Changes

  | part    = Critical elements of the petroleum system

  | part    = Critical elements of the petroleum system

  | chapter = Evaluating source rocks

  | chapter = Evaluating source rocks

  | frompg  = 6-1

  | frompg  = 6-28

  | topg    = 6-41

  | topg    = 6-29

  | author  = Carol A. Law

  | author  = Carol A. Law

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch06/ch06.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch06/ch06.htm

==Measuring maturity with apatite fission tracks==

==Measuring maturity with apatite fission tracks==

The continued existence of apatite fission tracks is temperature dependent. At temperatures below [[temperature::60°C]] the fission tracks exist as they were formed. However, as the temperature increases from [[temperature::60-120°C]], the length of the fission tracks in apatite crystals will decrease due to thermally induced “healing” of the crystal structure. This process is referred to as '''annealing'''.

The continued existence of apatite fission tracks is temperature dependent. At temperatures below [[temperature::60°C]] the fission tracks exist as they were formed. However, as the temperature increases from 60-[[temperature::120°C]], the length of the fission tracks in apatite crystals will decrease due to thermally induced “healing” of the crystal structure. This process is referred to as '''annealing'''.

The mean length of a fission track at the time of formation and up to [[temperature::60°C]] is 15μ. Fission tracks will completely anneal and disappear at approximately [[temperature::120°C]]. Therefore, the length of apatite fission tracks can be used as a measure of the maximum temperature that a rock has been subjected to and provides information related to thermal history.

The mean length of a fission track at the time of formation and up to [[temperature::60°C]] is 15μ. Fission tracks will completely anneal and disappear at approximately [[temperature::120°C]]. Therefore, the length of apatite fission tracks can be used as a measure of the maximum temperature that a rock has been subjected to and provides information related to thermal history.

==Using analysis with r_o==

==Using analysis with r_o==

Apatite grains are commonly found in sandstones. The amount of information provided from fission track analysis can be significant and is very complementary to vitrinite data in portions of the geologic section that are lean in organic matter. See <ref name=ch06r4>Duddy, I., R., Green, P., F., Laslett, G., M., 1988, Thermal annealing of fission tracks in apatite—3, variable temperature behaviour: Chemical Geology (Isotopic Geoscience Section), vol. 73, p. 25–38. Mathematical presentation and discussion of the thermal annealing equations of apatite., 10., 1016/0168-9622(88)90019-X</ref> and Gleadow et al., 1986, for a general overview of the interpretation and application of apatite fission track data in petroleum exploration.

Apatite grains are commonly found in sandstones. The amount of information provided from fission track analysis can be significant and is very complementary to [[Thermal maturation#Vitrinite|vitrinite]] data in portions of the geologic section that are lean in organic matter. See Duddy et al<ref name=ch06r4>Duddy, I. R., P. F. Green, and G. M. Laslett, 1988, Thermal annealing of fission tracks in apatite—3, variable temperature behaviour: Chemical Geology (Isotopic Geoscience Section), vol. 73, p. 25–38. Mathematical presentation and discussion of the thermal annealing equations of apatite, DOI: 10.1016/0168-9622(88)90019-X.</ref> and Gleadow et al<ref name=Gleadow86>Gleadow, J. W., I. R. Duddy, P. F. Green, J. E. and Lovering, 1986, Confined track lengths in apatite—a diagnostic tool for thermal history analysis: Contrib. Mineral. Petrol, 94:405-415.</ref> for a general overview of the interpretation and application of apatite fission track data in petroleum exploration.

==Caveat==

==Caveat==

If the mean age distribution of a sample as determined from apatite fission track analysis is less than the geologic age of that sample determined by other means, then that sample is interpreted to have been exposed to temperatures of > [[temperature::120&deg;C]] during its burial history. For example, a sample might have been buried to a depth where temperatures were > [[temperature::120&deg;C]]. At these high temperatures, all the fission tracks would have annealed. Then the sample might have been subsequently uplifted and exposed to temperatures

If the mean age distribution of a sample as determined from apatite fission track analysis is less than the geologic age of that sample determined by other means, then that sample is interpreted to have been exposed to temperatures of > [[temperature::120&deg;C]] during its burial history. For example, a sample might have been buried to a depth where temperatures were > [[temperature::120&deg;C]]. At these high temperatures, all the fission tracks would have annealed. Then the sample might have been subsequently uplifted and exposed to temperatures

This application of fission track data makes the technique extremely useful for evaluating the magnitude and timing of major unconformities in an area of interest.

This application of fission track data makes the technique extremely useful for evaluating the magnitude and timing of major [[Unconformity|unconformities]] in an area of interest.

==Example of fission track length interpretation==

==Example of fission track length interpretation==

[[file:evaluating-source-rocks_fig6-13.png|thumb|{{figure number|6-13}}See text for explanation.]]

[[file:evaluating-source-rocks_fig6-13.png|thumb|300px|{{figure number|1}}Unimodal distribution of relatively long (unannealed) fission tracks.]]

 

The length data in itself may not be adequate to evaluate the thermal history. For example, [[:file:evaluating-source-rocks_fig6-13.png|Figure 1]] shows a unimodal distribution of relatively long (unannealed) fission tracks. These data would need to be combined with the fission track age determination to derive thermal history information. If the fission track age of this sample was older than the depositional age, the implication would be that the sample had never been exposed to temperature in the annealing zone and most likely never experienced temperatures greater than [[temperature::60&deg;C]] to [[temperature::70&deg;C]]. In contrast, if the fission track age was less than the depositional age, the sample would have been subjected to temperatures above [[temperature::110&deg;C]] to [[temperature::120&deg;C]], where all of the original tracks would have been annealed. The sample would then have been uplifted rapidly and all of the remaining tracks would have formed since the uplift to a temperature zone less than or equal to the [[temperature::60&deg;C]] to [[temperature::70&deg;C]]. The fission track age date would provide information on the timing of this rapid uplift event.

==See also==

==See also==

[[Category:Critical elements of the petroleum system]]  

[[Category:Critical elements of the petroleum system]]  

[[Category:Evaluating source rocks]]

[[Category:Evaluating source rocks]]