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  = 21-56

  | topg    = 21-68

  | topg    = 21-59

  | 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

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

We can reasonably predict the location of stratigraphic or combination traps using the cross sections, seismic sections, and maps generated during an analysis of the seismic stratigraphy of a basin. This is especially true in basins containing oil or gas traps that can be used as analogs. [[Sequence stratigraphy]], interpreted from seismic, well, and outcrop data, is an effective concept for assessing the quality and location of source, seal, and reservoir rocks. However, most researchers caution against blindly applying published sequence stratigraphic models.<ref name=ch21r24>Handford, C., R., Loucks, R., G., 1993, Carbonate depositional sequences and systems tracts—responses of carbonate platforms to relative sea-level changes, in Loucks, R., G., Sarg, J., F., eds., Carbonate [[Sequence stratigraphy]]: Recent Developments and [[Applications]]: AAPG Memoir 57, p. 3–42.</ref><ref name=ch21r49>Weimer, P., Posamentier, H., W., eds., 1993, Siliciclastic Sequence Stratigraphy, Recent Developments and [[Applications]]: AAPG Memoir 58, 492 p.</ref> Exxon workers<ref name=ch21r47>Van Wagoner, J., C., Mitchum, R., M., Campion, K., M., Rahmanian, V., D., 1990, Siliciclastic Sequence Stratigraphy in Well Logs, Cores and Outcrops: Concepts for High-Resolution Correlation of Time and Facies: AAPG Methods in Exploration Series No. 7, 55 p.</ref><ref name=ch21r39>Sarg, J., F., 1988, Carbonate [[sequence stratigraphy]], in Wilgus, C., K., eds., Sea-Level Changes—An Integrated Approach: SEPM Special Publication 42, p. 155–181.</ref> made [[assumptions]] in the models they developed, mainly based on Gulf Coast geology, that might not have universal application. Any model of sequence stratigraphy used for exploration purposes should be based on local geology. Locally based models make more effective exploration tools.

We can reasonably predict the location of stratigraphic or combination traps using the [[cross section]]s, seismic sections, and maps generated during an analysis of the seismic stratigraphy of a basin. This is especially true in basins containing oil or gas traps that can be used as analogs. [[Sequence stratigraphy]], interpreted from [[Seismic data|seismic]], well, and [http://www.merriam-webster.com/dictionary/outcrop outcrop] data, is an effective concept for assessing the quality and location of source, seal, and reservoir rocks. However, most researchers caution against blindly applying published sequence stratigraphic models.<ref name=ch21r24>Handford, C. R., and R. G. Loucks, 1993, [http://archives.datapages.com/data/specpubs/seismic2/data/a168/a168/0001/0000/0003.htm Carbonate depositional sequences and systems tracts—responses of carbonate platforms to relative sea-level changes], in R. G. Loucks, and J. F. Sarg, J. F., eds., Carbonate Sequence Stratigraphy: Recent Developments and Applications: [http://store.aapg.org/detail.aspx?id=585 AAPG Memoir 57], p. 3–42.</ref><ref name=ch21r49>Weimer, P., and H. W. Posamentier, eds., 1993, Siliciclastic Sequence Stratigraphy, Recent Developments and Applications: [http://store.aapg.org/detail.aspx?id=1202 AAPG Memoir 58], 492 p.</ref> Exxon workers<ref name=ch21r47>Van Wagoner, J. C., R. M. Mitchum, K. M. Campion, and V. D. Rahmanian, 1990, Siliciclastic Sequence Stratigraphy in Well Logs, Cores and Outcrops: Concepts for High-Resolution Correlation of Time and Facies: [http://store.aapg.org/detail.aspx?id=1196 AAPG Methods in Exploration Series No. 7], 55 p.</ref><ref name=ch21r39>Sarg, J. F., 1988, Carbonate [[sequence stratigraphy]], in C. K. Wilgus, ed., Sea-Level Changes—An Integrated Approach: SEPM Special Publication 42, p. 155–181.</ref> made [[assumptions]] in the models they developed, mainly based on Gulf Coast geology, that might not have universal application. Any model of sequence stratigraphy used for exploration purposes should be based on local geology. Locally based models make more effective exploration tools.

