Changes

  | part    = Critical elements of the petroleum system

  | part    = Critical elements of the petroleum system

  | chapter = Sedimentary basin analysis

  | chapter = Sedimentary basin analysis

  | frompg  = 4-1

  | frompg  = 4-53

  | topg    = 4-123

  | topg    = 4-54

  | author  = John M. Armentrout

  | author  = John M. Armentrout

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Each systems tract—highstand, lowstand, and transgressive—has a different trapping potential based on the vertical and lateral distribution of lithofacies deposited within specific depositional environments. White<ref name=ch04r112>White, D., A., 1980, Assessing oil and gas plays in facies-cycles wedges: AAPG Bulletin, vol. 64, p. 1158–1178.</ref> presents an excellent review of trap types within facies-cycle wedges, which are related to transgressive-regressive cycles and can be related most specifically to the transgressive systems tract and the highstand systems tract. In White's classification, prograding lithofacies of the lowstand systems tract might occur as subunconformity traps or might be mistakenly identified as highstand systems tract deposits. Gravity-flow deposits of slope and basin-floor fan systems are most often placed into the lowstand systems tract because they are deposited basinward of the shelf/slope inflection.

Each systems tract—highstand, lowstand, and transgressive—has a different trapping potential based on the vertical and [[lateral]] distribution of [[lithofacies]] deposited within specific depositional environments. White<ref name=ch04r112>White, D., A., 1980, Assessing oil and gas plays in facies-cycles wedges: AAPG Bulletin, vol. 64, p. 1158–1178.</ref> presents an excellent review of trap types within facies-cycle wedges, which are related to transgressive-regressive cycles and can be related most specifically to the transgressive systems tract and the highstand systems tract. In White's classification, [[Well_log_sequence_analysis#Parasequence_stacking_patterns|prograding]] lithofacies of the lowstand systems tract might occur as subunconformity traps or might be mistakenly identified as highstand systems tract deposits. [[Gravity]]-flow deposits of slope and basin-floor fan systems are most often placed into the lowstand systems tract because they are deposited basinward of the shelf/slope inflection.

White<ref name=ch04r112 /> discusses both siliciclastic and carbonate systems. Sarg<ref name=ch04r84>Sarg, J., F., 1988, Carbonate [[sequence stratigraphy]]: SEPM Special Publication 42, p. 155–181.</ref> provides an excellent discussion of carbonate systems. Only siliciclastic systems, similar to those of the Cenozoic of the central and western Gulf of Mexico, are discussed here.

White<ref name=ch04r112 /> discusses both siliciclastic and carbonate systems. Sarg<ref name=ch04r84>Sarg, J., F., 1988, Carbonate sequence stratigraphy: SEPM Special Publication 42, p. 155–181.</ref> provides an excellent discussion of carbonate systems. Only siliciclastic systems, similar to those of the [[Cenozoic]] of the central and western [[Gulf of Mexico]], are discussed here.

==Lowstand systems tract traps==

==Lowstand systems tract traps==

Lowstand gravity-flow, sand-prone reservoirs occur in basin-floor and slope systems. They are most often encased within marine hemipelagic mudstones, which serve as seal and sometimes potential [[source rock]]. Traps are often stratigraphic, but postdepositional deformation that places the gravity-flow sand deposit in a structurally high position enhances the potential for focused [[migration]] of hydrocarbon fluids to the reservoir facies.<ref name=ch04r69>Mitchum, R., M., Jr., 1985, Seismic stratigraphic expression of submarine fans: AAPG Memoir 39, p. 117–136.</ref>

Lowstand gravity-flow, sand-prone reservoirs occur in basin-floor and slope systems. They are most often encased within marine hemipelagic mudstones, which serve as seal and sometimes potential [[source rock]]. Traps are often stratigraphic, but postdepositional [[deformation]] that places the gravity-flow sand deposit in a structurally high position enhances the potential for focused [[migration]] of hydrocarbon fluids to the reservoir facies.<ref name=ch04r69>Mitchum, R., M., Jr., 1985, Seismic stratigraphic expression of submarine fans: AAPG Memoir 39, p. 117–136.</ref>

==Lowstand prograding complex traps==

==Lowstand prograding complex traps==

Siliciclastic lowstand prograding complexes, imaged on seismic reflection profiles as clinoforms, are often fluvial-deltaic complexes with abundant sand in the depositional topsets (Figures 4-19 and 4-21). As the relative sea level cycle turns around from low to rising, the coarse-grained sediment supply decreases. The fine-grained sediments of the transgressive systems tract overlie the lowstand systems tract–prograding complex sand-prone facies, providing excellent top seal to the underlying sandy reservoir. If the transgressive shales are organic rich and buried in the thermal regime for kerogen cracking, hydrocarbons will be generated. If the lithofacies forming the preceding shelf edge can provide lateral seal, the prograding complex reservoir facies may become charged with hydrocarbons even without structural enhancement of the trap.<ref name=ch04r16>Armentrout, J., M., Rodgers, B., K., Fearn, L., B., Block, R., B., Snedden, J., W., Lyle, W., D., Herrick, D., C., Nwankwo, B., 1997, Application of high resolution biostratigraphy, Oso field, Nigeria: Proceedings, Gulf Coast Section SEPM 18th Annual Research conference, p. 13–20.</ref>

