Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Exploring for stratigraphic traps

  | chapter = Exploring for stratigraphic traps

  | frompg  = 21-1

  | frompg  = 21-27

  | topg    = 21-68

  | topg    = 21-28

  | author  = John C. Dolson, Mike S. Bahorich, Rick C. Tobin, Edward A. Beaumont, Louis J. Terlikoski, Michael L. Hendricks

  | author  = John C. Dolson, Mike S. Bahorich, Rick C. Tobin, Edward A. Beaumont, Louis J. Terlikoski, Michael L. Hendricks

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch21/ch21.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch21/ch21.htm

==Objective==

==Objective==

The objective of seismic sequence analysis is to identify depositional sequences and systems tracts on seismic sections by interpreting the location of their boundaries. Boundaries are manifested as discontinuities in seismic sections and are located mainly by finding reflection terminations.<ref name=ch21r44>Vail, P., R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A., W., ed., Atlas of Seismic Stratigraphy: AAPG Studies in Geology No. 27, p. 2.</ref>

The objective of seismic sequence analysis is to identify [[Depositional sequence identification|depositional sequence]]s and [[Systems tracts identification|systems tracts]] on [[Seismic data|seismic sections]] by interpreting the location of their boundaries. Boundaries are manifested as discontinuities in seismic sections and are located mainly by finding [http://wiki.seg.org/wiki/Dictionary:Reflection reflection] terminations.<ref name=ch21r44>Vail, P. R., 1987, [http://archives.datapages.com/data/specpubs/oversiz2/data/a188/a188/0001/0000/0001.htm Seismic stratigraphy interpretation procedure], in A. W. Bally, ed., Atlas of seismic stratigraphy: [http://store.aapg.org/detail.aspx?id=478 AAPG Studies in Geology 27], p. 1-10.</ref>

==Procedure==

==Procedure==

Follow the steps listed in the table below to perform a sequence analysis of a seismic section (adapted from <ref name=ch21r44 />).

Follow the steps listed below to perform a sequence analysis of a seismic section (adapted from Vail<ref name=ch21r44 />).

# Look for places where two [http://wiki.seg.org/wiki/Dictionary:Reflector reflectors] converge. Where reflectors converge, there will be terminations.

# Mark the [http://wiki.seg.org/wiki/Dictionary:Reflection reflection] terminations with arrows.

# Draw in the discontinuity surface between the [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=onlap onlapping] and [http://www.glossary.oilfield.slb.com/en/Terms/d/downlap.aspx downlapping] reflections above, and the truncating and [http://homepage.ufp.pt/biblioteca/Seismic/Pages/Page22.htm toplapping] reflectors above. If the discontinuity surface becomes conformable, trace its position across the section by reflection correlation.

# Continue the process described in steps 1, 2, and 3 for all the seismic lines in the grid.

# Close all seismic grid loops by checking the loop ties for each discontinuity or its correlative equivalent.

# Categorize each discontinuity.

| Look for places where two reflectors converge. Where reflectors converge, there will be terminations.

#* '''If''' it is characterized by regional onlap above and truncation below '''then''' it is probably a [[Definitions_of_depositional_system_elements#Sequence_boundaries|sequence boundary]]

#* '''If''' it is characterized by regional downlap '''then''' it is most likely a downlap surface

| Mark the reflection terminations with arrows.

| Draw in the discontinuity surface between the onlapping and downlapping reflections above, and the truncating and toplapping reflectors above. If the discontinuity surface becomes conformable, trace its position across the section by reflection correlation.

| Continue the process described in steps 1, 2, and 3 for all the seismic lines in the grid.

| Close all seismic grid loops by checking the loop ties for each discontinuity or its correlative equivalent.

| Categorize each discontinuity.

 

* <table-wrap id="ch21utbl8a" position="float"> <table frame="hsides"> <colgroup> <col align="left"></col> <col align="left"></col> </colgroup> <thead> <tr> <th align="center"> If </th> <th align="center"> Then... </th> </tr> </thead> <tbody> <tr> <td align="left"> It is characterized by regional onlap above and truncation below It is probably a sequence boundary </td></tr> <tr> It is characterized by regional downlap It is most likely a downlap surface </tr> </tbody> </table> </table-wrap>

 

| <th align="center"> If</th>

| It is characterized by regional downlap

| It is most likely a downlap surface

==Marking a seismic section, marking==

==Marking a seismic section, marking==

The table below shows suggested colors for marking seismic sections during sequence analysis (after <ref name=ch21r44 />).

The table below shows suggested colors for marking seismic sections during sequence analysis (after Vail<ref name=ch21r44 />).

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Feature

! Feature || Color

! Color

|-

|-

| Reflection terminations and reflection patterns

| Reflection terminations and reflection patterns || Red

| Red

|-

|-

| Downlap surfaces

| Downlap surfaces || Green

| Green

|-

|-

| Transgressive surfaces

| Transgressive surfaces || Blue

| Blue

|-

|-

| Sequence boundaries

| Sequence boundaries || Miscellaneous

| Miscellaneous

|}

|}

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Reflection termination point

! Reflection termination point || Pattern || Associated discontinuity

! Pattern

! Associated discontinuity

|-

|-

| Above a discontinuity

| rowspan=2 | Above a discontinuity || [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=onlap Onlap] || [[Definitions_of_depositional_system_elements#Sequence_boundaries|Sequence boundary]] ([[unconformity]])

| Onlap

| Sequence boundary (unconformity)

|-

|-

| Downlap

| [http://www.glossary.oilfield.slb.com/en/Terms/d/downlap.aspx Downlap] || Downlap surface (condensed section)

| Downlap surface (condensed section)

|-

|-

| Below a discontinuity

| rowspan=3 | Below a discontinuity || Truncation || Sequence boundary

| Truncation

| Sequence boundary

|-

|-

| Toplap

| [http://homepage.ufp.pt/biblioteca/Seismic/Pages/Page22.htm Toplap] || Sequence boundary

| Sequence boundary

|-

|-

| Apparent truncation

| Apparent truncation || Downlap surface

| Downlap surface

|}

|}

==Locating reflection terminations==

==Locating reflection terminations==

Locating reflection termination is a matter of finding the patterns described in the table above. The figure below shows these patterns and the associated discontinuity surfaces. HST is highstand systems tract, LST is lowstand systems tract, and TST is transgressive systems tract.

Locating reflection termination is a matter of finding the patterns described in the table above. [[:file:exploring-for-stratigraphic-traps_fig21-18.png|Figure 1]] shows these patterns and the associated discontinuity surfaces.

 

[[file:exploring-for-stratigraphic-traps_fig21-18.png|500px|thumb|center|{{figure number|1}}Reflection termination patterns and the associated discontinuity surfaces.]]

[[file:exploring-for-stratigraphic-traps_fig21-18.png|thumb|{{figure number|21-18}}.]]

==See also==

==See also==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Exploring for stratigraphic traps]]

[[Category:Exploring for stratigraphic traps]]