|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|
Molecular fossils include any distinctive organic molecule or molecular fragment recovered from rocks, sediments, fossils, or oils that can be reliably associated with one or a few biotic antecedents. The term “molecular fossil” is limited here to compounds used to identify source organisms and to interpret corresponding paleoenvironments; it is not synonymous with “biomarker.”
Utility of molecular fossils
Some molecular fossils can be correlated with source organisms that were restricted to a limited range of environments. Recognizing these molecules in sediments or in oil provides information about the paleoenvironment of the sediment or source.
Recognition of biotic antecedent
Extracting high abundances of specific molecular fossils from source rocks dominated by a single type of organism is a reliable means of demonstrating connection. A less certain approach, apparently valid in select cases, is based on analogy: we can infer a molecular fossil in a modern organism, identical to a compound from ancient strata, was derived from a closely related ancient organism.
The match of molecular fossil to biotic antecedent and the use of this knowledge for paleo-environmental interpretation is often a challenge.
- Data are dispersed in the literature of diverse scientific disciplines.
- Modern gas chromatograph-mass spectrometer analyses are not available for many important groups of living organisms.
- Only a few molecular fossils can be correlated to biotic antecedents confined to a narrow range of environments.
Thus, Treibs' discovery that derivatives of chlorophyll are widespread in oils is not useful in defining paleoenvironments because chlorophyll is environmentally ubiquitous. Volkman dispells an earlier concept that an abundance of C29 sterols is evidence of terrigenous organic matter by finding abundant C29 sterols in lacustrine environments.
Molecular fossils useful in diagnosing paleoenvironments in select cases include those that reflect fundamental metabolic processes and have a unique biotic antecedent. Some of these molecular fossils show great durability in terms of geologic time and can be recognized from oils and rocks as old as Precambrian. Three groups of molecular fossils meet these criteria:
- Phytanyl glycerol ethers
Major evolutionary changes in the biosphere have had geochemical expression. For this reason, the molecular fossil content of many Paleozoic oils stands in distinct contrast to that of younger oils, and oils from some geological intervals are characterized by distinctive molecular fossil associations.
Molecular fossils can be particularly useful in discriminating among marine, lacustrine, and terrestrial source rocks.
- Marine source rocks are typified by a high diversity of molecular fossils, particularly sterols. Certain carotenoids, ketones, and alkenes are only known to occur in marine plankton.
- Lacustrine sources are characterized by the presence of botryococcane, which originates in the lacustrine alga Botryococcus, and of abundant carotenoids and cyanobacterial products.
- Terrestrial molecular fossils are derived from terrestrial plants, particularly as part of the durable cutin and associated waxes in plant leaves.
- Azevedo, D., F. Aquino Mato, B. Simoneit, and A. Pinto, 1992, Novel series of tricyclic aromatic terpanes characterized in Tasmanian tasmanite: Organic Geochemistry, v. 18, p. 9–16, DOI: 10.1016/0146-6380(92)90138-N.
- Treibs, A., 1934, On the occurrence of chlorophyll derivates in an oil shale of the Upper Triassic Period: Annales Chimie 509, p. 103–114.
- Volkman, J. K., 1986, A review of sterol markers for marine and terrigenous organic matter: Organic Geochemistry, vol. 9, p. 83–99, DOI: 10.1016/0146-6380(86)90089-6.