Palynomorphs (organic-walled microfossils)
|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|
Organic-walled microfossils are composed of entirely unmineralized proteinaceous material. There are four types of organic-walled microfossils: chitinozoans, spores and pollen, acritarchs, and dinoflagellates.
Chitinozoans are marine organic-walled, flask-shaped microfossils (50 μm to 2 mm0.002 m
0.0787 in in size) that occur in rocks of Ordovician to Devonian age. The biological affinities of chitinozoans are poorly understood, but they may be eggs of marine metazoans. They are excellent biostratigraphic indices and useful paleoenvironmental markers. They also have potential as thermal maturity indices (see Thermal maturation).
Figure 1 shows some typical chitinozoans.
Spores and pollen
Spores and pollen are parts of the reproductive cycle of plants and range in age from Late Ordovician and Carboniferous, respectively, to Holocene. Although land derived, the grains can be carried by wind and water currents into marine and nonmarine (particularly lacustrine and fluviatile) environments. The type and relative abundance of spores and pollen provide useful paleoenvironmental and paleoclimatic information, and they are widely used for basinal and regional stratigraphic correlation. Spores and pollen are also very useful in estimating thermal maturity, especially at temperature levels associated with hydrocarbon generation (see “Thermal Maturity”).
Figure 2 shows some typical spores and pollen. The first, second, and last drawings are pollen; the third is fungal (spore); and the fourth is fern (spore).
Acritarchs are marine microplankton of unknown biological affinity ranging from Precambrian to Holocene in age. They are excellent biostratigraphic indices for Proterozoic through Devonian strata but are less important in the Mesozoic and Cenozoic. Acritarchs occur abundantly in fine-grained rocks and are geographically widespread. They have been used for paleoecology, paleogeography and thermal maturity analysis.
Figure 3 shows some typical acritarchs.
Dinoflagellates are the resting cysts of marine, unicellular red algae. They occur abundantly in Upper Triassic to Holocene sediments and are excellent biostratigraphic indices because of their rapid evolution and widespread geographic distribution. Dinoflagellate cysts occur predominantly in marine rocks but also are present in Cretaceous and Cenozoic lacustrine facies. The morphology and diversity of dinoflagellate assemblages can be used to differentiate marine environments.
Figure 4 shows some typical dinoflagellates.
- Microfossils in exploration
- Calcareous microfossils
- Agglutinated microfossils
- Siliceous microfossils
- Phosphatic microfossils