Geology Reference
In-Depth Information
Fig. 1.2.
Limestones as potential reservoir rocks
. The sample has been impregnated with blue-dye-resin that fills open pores
between and within skeletal grains (miliolid foraminifera, MF; bryozoans, B). Primary inter- and intra-particle porosity has
been somewhat reduced by syntaxial overgrowth cements (O) on echinoderm fragments, dogtooth and granular cements
within foraminiferal shells. Highly porous limestones may be good reservoir rocks depending on their permeability, and the
geometry and distribution of pores. Tertiary: Paris Basin, France.
changed with time. Modern carbonates, however, dem-
onstrate how organisms are involved in carbonate pro-
duction and which physical, biological and chemical
processes may be recorded in ancient carbonate rocks.
Current microfacies interpretations are strongly influ-
enced by new data both on shallow- and deep-marine
sedimentation (see Chap. 2).
may reflect short-term environmental changes and high-
frequency sea-level fluctuations as well as long-term
patterns in the formation of carbonate buildups (Sect.
15.6). Platform-basin relations are recorded in alloch-
thonous carbonates formed in shallow-water environ-
ments and deposited on the slope and within the basins
(Sect. 15.7.5). The reconstruction of 'vanished' shal-
low-marine depositional environments is a very prom-
ising tool for microfacies studies (Sect. 16.3).
Carbonate platforms and ramps.
Facies interpreta-
tions for ancient shallow-marine carbonates have been
heavily dependent on comparisons with sedimentation
patterns of the Great Bahama Bank, the Florida shelf
and parts of the Persian Gulf. Whilst 'platform carbon-
ates' have previously been regarded as the normal type
of shallow-marine sedimentation, case studies of an-
cient shallow-marine carbonates together with the in-
vestigation of modern examples now indicate that gen-
tly sloping carbonate ramps were also of major impor-
tance during the Phanerozoic. Ramps and platforms dif-
fer in their geometry, depositional depths and the dis-
tribution patterns of facies zones: microfacies reflect
these differences (Sect. 14.3). Platform and ramp car-
bonates are controlled by variations in biogenic pro-
duction as well as by fluctuations in both sea level and
in accommodation and sedimentation rates. Microfacies
Warm-water, temperate-water and cold-water car-
bonates.
The paradigm of a predominantly warm-wa-
ter origin for 'sun-born' shallow-marine carbonates has
become obsolete as a result of current investigations
of temperate, boreal, subarctic and even polar shelf car-
bonates (Sect. 2.4.4.3; Sect. 16.4). High-latitude ma-
rine carbonate production, cold-water organic reefs,
deep-marine seep and vent communities (Sect. 16.5)
as well as high bioclastic sedimentation rates in cold
ocean waters are factors which need to be considered
in the environmental interpretation of ancient carbon-
ates. Increasing numbers of ancient examples of these
types of carbonates are being recognized.
The ability to distinguish ancient 'tropical' warm-
water, and 'non-tropical' temperate and cold-water car-
