Thermal maturity is the extent of heat-driven reactions that alter the composition of organic matter (e.g., conversion of sedimentary organic matter to petroleum or cracking of oil to gas.) Different geochemical scales, such as vitrinite reflectance, pyrolysis Tmax, and biomarker maturity ratios can be used to indicate the level of thermal maturity of organic matter.
Many of the elements of basin modeling programs—maturation of source rocks, reservoir diagenesis, and porosity evolution—are affected by thermal and burial history. Thermal maturation data used to model these parameters are usually derived from fossils.
The following table shows which fossil material changes appearance due to thermal stress and therefore can be used as organic geothermometers.
|Fossil Material||Thermal Maturation Scale|
|Vitrinite||Vitrinite reflectance (Vr or Ro )|
|Pollen and spores||Thermal Alteration Index (TAI)|
|Ostracods||Ostracod Alteration Index (OAI)|
|Conodonts||Conodont Alteration Index (CAI)|
|Foraminifera||Foraminifera Alteration Index (FAI)|
|Graptolites||Calibrated to Ro scale|
|Scolecodonts||Calibrated to Ro scale|
|Chitinozoans||Calibrated to Ro scale|
Vitrinite is a coaly organic maceral derived from the connective tissue of vascular plants. The reflectance of vitrinite changes with heat. Vitrinite reflectance (Vr or Ro), the most commonly used thermal indicator, is the benchmark for maturation studies in the petroleum and coal industries. This technique is primarily useful for Devonian and younger clastic sediments and coals.
Pollen and spores
Pollen and spores are the organic-walled microfossils most commonly used for gauging paleotemperature. Fossil color, which changes with heating, is used to estimate a thermal alteration index, or TAI. The use of pollen and spores lets us examine in situ fossils rather than evaluate an aggregate “kerogen soup.” Other organic-walled fossils—acritarchs, chitinozoans, graptolites, scolecodonts (annelid worm jaws), and dinoflagellates—have been examined for their visual and reflected values, but these fossil groups have not been rigorously calibrated to the standard vitrinite reflectance scale.
Fossils composed of phosphate (conodonts), carbonate (ostracods), and agglutinated grains (agglutinated foraminifera) are also used for geothermometry. The organic framework of these fossils responds to thermal stress with color change. Of these, conodonts (conodont alteration index, or CAI) are the most widely used and conodont alteration values are calibrated to the vitrinite reflectance scale. The use of ostracods and foraminifera is a newly emerging approach and is not yet calibrated to vitrinite reflectance standards. The potential of these fossils is important because they commonly occur in lithologies devoid of organic-walled fossil remains (e.g., limestones, dolomites, fine-grained sands).
Whole kerogen analysis
Geochemists use a wide variety of organic compounds, derived from both terrestrial (land and aquatic) and marine organisms, to determine depositional environments and thermal maturation. These analyses assay whole kerogen assemblages and include elemental analyses (gas chromatography–mass spectrometry, or GC-MS), Fourier transform infrared spectroscopy (FTIR), biomarkers, spectral fluorescence, and Rock-Eval pyrolysis.
Use of multiple techniques
Since none of these techniques is infallible in quantifying thermal maturation history for all conditions, two or more should be used when possible as a cross-check of the maturation data. Caution is necessary, however, when equating thermal values to petroleum generation windows because different organic materials generate at different times during thermal exposure (e.g., early generators vs. late generators). This affects transformation ratios used in hydrocarbon systems modeling.
- Applications of biostratigraphy
- Biostratigraphic correlation and age determination
- Paleoenvironmental analysis
- Quantitative paleoenvironmental analysis
- Palynofacies and kerogen analysis
- Biostratigraphy in sequence stratigraphy
- Peters, Kenneth E., David J. Curry, and Marek Kacewicz, 2012, An overview of basin and petroleum system modeling: Definitions and concepts, in Peters, Kenneth E., David J. Curry, and Marek Kacewicz, eds., Basin modeling: New horizons in research and applications: AAPG Hedberg Series no. 4, p. 1-16.
- van Gizjel, P., 1980, Characterization and identification of kerogen and bitumen and determination of thermal maturation by means of qualitative and quantitative microscopical techniques, in How to Assess Maturation and Paleotemperatures: SEPM Short Course Notes, p. 1–56.
- Pradier, B., Bertrand, P., Martinez, L., Laggoun-Defarge, F., 1991, Fluorescence of organic matter and thermal maturity assessment: Organic Geochemistry, vol. 17, no. 4, p. 511–524, DOI: 10.1016/0146-6380(91)90115-Z.
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- Staplin, F., L., 1969, Sedimentary organic matter, organic metamorphism, and oil and gas occurrence: Bulletin of Canadian Petroleum Geology, vol. 17, no. 1, p. 47–66.
- Lerche, I., McKenna, T., C., 1991, Pollen translucency as a thermal maturation indicator: Journal of Petroleum Geology, vol. 14, no. 1, p. 19–36, DOI: 10.1111/jpg.1991.14.issue-1
- Marshall, J., E., A., 1991, Quantitative spore colour: Journal of the Geological Society of London, vol. 148, p. 223–233, DOI: 10.1144/gsjgs.148.2.0223
- Bertrand, R., Heroux, Y., 1987, Chitinozoan, graptolite and scolecodont reflectance as an alternative to vitrinite and pyrobitumen reflectance in Ordovician and Silurian strata, Anticosti Island, Quebec, Canada: AAPG Bulletin, vol. 41, p. 951–957.
- Epstein, A. G., Epstein, J. B., Harris, L. D., 1977, Conodont color alteration—an index to organic metamorphism: U., S. Geological Survey Professional Paper 995, p. 1–27.
- Ainsworth, N., R., Burnett, R., D., Kontrovitz, M., 1990, Ostracod colour change by thermal alteration, offshore Ireland and Western UK: Marine and Petroleum Geology, vol. 7, p. 288–297, DOI: 10.1016/0264-8172(90)90006-3
- McNeil, D., H., Issler, D., R., 1992, Correlation of foraminiferal coloration (FCI) and time-temperature (TTI) indices from Beaufort Sea exploration data: AAPG Annual Convention Abstracts, p. 87.
- Nuccio, V., F., 1991, Combining methods yields best source rock maturity: World Oil, vol. 212, no. 9, p. 63–72.