Geology Reference
In-Depth Information
7 Diagenesis, Porosity, and Dolomitization
The preceding chapters provided information on the
diagnostic criteria for
'depositional microfacies'
. This
chapter deals with
'diagenetic microfacies'
that reflects
changes during lithification and rock history (Pl. 28).
Diagenesis
refers to physical, chemical and biological
processes. The understanding of these processes and
their products has high economic importance, because
diagenetic criteria account for many of the petrophysical
properties of carbonate rocks and determine their value
as reservoir rocks and use in industry. Diagenetic stud-
ies require a combination of various methods, includ-
ing standard optical petrography, cathodoluminescence,
SEM observations, stable isotope analyses and rare el-
ement composition. The following text concentrates on
diagenetic features that can be studied in thin sections.
Comprehensive reviews of carbonate diagenesis can be
found in textbooks listed under Basics in the reference
list at the end of this chapter.
The chapter starts with a summary of carbonate min-
eralogy and diagenetic processes, followed by a sur-
vey of the major diagenetic environments. The treat-
ment of porosity focuses on thin-section criteria and
porosity types. We then take a look at how to recognize
and interpret carbonate cements and diagenetic textures,
discuss recrystallization and neomorphism, and exam-
ine the criteria of calcite spar formed by cementation
or recrystallization. The analysis of dolomitization is
limited to descriptive criteria, classifications, and an
overview of some of the more common dolomitization
models. The last part of this chapter deals with meta-
morphic carbonates and marbles.
7.1.1 Modern Carbonate Sediments and
Ancient Carbonate Rocks
Modern carbonates:
Two carbonate minerals prevail
in recent sediments, orthorhombic aragonite and trigo-
nal calcite. Calcite shows marked differences in the
magnesium content. Low-Mg calcite (LMC) and High-
Mg calcite (HMC) are usually separated by a value of
4 mol% MgCO
3
. The mineralogical composition of
modern carbonate sediments depends on that of the skel-
etal and non-skeletal grains and the mineralogy of early
cements. Common minerals are aragonite, magnesian
calcite, followed by subordinate minerals like calcite
and dolomite. Stable Low-Mg calcite dominates in
many non-marine carbonates and is by far the most
abundant carbonate mineral in deep-sea carbonates. The
mineralogical composition of tropical and non-tropi-
cal shelf carbonates is strongly controlled by water tem-
peratures. Shallow-marine
tropical carbonates
are
mainly composed of metastable aragonite and calcite
with high Mg concentrations.
Non-tropical carbonates
consist predominantly of High-Mg calcite (and minor
aragonite) in shallow warm-temperate settings, and
HMC and LMC in cool-temperate environments.
Carbonate rocks:
Since aragonite is metastable and
HMC looses Mg with time, all carbonate sediments are
eventually transformed to LMC. Carbonate rocks there-
fore consist of calcite and to a lesser extent, of dolo-
mite. The mineralogical composition controls the
di-
agenetic potential
that characterizes the varying man-
ner in which carbonates respond to diagenetic processes
(Schlanger and Douglas 1974). Sediments with differ-
ent mineralogical composition require different time
intervals to reach equal stages in lithification. Sediments
consisting of a mixture of aragonite and HMC (e.g. shal-
low-marine sands) as well as evaporite sediments have
a high diagenetic potential because these minerals are
easily and intensively dissolved or transformed. In con-
trast, sediments dominated by HMC and/or LMC (e.g.
pelagic chalks) have a lower diagenetic potential. Be-
cause the diagenesis of carbonate and silica is interre-
lated at least in pelagic carbonates, the role of siliceous
7.1 Carbonate Mineralogy and
Diagenetic Processes
This section summarizes attributes of common carbon-
ate minerals and discusses diagenetic processes affect-
ing these minerals. The prevailing mineralogies of ma-
rine carbonate constituents have changed during the
Phanerozoic; the last part of the section emphasizes the
relevance of these secular variations for understanding
cementation and porosity development.
