Microfossils in exploration
|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|
Durable microscopic fossils have a wide distribution in time and space, and their rapid and irreversible evolution and morphologically distinctive evolutionary stages make them excellent tools for measuring relative geologic time. They are particularly useful in hydrocarbon exploration because they can be recovered from both cuttings and cores. Microfossil groups that are too rare in most sedimentary rocks, too limited in overall stratigraphic or paleoenvironmental range, or too poorly understood to be broadly useful in industrial application are not considered in this section.
Utility of microfossils in exploration
In an operational environment, microfossils can be examined shortly after being brought to the surface in cuttings. Well-site analysis permits immediate identification of stratigraphic levels and drilling objectives, minimizing drilling time. Microfossils can also be used to accurately predict overpressured zones in advance of the drill bit.
In the office, microfossil studies allow precise local, regional, and global time-stratigraphic correlations that help in hydrocarbon prospect and trend delineation, regional stratigraphic and geologic studies, and exploitation evaluations. Analysis of microfossils helps scientists recognize paleoenvironmental distributions, which in turn helps us interpret sequence stratigraphy and reconstruct the paleogeography and paleoclimate. Some microfossils function as “paleothermometers” by undergoing irreversible color changes with postburial heating. As such, they indicate hydrocarbon maturity levels.
Microfossils useful in hydrocarbon exploration can be divided into five principal microfossil groups based on the composition of the shell (test) or hard parts. The table below shows the principal microfossil groups.
|Calcareous||Foraminifera, ostracods, calcareous nannofossils|
|Siliceous||radiolarians, diatoms, silicoflagellates|
|Organic walled||chitinozoans, pollen and spores, acritarchs, dinoflagellates (collectively, palynomorphs)|
- Calcareous microfossils
- Agglutinated microfossils
- Siliceous microfossils
- Phosphatic microfossils
- Palynomorphs (organic-walled microfossils)
- Thermal maturation
- Paleoenvironmental analysis
- Quantitative paleoenvironmental analysis
- Palynofacies and kerogen analysis