==Procedure==

==Procedure==

Analyzing sequences for stratigraphic or combination traps is simply looking for stratigraphic changes, such as updip pinch-outs of rocks with reservoir potential or mounds of reservoir-quality rocks, in the context of a depositional sequence. Knowing where the target interval and area are within a depositional sequence gives us the ability to predict the presence of certain trap types. Follow the procedure outlined below to predict the location of traps within a sequence.

Analyzing sequences for stratigraphic or combination traps is simply looking for stratigraphic changes, such as updip pinch-outs of rocks with reservoir potential or mounds of reservoir-quality rocks, in the context of a depositional sequence. Knowing where the target interval and area are within a depositional sequence gives us the ability to predict the presence of certain trap types. Follow the procedure outlined below to predict the location of traps within a sequence.

# Using seismic lines and/or log [[cross section]]s, determine the systems tract type for intervals of interest, i.e., lowstand, transgressive, or highstand.

# Identify potential seal- and reservoir-quality rocks using seismic facies and [[lithofacies]] shown on maps and cross sections.

# In areas with juxtaposed reservoir- and seal-quality rocks, look for trapping geometries.

| Using seismic lines and/or log cross sections, determine the systems tract type for intervals of interest, i.e., lowstand, transgressive, or highstand.

| Identify potential seal- and reservoir-quality rocks using seismic facies and lithofacies shown on maps and cross sections.

| In areas with juxtaposed reservoir- and seal-quality rocks, look for trapping geometries.

==Transgressive and highstand systems tracts==

==Transgressive and highstand systems tracts==

Accommodation rates are high during transgressive–early highstand episodes of sea level, forming thick reservoirs of excellent quality. Shales in the upper transgressive systems tract and lower highstand systems tract are generally high-quality seals. Updip and bottom seals can be a problem for stratigraphic traps. Unconformity truncations, onlapping sands, and mounded shoreline sands form stratigraphic traps. Siliciclastics of the late highstand generally are poor reservoirs. Excellent source rocks are associated with the starved portion of the transgressive and early highstand systems tracts. Coals and terrestrial source rocks also are associated with the transgressive and early highstand systems tracts.

Accommodation rates are high during transgressive–early highstand episodes of sea level, forming thick reservoirs of excellent quality. Shales in the upper transgressive systems tract and lower highstand systems tract are generally high-quality seals. Updip and bottom seals can be a problem for stratigraphic traps. [[Unconformity]] truncations, onlapping sands, and mounded shoreline sands form stratigraphic traps. Siliciclastics of the late highstand generally are poor reservoirs. Excellent source rocks are associated with the starved portion of the transgressive and early highstand systems tracts. Coals and terrestrial source rocks also are associated with the transgressive and early highstand systems tracts.

 

[[file:exploring-for-stratigraphic-traps_fig21-37.png|left|thumb|{{figure number|1}.]]

[[file:exploring-for-stratigraphic-traps_fig21-37.png|350px|thumb|{{figure number|1}}Diagrammatic cross section showing six potential trap types associated with the lowstand systems tract. From Vail;<ref name=ch21r44 /> courtesy AAPG.]]

During lowstands of sea level, sedimentation rates are high. Therefore, organic source potential is generally low. Where depositional sites are euxinic, source potential is higher. Even so, total organic carbon rarely exceeds 1%.<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 No. 27], p. 2.</ref> Reservoir sands can be thick because they tend to [[Well_log_sequence_analysis#Parasequence_stacking_patterns|aggrade as well as prograde]].

During lowstands of sea level, sedimentation rates are high. Therefore, organic source potential is generally low. Where depositional sites are euxinic, source potential is higher. Even so, total organic carbon rarely exceeds 1%.<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> Reservoir sands can be thick because they tend to aggrade as well as prograde.

==Lowstand systems tract traps==

==Lowstand systems tract traps==

The diagrammatic cross section ([[:file:exploring-for-stratigraphic-traps_fig21-37.png|Figure 1]] and the corresponding table describe six potential trap types associated with the lowstand systems tract.

 

 

The diagrammatic [[cross section]] ([[:file:exploring-for-stratigraphic-traps_fig21-37.png|Figure 1]]) and the corresponding table describe six potential trap types associated with the lowstand systems tract.

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! No.

