Perplexing Mineral

part of a two-step process in which microorgan-
isms first weather the basalt and incidentally 
release Mg, Ca and Fe. Microbial consumption of 
CO
2
then results in dolomite crystal nucleation 
on their cell walls. 
Field observations showed low-temperature 
dolomite precipitation by microbial action after 
three months. Subsequent experiments were 
conducted in a controlled laboratory environ-
ment in which methanogenic bacteria were incu-
bated in an anaerobic chamber at 25°C for eight 
months. Once again, dolomite crystals nucleated 
on microbe cells that colonized the basalt sur-
faces. This study expands the range of environ-
ments in which dolomite precipitation is found to 
occur at low temperature, opening the possibility 
for new models to explain the origin and diage-
netic history of ancient dolomites.
The preceding review of dolomite formation 
provides only a general orientation and does not 
cover the full range of environments that support 
the creation of dolomite. Variations and combina-
tions of different modes are espoused by many 
researchers. Furthermore, a survey of dolomite 
literature would reveal that each model is only as 
good as the latest technical paper, and all models 
have been roundly debated, criticized and in 
some cases, rebuked. 
Each dolomite reservoir is created under 
unique circumstances, and some reservoirs may 
consist of multiple generations of dolomite 
formed by different flow systems and mecha-
nisms. All dolomite reservoirs should be investi-
gated and characterized on a case-by-case basis. 
Reservoir Evaluation
Evaluation of dolomite reservoirs is never 
straightforward. The heterogeneous pore systems 
in dolomitic rock can easily confound petrophysi-
cal evaluation efforts. Even the quantification of 
dolomite can be difficult. The carbonate precur-
sors of dolomite, deposited primarily as a result 
of biological activity and composed of fossil frag-
ments and assorted rock grains, tend to create 
rocks with very complex textures and a wide 
range of pore shapes and sizes. These rocks may 
be further beset by multiple physical, biological 
and chemical processes, each operating at differ-
ent scales. Once converted, dolomite may later 
be subjected to multiple stages of dissolution, 
precipitation and recrystallization. 
Dolomite reservoir evaluation must account 
for heterogeneity in lithology, rock pores, grains 
and textures. The Carbonate Advisor petrophys-
ics and productivity analysis process was devel-
oped to help geoscientists evaluate these complex 
reservoirs. Carbonate Advisor analysis relates 
logging data to producibility using texture-sensi-
tive logs and borehole imaging to characterize 
pore geometry.
44
The interpretation methodology 
involves an integrated sequence to determine 
lithology, porosity, pore type, permeability, rela-
tive permeability and saturation 
(above)
.
Lithology and porosity are derived by combin-
ing measurements from various tools, each with 
sensitivity to different factors, including rock 
matrix, fluid properties and porosity. Neutron cap-
ture spectroscopy and photoelectric factor (PEF) 
data are used to quantify rock mineralogy. Bulk 
density and neutron porosity measurements are 
sensitive to both the lithology and the fluids 
contained in their pore spaces. Nuclear magnetic 
resonance (NMR) porosity and bound-fluid volume 
are sensitive to fluid type and pore-space geome-
try, but less sensitive to the rock matrix. Relative 
permeability, which pertains to the effective flow 
of oil or gas and water, affects shallow resistivity 
measurements more than deep resistivity mea-
surements. All these measurements are inte-
grated with others into a simultaneous solution.
Porosity, in particular, is a focal point of any 
reservoir evaluation. However, calculating poros-
ity values in carbonates, which include calcite 
38. Hurley NF and Budros R: “Albion-Scipio and Stoney 
Point Fields, U.S.A., Michigan Basin,” in Beaumont 
EA and Foster NH (eds):