Geology Reference
In-Depth Information
of pre-cementation porosity and expressed by the total
of pore-filling cement percentage of the bulk volume,
is necessary for understanding and describing porosity
evolution during time (see Pl. 30/1, 3).
Thorough reviews of the creation and destruction
of carbonate porosity were published by Jodry (1972),
Bebout et al. (1979), Wardlaw (1979), Roehl and
Choquette (1985), Moore (1979, 1989, 2001), Purser et
al. (1994), Budd et al. (1995), and Lucia (1999).
and brecciation. Secondary porosity formation by dis-
solution occurs at any point in the burial history and
can substantially enhance reservoir properties. Prime
prerequisites for the formation of secondary porosity
are the existence of a solution undersaturated with re-
spect to carbonate, and fluids that transport the solu-
tions. Deep subsurface processes resulting in under-
saturation are mixing of waters of dissimilar composi-
tion, dissolution by acidic waters through decarboxy-
lation, and thermal maturation of organic matter related
to igneous intrusions and thermal metamorphism (Al-
Shaleb and Shelton 1981; Surdam et al. 1984). Ther-
mally driven diagenesis of shales associated with de-
watering of clays and transformation of smectite to il-
lite may form dilute solutions that lower the saturation
state and thus cause fluid flow.
Enhanced porosity
originates from the enlargement
of primary interparticle porosity, and the creation of
secondary porosity, predominantly by selective disso-
lution of carbonate grains (e.g. aragonitic shells) dur-
ing subaerial exposure, or dissolution of evaporite min-
erals.
Solution porosity
(Pl. 15/4, Pl. 34/6, Pl. 36/2, Pl.
123/2) refers to moldic pores, vugs, channels and cav-
erns. Vugs and channels are crosscutting non-fabric
Primary and secondary porosity:
The porosity of sedi-
mentary rocks falls into two major groups:
•
Primary porosity
forms during the predepositional
stage ( e.g. intragranular pores in foraminifers, corals,
or ooids) and during the depositional stage (depositional
porosity), e.g. intergranular porosity, framework growth
porosity.
•
Secondary porosity
is formed during diagenesis at
any time after deposition. The time involved in the for-
mation of secondary porosity may be tremendously
long, and can be subdivided into three stages called
eogenetic, mesogenetic, and telogenetic by Choquette
and Pray (1970) and summarized in Box 7.3. Impor-
tant processes generating secondary porosity are dis-
solution, dolomitization/dedolomitization, fracturing
Box 7.3.
Porosity and the timing of diagenetic processes
(after Choquette and Pray 1970). During their geological
history carbonate rocks can be affected several times by mesogenetic and telogenetic processes causing modification,
destruction and renewed construction of secondary porosity.
Eogenetic (near-surface)
Near-surface diagenetic processes of relatively short duration occur in the time between the deposition of sediments
and their burial within the zone of active surface-related processes and surface-promoted fluid migration. The upper
limit is the subaerial or subaqueous interface, the lower limit lies at the point where surface-related meteoric or
marine waters cease to circulate by gravitation or convection. Sediments are mineralogically unstable; their porosity
is modified by dissolution, cementation and dolomitization. Diagenetic environments active within the eogenetic
zone comprise the meteoric vadose, the meteoric phreatic zone, and the mixing zone. Porosity in the eogenetic zone
is medium to high.
Mesogenetic (burial)
Diagenetic processes taking place during burial, away from the zone of major influence of surface-related processes
are characterized by rather slow porosity modifications, but often radical porosity destruction due to compaction and
compaction-related processes. The mesogenetic zone corresponds roughly to the deep burial diagenetic environ-
ment.
Telogenetic
The term refers to the time during which long-buried mineralogically stable limestones and dolomites of the
mesogenetic zone are exhumed in connection with unconformities (
unconformity-related diagenesis
), following
tectonic uplift and controlled by surface-related fluid migration. The rocks are affected by surface-related meteoric
processes. Long-buried and exhumed carbonate rocks are significantly influenced by solution and precipitation
processes, often in association with the formation of unconformities as seen in paleokarst systems. Karstification
modifies porosity and produces solution pores ranging in size from tiny voids to extremely large caverns (Sect.
2.4.1.3).
Because sea-level changes can affect deposits from the surface down to a burial of several hundreds of
meters, eogenetic criteria can overprint mesogenetic and telogenetic signatures, as seen, for example, in the karst
development in Florida and the Bahamas.
