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Using conventional well logs to estimate richness (view source)
Revision as of 13:34, 24 January 2014
, 13:34, 24 January 2014Initial import
{{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
}}
Conventional well logs are useful for estimating source rock richness both qualitatively and quantitatively. Well logs allow a qualitative identification of organic-rich formations and a quantitative analysis of the amount of organic matter. The advantages of using well logs over cuttings are continuous sampling, more accurate depth control, and greater vertical resolution.<ref name=ch06r7>Herron, S., L., 1991, In situ evaluation of potential source rocks by wireline logs, in Merrill, R., K., ed., Source and Migration Processes and Evaluation Techniques: AAPG Treatise of Petroleum Geology, Handbook of Petroleum Geology, p. 127–134.</ref>
==Using well logs==
The use of conventional well logs to predict organic richness of a unit of rock requires calibrating well log intervals to samples measured for TOC using pyrolysis or another suitable measurement technique. Using the calibrated logs, we can extrapolate TOC values to uncalibrated logs to estimate source rock richness over large areas.
'''Note:''' Consider changes in depositional environment and maturity when applying these techniques.
==Effects of organic matter on well logs==
The increasing concentration of organic matter in a rock directly affects its properties by lowering density slowing sonic velocity increasing radioactivity raising resistivity and raising hydrogen and carbon contents. All of these [[attributes]] can be measured using density sonic, neutron, gamma ray, and resistivity logs. The table below (from <ref name=ch06r7 />) summarizes log responses to organic matter.
{| class = "wikitable"
|-
! Log/Property
! Response/Value for Organic Matter (OM)
! Comments
|-
| Gamma ray (GR) or uranium (U)
| High
| High GR caused by U; can be linear with OM; U not always present
|-
| Density
| Low (approx. 1 g/cm<sup>3</sup> )
| Similar to pore fluids
|-
| Neutron
| High
| Due to hydrogen in OM
|-
| Sonic
| High transit time
| Estimates vary from 150 to more than 200 μsec/ft
|-
| Resistivity
| High
| May not affect log response unless generated hydrocarbons occupy pores
|-
| Pulsed neutron
| High carbon-oxygen ratio
| Most direct measurement of carbon; needs inorganic correction
|}
==Shale response example==
The following composite log of Kimmeridge shale, North Sea, is a typical log response to a high concentration of organic matter in a shale section. The underlying Heather Formation is very silty. The source rock interval is indicated by dots with dashes through them. Note the high sonic and gamma ray readings and the low density readings. The resistivity log reads only slightly higher than the overlying shale section.
[[file:evaluating-source-rocks_fig6-2.png|thumb|{{figure number|6-2}}.]]
==Limestone response example==
The following composite log of the Upper Jurassic Hanifa limestone in the Middle East displays a typical response of organic matter in a limestone section. No shale is present in this interval. Note the high gamma ray and resistivity readings. Core analysis of this well indicates the source rock (indicated by dots with dashes through them) is mature and generating oil.
[[file:evaluating-source-rocks_fig6-3.png|thumb|{{figure number|6-3}}.]]
==Limitations==
Well-log signals are recorded for an interval thickness, which varies depending on the type of logging tool. Averaged values may not give a true picture of the richness of the source rock.
==See also==
* [[Evaluating source rock richness]]
* [[Measuring a rock's ability to generate hydrocarbons]]
* [[Using pyrolysis to estimate richness]]
* [[Using the LECO method 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
}}
Conventional well logs are useful for estimating source rock richness both qualitatively and quantitatively. Well logs allow a qualitative identification of organic-rich formations and a quantitative analysis of the amount of organic matter. The advantages of using well logs over cuttings are continuous sampling, more accurate depth control, and greater vertical resolution.<ref name=ch06r7>Herron, S., L., 1991, In situ evaluation of potential source rocks by wireline logs, in Merrill, R., K., ed., Source and Migration Processes and Evaluation Techniques: AAPG Treatise of Petroleum Geology, Handbook of Petroleum Geology, p. 127–134.</ref>
==Using well logs==
The use of conventional well logs to predict organic richness of a unit of rock requires calibrating well log intervals to samples measured for TOC using pyrolysis or another suitable measurement technique. Using the calibrated logs, we can extrapolate TOC values to uncalibrated logs to estimate source rock richness over large areas.
'''Note:''' Consider changes in depositional environment and maturity when applying these techniques.
==Effects of organic matter on well logs==
The increasing concentration of organic matter in a rock directly affects its properties by lowering density slowing sonic velocity increasing radioactivity raising resistivity and raising hydrogen and carbon contents. All of these [[attributes]] can be measured using density sonic, neutron, gamma ray, and resistivity logs. The table below (from <ref name=ch06r7 />) summarizes log responses to organic matter.
{| class = "wikitable"
|-
! Log/Property
! Response/Value for Organic Matter (OM)
! Comments
|-
| Gamma ray (GR) or uranium (U)
| High
| High GR caused by U; can be linear with OM; U not always present
|-
| Density
| Low (approx. 1 g/cm<sup>3</sup> )
| Similar to pore fluids
|-
| Neutron
| High
| Due to hydrogen in OM
|-
| Sonic
| High transit time
| Estimates vary from 150 to more than 200 μsec/ft
|-
| Resistivity
| High
| May not affect log response unless generated hydrocarbons occupy pores
|-
| Pulsed neutron
| High carbon-oxygen ratio
| Most direct measurement of carbon; needs inorganic correction
|}
==Shale response example==
The following composite log of Kimmeridge shale, North Sea, is a typical log response to a high concentration of organic matter in a shale section. The underlying Heather Formation is very silty. The source rock interval is indicated by dots with dashes through them. Note the high sonic and gamma ray readings and the low density readings. The resistivity log reads only slightly higher than the overlying shale section.
[[file:evaluating-source-rocks_fig6-2.png|thumb|{{figure number|6-2}}.]]
==Limestone response example==
The following composite log of the Upper Jurassic Hanifa limestone in the Middle East displays a typical response of organic matter in a limestone section. No shale is present in this interval. Note the high gamma ray and resistivity readings. Core analysis of this well indicates the source rock (indicated by dots with dashes through them) is mature and generating oil.
[[file:evaluating-source-rocks_fig6-3.png|thumb|{{figure number|6-3}}.]]
==Limitations==
Well-log signals are recorded for an interval thickness, which varies depending on the type of logging tool. Averaged values may not give a true picture of the richness of the source rock.
==See also==
* [[Evaluating source rock richness]]
* [[Measuring a rock's ability to generate hydrocarbons]]
* [[Using pyrolysis to estimate richness]]
* [[Using the LECO method 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]]