! No. || Facies || Trap description

! Facies

! Trap description

|-

|-

| 1

| 1 || Incised valley sands || Excellent reservoirs. Traps form where valley incises underlying coastal plain shales.

| Incised valley sands

| Excellent reservoirs. Traps form where valley incises underlying coastal plain shales.

|-

|-

| 2

| 2 || Coastal belt sands || Good reservoirs, commonly very thick. Rollover traps common. Strat traps depend on undip seal. If underlying unit is impermeable, they are present where onlapping sands pinch out below preceding shoreline break.

| Coastal belt sands

| Good reservoirs, commonly very thick. Rollover traps common. Strat traps depend on undip seal. If underlying unit is impermeable, they are present where onlapping sands pinch out below preceding shoreline break.

|-

|-

| 3

| 3 || Channel/overbank channel sands || Excellent reservoirs. Seal provided by toes of overlying low-stand wedge.

| Channel/overbank channel sands

| Excellent reservoirs. Seal provided by toes of overlying low-stand wedge.

|-

|-

| 4

| 4 || Overbank sands || Poor reservoirs. Seal provided by toes of overlying lowstand wedge.

| Overbank sands

| Poor reservoirs. Seal provided by toes of overlying lowstand wedge.

|-

|-

| 5

| 5 || Mounded basin floor fan sands || Sands thin or pinchout over contemporaneous highs. Strat traps depend on top and bottom seal. Overlying slope fan not a good seal. Best traps pinch out in a basinward direction.

| Mounded basin floor fan sands

| Sands thin or pinchout over contemporaneous highs. Strat traps depend on top and bottom seal. Overlying slope fan not a good seal. Best traps pinch out in a basinward direction.

|-

|-

| 6

| 6 || Shingled toe of lowstand prograding wedge sands || Good reservoirs. In sandy systems, basin floor fans are shingled and pinch out between the shale toes of lowstand prograding wedge.

| Shingled toe of lowstand prograding wedge sands

| Good reservoirs. In sandy systems, basin floor fans are shingled and pinch out between the shale toes of lowstand prograding wedge.

|}

|}

[[file:exploring-for-stratigraphic-traps_fig21-39.png|300px|thumb|{{figure number|3}}Cross section representing the reservoir properties from representative [[capillary pressure]] data.]]

Different margin types in basins have different play types determined by the geometry and history of the margin. [[:file:exploring-for-stratigraphic-traps_fig21-38.png|Figure 2]] shows play types for shelf-edge and ramp margins.

[[file:exploring-for-stratigraphic-traps_fig21-39.png|left|thumb|{{figure number|3}}See text for explanation.]]

Different margin types in basins have different play types determined by the geometry and history of the margin. [[:file:exploring-for-stratigraphic-traps_fig21-38.jpg|Figure 2]] shows play types for shelf-edge and ramp margins.

==Example: integrating petrophysics and geology==

==Example: integrating petrophysics and geology==

Unpublished data (courtesy Amoco Production Company) derived from cores and [[seismic data]] were used to build an integrated lithofacies map. [[:file:exploring-for-stratigraphic-traps_fig21-39.png|Figure 3]] is a cross section representing the reservoir properties from representative [[capillary pressure]] data. The facies belts shown in the map above the cross section were deposited during maximum highstand of the Ismay (Pennsylvanian) carbonates. The fades are superimposed on an isopach map of the highstand systems tract. Test and show data overlain on the map show that significant reservoirs are restricted generally in the ''Ivanovia'' algal mound buildups, which flank a highstand basin shown in gray.

 

Unpublished data (courtesy Amoco Production Company) derived from cores and [[seismic data]] were used to build an integrated lithofacies map. [[:file:exploring-for-stratigraphic-traps_fig21-39.png|Figure 3]] is a [[cross section]] representing the reservoir properties from representative [[capillary pressure]] data. The facies belts shown in the map above the cross section were deposited during maximum highstand of the Ismay (Pennsylvanian) carbonates. The facies are superimposed on an isopach map of the highstand systems tract. Test and show data overlain on the map show that significant reservoirs are restricted generally in the ''Ivanovia'' algal mound buildups, which flank a highstand basin shown in gray.

==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]]

[[Category:Pages with missing figures]]

[[Category:Sedimentology and stratigraphy – Clastics]]

[[Category:Treatise Handbook 3]]