Siliciclastic lowstand [[Well_log_sequence_analysis#Parasequence_stacking_patterns|prograding]] complexes, imaged on seismic reflection profiles as clinoforms, are often fluvial-deltaic complexes with abundant sand in the depositional topsets (Figures 4-19 and 4-21). As the relative sea level cycle turns around from low to rising, the coarse-grained sediment supply decreases. The fine-grained sediments of the transgressive systems tract overlie the lowstand systems tract–prograding complex sand-prone facies, providing excellent top seal to the underlying sandy reservoir. If the transgressive shales are organic rich and buried in the thermal regime for [[kerogen]] [[cracking]], hydrocarbons will be generated. If the [[lithofacies]] forming the preceding shelf edge can provide lateral seal, the prograding complex reservoir facies may become charged with hydrocarbons even without structural enhancement of the trap.<ref name=ch04r16>Armentrout, J., M., Rodgers, B., K., Fearn, L., B., Block, R., B., Snedden, J., W., Lyle, W., D., Herrick, D., C., Nwankwo, B., 1997, Application of high resolution biostratigraphy, Oso field, Nigeria: Proceedings, Gulf Coast Section SEPM 18th Annual Research conference, p. 13–20.</ref>

==Transgressive systems tract traps==

==Transgressive systems tract traps==

==Highstand systems tract traps==

==Highstand systems tract traps==

Highstand systems tracts step toward the basin center and often prograde at the expense of the preceding parasequence due to erosion during relative fall of sea level. The falling sea level also decreases the space into which the sediment can accumulate, resulting in potentially rapid lateral shifting of the prograding deltaic lobes. This results in relatively thin but widespread sand-prone facies. An effective top seal for such a highstand system would require a very major transgression well landward of the updip end of the sandy facies of the prograding coastal plain.<ref name=ch04r112 /> Such a transgression could be eustatic or tectonic in nature, as in a rapidly subsiding foreland basin setting. Postdepositional deformation forming anticlines enhances the potential for entrapping hydrocarbons in sheet-like highstand systems tract reservoirs.

Highstand systems tracts step toward the basin center and often [[Well_log_sequence_analysis#Parasequence_stacking_patterns|prograde]] at the expense of the preceding parasequence due to erosion during relative fall of sea level. The falling sea level also decreases the space into which the sediment can accumulate, resulting in potentially rapid lateral shifting of the prograding deltaic lobes. This results in relatively thin but widespread sand-prone facies. An effective top seal for such a highstand system would require a very major transgression well landward of the updip end of the sandy facies of the prograding coastal plain.<ref name=ch04r112 /> Such a transgression could be eustatic or tectonic in nature, as in a rapidly subsiding foreland basin setting. Postdepositional deformation forming anticlines enhances the potential for entrapping hydrocarbons in sheet-like highstand systems tract reservoirs.

==Shelf margin systems tract traps==

==Shelf margin systems tract traps==

==Systems tracts with greatest trapping potential==

==Systems tracts with greatest trapping potential==

White<ref name=ch04r112 />) compiles data on the depositional setting of more than 2000 major oil and gas fields in 200 transgressive and regressive wedges within 80 basins. With clearly stated qualifications, White shows that most hydrocarbons found in siliciclastic reservoirs occur in the base to middle of the wedge in generally lowstand to transgressive depositional facies. This can be attributed to the greater probability of effective top seal in contrast to the highstand systems tract. By using the stratal stacking pattern, supplemented by lithofacies and biofacies data, depositional environments can be properly identified and paleogeographic maps constructed for each systems tract to predict between and beyond data points.

White<ref name=ch04r112 />) compiles data on the depositional setting of more than 2000 major oil and gas fields in 200 transgressive and regressive wedges within 80 basins. With clearly stated qualifications, White shows that most hydrocarbons found in siliciclastic reservoirs occur in the base to middle of the wedge in generally lowstand to transgressive depositional facies. This can be attributed to the greater probability of effective top seal in contrast to the highstand systems tract. By using the stratal stacking pattern, supplemented by lithofacies and [[Fossil assemblage|biofacies]] data, depositional environments can be properly identified and paleogeographic maps constructed for each systems tract to predict between and beyond data points.

==See also==

==See also==

* [[Determining sea level cycle order]]

* [[Determining sea level cycle order]]

* [[Sea level cycle phase and systems tracts]]

* [[Sea level cycle phase and systems tracts]]

* [[Identifying systems tracts]]

* [[Systems tracts identification]]

* [[Identifying sea level cycle phase with biostratigraphy]]

* [[Identifying sea level cycle phase with biostratigraphy]]

* [[Biofacies and changing sea level]]

* [[Biofacies and changing sea level]]

* [[Constructing age model charts]]

* [[Constructing age model charts]]

* [[Superimposed sea level cycles]]

* [[Superimposed sea level cycles]]

==References==

==References==

[[Category:Critical elements of the petroleum system]]  

[[Category:Critical elements of the petroleum system]]  

[[Category:Sedimentary basin analysis]]

[[Category:Sedimentary basin analysis]]