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{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Evaluating source rocks
| frompg = 6-1
| topg = 6-41
| author = Carol A. Law
| link = http://archives.datapages.com/data/specpubs/beaumont/ch06/ch06.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==What is pyrolysis?==
Pyrolysis is the decomposition of organic matter by heating in the absence of oxygen. Organic geochemists use pyrolysis to measure richness and maturity of potential source rocks. In a pyrolysis analysis, the organic content is pyrolyzed in the absence of oxygen, then combusted. The amount of hydrocarbons and carbon dioxide released is measured. The most widely used pyrolysis technique is Rock-Eval.
==Rock-eval pyrolysis==
In Rock-Eval pyrolysis, a sample is placed in a vessel and is progressively heated to [[temperature::550°C]] under an inert atmosphere. During the analysis, the hydrocarbons already present in the sample are volatized at a moderate temperature. The amount of hydrocarbons are measured and recorded as a peak known as S<sub>1</sub> Next pyrolyzed is the kerogen present in the sample, which generates hydrocarbons and hydrocarbon-like compounds (recorded as the S<sub>2</sub> peak), CO<sub>2</sub>, and water.<ref name=ch06r12>Tissot, B., P., Welte, D., H., 1984, Petroleum Formation and Occurrence, 2 ed.: New York, Springer-Verlag, 699 p. The best overall reference for petroleum geochemistry.</ref> The CO<sub>2</sub> generated is recorded as the S<sub>3</sub> peak. Residual carbon is also measured and is recorded as S<sub>4</sub>.
The diagram below shows the cycle of analysis and the corresponding recording.
[[file:evaluating-source-rocks_fig6-1.png|thumb|{{figure number|6-1}}. Copyright: Tissot and Weite, 1984; courtesy Springer-Verlag,.]]
==Pyrolysis indices==
The table below lists the Rock-Eval pyrolysis peaks and explains what they represent.
{| class = "wikitable"
|-
! Peak
! Is a measurement of…
! Comment
|-
| S<sub>1</sub> mg Hc/g rock
| The free hydrocarbons present in the sample before the analysis
| Can be thought of as a residual hydrocarbon phase. When S<sub>1</sub> is large relative to S<sub>2</sub> , an alternative source such as migrated hydrocarbons or contamination should be suspected
|-
| S<sub>2</sub> mg Hc/g rock
| The volume of hydrocarbons that formed during thermal pyrolysis of the sample
| Used to estimate the remaining hydrocarbon generating potential of the sample
|-
| S<sub>3</sub> mg Co2/g rock
| The CO<sub>2</sub> yield during thermal breakdown of kerogen
| Most prevalent in calcareous source rocks.
|-
| S<sub>4</sub> mg carbon/g rock
| The residual carbon content of the sample
| Residual carbon content of sample has little or no potential to generate hydrocarbons due to a lack of hydrogen and the chemical structure of the molecule
|}
==Estimating TOC with pyrolysis==
The percent TOC is actually a value that is calculated, not measured directly, using the following formula:
:<math>\%\mbox{TOC} = [0.082(\mbox{S}_{1} + \mbox{S}_{2}) + \mbox{S}_{4}]/10</math>
Units are usually given as wt % organic carbon per weight of dry rock (milligrams hydrocarbon per gram of rock).
==See also==
* [[Evaluating source rock richness]]
* [[Measuring a rock's ability to generate hydrocarbons]]
* [[Using the LECO method to estimate richness]]
* [[Using conventional well logs to estimate richness]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch06/ch06.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Evaluating source rocks]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Evaluating source rocks
| frompg = 6-1
| topg = 6-41
| author = Carol A. Law
| link = http://archives.datapages.com/data/specpubs/beaumont/ch06/ch06.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==What is pyrolysis?==
Pyrolysis is the decomposition of organic matter by heating in the absence of oxygen. Organic geochemists use pyrolysis to measure richness and maturity of potential source rocks. In a pyrolysis analysis, the organic content is pyrolyzed in the absence of oxygen, then combusted. The amount of hydrocarbons and carbon dioxide released is measured. The most widely used pyrolysis technique is Rock-Eval.
==Rock-eval pyrolysis==
In Rock-Eval pyrolysis, a sample is placed in a vessel and is progressively heated to [[temperature::550°C]] under an inert atmosphere. During the analysis, the hydrocarbons already present in the sample are volatized at a moderate temperature. The amount of hydrocarbons are measured and recorded as a peak known as S<sub>1</sub> Next pyrolyzed is the kerogen present in the sample, which generates hydrocarbons and hydrocarbon-like compounds (recorded as the S<sub>2</sub> peak), CO<sub>2</sub>, and water.<ref name=ch06r12>Tissot, B., P., Welte, D., H., 1984, Petroleum Formation and Occurrence, 2 ed.: New York, Springer-Verlag, 699 p. The best overall reference for petroleum geochemistry.</ref> The CO<sub>2</sub> generated is recorded as the S<sub>3</sub> peak. Residual carbon is also measured and is recorded as S<sub>4</sub>.
The diagram below shows the cycle of analysis and the corresponding recording.
[[file:evaluating-source-rocks_fig6-1.png|thumb|{{figure number|6-1}}. Copyright: Tissot and Weite, 1984; courtesy Springer-Verlag,.]]
==Pyrolysis indices==
The table below lists the Rock-Eval pyrolysis peaks and explains what they represent.
{| class = "wikitable"
|-
! Peak
! Is a measurement of…
! Comment
|-
| S<sub>1</sub> mg Hc/g rock
| The free hydrocarbons present in the sample before the analysis
| Can be thought of as a residual hydrocarbon phase. When S<sub>1</sub> is large relative to S<sub>2</sub> , an alternative source such as migrated hydrocarbons or contamination should be suspected
|-
| S<sub>2</sub> mg Hc/g rock
| The volume of hydrocarbons that formed during thermal pyrolysis of the sample
| Used to estimate the remaining hydrocarbon generating potential of the sample
|-
| S<sub>3</sub> mg Co2/g rock
| The CO<sub>2</sub> yield during thermal breakdown of kerogen
| Most prevalent in calcareous source rocks.
|-
| S<sub>4</sub> mg carbon/g rock
| The residual carbon content of the sample
| Residual carbon content of sample has little or no potential to generate hydrocarbons due to a lack of hydrogen and the chemical structure of the molecule
|}
==Estimating TOC with pyrolysis==
The percent TOC is actually a value that is calculated, not measured directly, using the following formula:
:<math>\%\mbox{TOC} = [0.082(\mbox{S}_{1} + \mbox{S}_{2}) + \mbox{S}_{4}]/10</math>
Units are usually given as wt % organic carbon per weight of dry rock (milligrams hydrocarbon per gram of rock).
==See also==
* [[Evaluating source rock richness]]
* [[Measuring a rock's ability to generate hydrocarbons]]
* [[Using the LECO method to estimate richness]]
* [[Using conventional well logs to estimate richness]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch06/ch06.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Evaluating source rocks]]