Page 150 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 131

Figure 5.6 Scanning electron micrograph of a Miocene chalk showing the characteristic
microstructure of cocolith fragments. Scattered discoasters (star - shaped objects) are also

present. The width of the micrograph is 22 μ m. (From Scholle (1978) . Reproduced with per-
mission from the American Association of Petroleum Geologists.)
failures. Typical basinal facies associations include laminites that represent the
ambient condition on the basinal floor of ramps and shelves, rhythmites that mark

the distal parts of event bedding on any platform type, and distal turbidites that
extended long distances from their source, mainly slopes and shelf margins. Depend-
ing on seawater and interstitial water chemistry, basinal facies may be rich in organic
carbon, chert or chalcedony, or evaporites.
Chalks may have been deposited in basinal or shallow subtidal environments
depending on the physiography of the basin. As we mentioned earlier, chalk deposi-
tion was particularly widespread during the Late Cretaceous and Early Tertiary in
Europe, North America, and the Middle East. Most of the North American and
Middle Eastern chalks are shallow subtidal deposits, but some of the European
chalks were deposited in shallow basins. Chalk consists mainly of calcitic microfos-
sils called cocoliths and discoasters along with various foraminifera (Figure 5.6 ).
Shallow - water chalks also contain benthic organisms such as mollusks and echino-
derms that add coarser grains to the chalk texture and aragonite to its mineralogical
composition. Some European chalks contain abundant chert, or flint, nodules that

are probably derived from siliceous sponge spicules. Metastable aragonite is more
reactive to dissolution and reprecipitation than calcite; therefore chalks with large
amounts of benthic megafossils may exhibit reduced porosity and permeability
compared to purely pelagic chalks (Neugebauer, 1975 ). The main cause of porosity
loss in chalks is compaction. A number of studies including those by Neugebauer
(1973) , Scholle (1977) , Herrington et al. (1991) , and Brasher and Vagle (1996) found