|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Predicting the occurrence of oil and gas traps|
|Author||Robert L. Fleisher, H. Richard Lane|
Calcareous microfossils have shells composed of calcite or aragonite. These organisms are present in most marine and in some nonmarine environments. At great oceanic depths characterized by low temperature and high hydrostatic pressure, however, calcareous remains are largely or completely dissolved. The depth below which this occurs, which varies in different oceanographic settings, is termed the carbonate compensation depth (CCD).
There are three principal types of calcareous microfossils: calcareous foraminifera, ostracods, and calcareous nannofossils.
Calcareous foraminifera are a group of unicellular organisms (protists) that secrete a rigid calcite or aragonite shell (or test). Fossils of these forms are found in sediments of brackish to marine origin from Silurian to Holocene in age. Most are benthic (bottom dwelling), but a significant group in the late Mesozoic and Cenozoic are planktonic (floating) forms.
Some stratigraphically important foraminifera developed complex internal structures and, frequently, large test size. Studied primarily in thin section, these include the fusulinids (Pennsylvanian to Permian) and several groups of so-called larger foraminifera (Triassic to Holocene). They occur primarily in carbonate or fine-grained clastic rocks and are excellent time markers.
Because many species have limited and well-known environmental ranges, they are excellent paleobathymetric and paleoenvironmental indicators, especially in younger Phanerozoic rocks.
Figure 1 shows some typical calcareous foraminifera.
Ostracods are microscopic crustaceans whose fossils are found in Cambrian to Holocene rocks. They occur in most marine and nonmarine depositional environments and are generally excellent environmental indicators. The paleontologic application of ostracods is limited because (1) they are rare in many sections and (2) many species are endemic to local basins, so their age and environmental ranges are poorly understood. Ostracods typically have rapid evolutionary rates and are useful biostratigraphic tools in some situations:
- In Paleozoic sequences
- In marine environments where wide-ranging species are present
- For local stratigraphy in basins of limited extent
- In lacustrine environments, where they are frequently one of the few microfossils present
Ostracods may also indicate thermal maturation of source rocks.
Figure 2 shows some typical ostracods.
The term calcareous nannofossils includes both fossil coccoliths and nannoliths. Coccoliths are minute (<25μm) calcite objects produced by unicellular marine plants (golden-brown algae). The origin of nannoliths is uncertain, but these calcite bodies are associated with fossil coccoliths assemblages in marine sediments and are also organically derived.
Calcareous nannofossils are an excellent biostratigraphic tool because of their rapid evolution and geographic dispersal (i.e., their entire life cycle is in the photic zone of the ocean) as well as their varied and distinct morphologies. The oldest known calcareous nannofossils are Late Triassic; they are a crucial microfossil group in calibrating the Jurassic-Holocene marine record. Relatively little has been published about the paleogeographic distributions of calcareous nannofossils; less is known about their exact paleoenvironmental preferences, although they have been shown occasionally to penetrate into shallow marine environments. Their main industrial application is their calibration to published time scales and sequence stratigraphic records, especially the association of high abundance with condensed marine sections.
Figure 3 shows some typical calcareous nannofossils.
- Microfossils in exploration
- Agglutinated microfossils
- Siliceous microfossils
- Phosphatic microfossils
- Palynomorphs (organic-walled microfossils)