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Created page with "The amount of organic carbon present in a rock is a determining factor in a rock's ability to generate hydrocarbons. ==Effect of depositional environment== Depositional envir..."
The amount of organic carbon present in a rock is a determining factor in a rock's ability to generate hydrocarbons.
==Effect of depositional environment==
Depositional environment controls the amount of organic carbon contained in a rock. Source rocks are generally associated with areas where high organic productivity is combined with (1) deposition in poorly oxygenated environments (anoxic to dysaerobic), (2) upwelling, and (3) rapid sedimentation.<ref name=ch06r3>Chinn, E., W., 1991, The role of organic geochemistry in petroleum exploration: Basin Research Institute Bulletin, Louisiana State University, Baton Rouge, LA, p. 15–23. ''Short paper that summarizes petroleum geochemistry and its analytical techniques.''</ref> These processes preserve organic matter.
==Average TOCs==
The table below (from <ref name=ch06r3 />) shows average TOC values for different sedimentary rock types.
{| class = "wikitable"
|-
! Rock type
! TOC value, %
|-
| Average for all shales
| 0.8
|-
| Average for shale source rocks
| 2.2
|-
| Average for calcareous shale source rocks
| 1.8
|-
| Average for carbonate source rocks
| 0.7
|-
| Average for all source rocks
| 1.8
|}
==Caveat==
Use the table above only as a guide. TOC average values in the real world are meaningless unless we know how the average was calculated. For example, suppose a potential source rock unit is [[length::100 m]] thick. What was the average calculated from:
* 1 sample, [[length::1 cm]] thick
* 10 samples, [[length::10 cm]] thick, taken every 10 m
* 100 samples, [[length::1 cm]] thick, taken every 1 m
Was the sampling high-graded, taking only the richest intervals? Without answers to these questions, TOC averages have no value.
==Guidelines for assessing richness==
The table below gives guidelines for assessing the richness of source rock intervals.
{| class = "wikitable"
|-
! Generation potential
! Wt % TOC, shales
! Wt % TOC, carbonates
|-
| Poor
| 0.0-0.5
| 0.0-0.2
|-
| Fair
| 0.5-1.0
| 0.2-0.5
|-
| Good
| 1.0-2.0
| 0.5-1.0
|-
| Very Good
| 2.0-5.0
| 1.0-2.0
|-
| Excellent
| > 5.0
| > 2.0
|}
==Caveat==
If a sample being analyzed for richness is not in an immature state, then the present-day maturation level of the interval needs to be determined to establish an initial (prematuration) organic carbon value for the interval. The measured TOC value is not indicative of the sample's source potential.
==See also==
* [[Source rock richness]]
* [[Using pyrolysis to estimate richness]]
* [[Using the LECO method to estimate richness]]
* [[Using conventional well logs to estimate richness]]
==References==
{{reflist}}
==Effect of depositional environment==
Depositional environment controls the amount of organic carbon contained in a rock. Source rocks are generally associated with areas where high organic productivity is combined with (1) deposition in poorly oxygenated environments (anoxic to dysaerobic), (2) upwelling, and (3) rapid sedimentation.<ref name=ch06r3>Chinn, E., W., 1991, The role of organic geochemistry in petroleum exploration: Basin Research Institute Bulletin, Louisiana State University, Baton Rouge, LA, p. 15–23. ''Short paper that summarizes petroleum geochemistry and its analytical techniques.''</ref> These processes preserve organic matter.
==Average TOCs==
The table below (from <ref name=ch06r3 />) shows average TOC values for different sedimentary rock types.
{| class = "wikitable"
|-
! Rock type
! TOC value, %
|-
| Average for all shales
| 0.8
|-
| Average for shale source rocks
| 2.2
|-
| Average for calcareous shale source rocks
| 1.8
|-
| Average for carbonate source rocks
| 0.7
|-
| Average for all source rocks
| 1.8
|}
==Caveat==
Use the table above only as a guide. TOC average values in the real world are meaningless unless we know how the average was calculated. For example, suppose a potential source rock unit is [[length::100 m]] thick. What was the average calculated from:
* 1 sample, [[length::1 cm]] thick
* 10 samples, [[length::10 cm]] thick, taken every 10 m
* 100 samples, [[length::1 cm]] thick, taken every 1 m
Was the sampling high-graded, taking only the richest intervals? Without answers to these questions, TOC averages have no value.
==Guidelines for assessing richness==
The table below gives guidelines for assessing the richness of source rock intervals.
{| class = "wikitable"
|-
! Generation potential
! Wt % TOC, shales
! Wt % TOC, carbonates
|-
| Poor
| 0.0-0.5
| 0.0-0.2
|-
| Fair
| 0.5-1.0
| 0.2-0.5
|-
| Good
| 1.0-2.0
| 0.5-1.0
|-
| Very Good
| 2.0-5.0
| 1.0-2.0
|-
| Excellent
| > 5.0
| > 2.0
|}
==Caveat==
If a sample being analyzed for richness is not in an immature state, then the present-day maturation level of the interval needs to be determined to establish an initial (prematuration) organic carbon value for the interval. The measured TOC value is not indicative of the sample's source potential.
==See also==
* [[Source rock richness]]
* [[Using pyrolysis to estimate richness]]
* [[Using the LECO method to estimate richness]]
* [[Using conventional well logs to estimate richness]]
==References==
{{reflist}}