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{{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 = Sedimentary basin analysis
| frompg = 4-1
| topg = 4-123
| author = John M. Armentrout
| link = http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==What is an age model?==
Detailed correlation of depositional sequences and the calculation of [[maturation]] and timing of generation vs. trap formation requires an age model for the stratigraphy of a study area. An age model is a chart showing the chronostratigraphic relationship of different depositional sequences and associated formations within a study area. Integration of biostratigraphy and depositional sequences and their correlation to a global geologic time scale provides such an age model. Using this age model to calibrate each depositional sequence lets us calculate geologic rates, such as rates of rock accumulation and burial and thermal heating rates of the stratigraphic section.
==Procedure==
The chart below outlines the procedure for constructing an age model.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Construct a depositional sequence chart for the study area. Use all available depositional sequence and biostratigraphic data.
|-
| 2
| Normalize all available sequence charts for the basin, including the study area sequence chart, to the same time scale using the bioevent marker taxa or zonal assemblages.
|-
| 3
| Make a sum of sequences curve by integrating the depositional sequence chart for the study area with the other sequence charts for the basin.
|-
| 4
| Calibrate the sum of sequences curve to a global time scale using global biostratigraphic zones, magnetostratigraphic polarity scales, oxygen isotope chronology, and global sea level cycle charts.
|}
==Example==
Constructing depositional cycle charts for the GOM basin extends back to at least Kolb and Van Lopik<ref name=ch04r55>Kolb, C., R., Van Lopik, J., R., 1958, Geology of the Mississippi River deltaic plain, southeastern Louisiana: U., S. Army Engineer Waterway Experiment Station, Corps of Engineers, Vicksburg, MS, Technical Report 3-483, 120 p.</ref> and Frasier<ref name=ch04r33>Frasier, D., E., 1974, Depositional episodes: their relationship to the Quaternary stratigraphic framework in the north-western portion of the Gulf basin: University of Texas at Austin, Bureau of Economic Geology Circular 74-1.</ref> with Beard et al..<ref name=ch04r18>Beard, J., H., Sangree, J., B., Smith, L., A., 1982, Quaternary chronology, paleoclimate, depositional sequences, and eustatic cycles: AAPG Bulletin, vol. 66, p. 158–169.</ref> demonstrating the link between depositional sequences and glacial eustasy. The following figure is a composite chronostratigraphic chart that serves as an age model for the GOM basin Pliocene and Pleistocene, summarizing nine studies published between 1982 and 1993. The local cycle charts from each of these studies have been calibrated to the same time scale using the same bioevent marker taxa and are in turn correlated to the global foraminiferal zones and magnetostratigraphic polarity scale as defined by Berggren et al.<ref name=ch04r19>Berggren, W., A., Kent, D., V., Van Couvering, J., A., 1985, The Neogene: part 2. Neogene geochronology and chronostratigraphy in Snelling, N., J., ed., The Chronology of the Geologic Record: Blackwell Scientific Publishing and Geological Society of London Memoir 10, p. 211–260.</ref> and the oxygen isotope chronology of Joyce et al.<ref name=ch04r51>Joyce, J., E., Tjalsma, L., R., C., Prutzman, J., M., 1990, High-resolution planktic stable isotope record and spectral analysis for the last 5., 35 myr: ODP site 625 northeast Gulf of Mexico: Paleoceanography, vol. 5, p. 507–529.</ref> The resulting sum of the depositional sequences and their associated condensed sections (Schaffer, 1987a, b, .<ref name=ch04r72>Pacht, J., A., Bowen, B., E., Bearn, J., H., Schaffer, B., L., 1990, [[Sequence stratigraphy]] of Plio–Pleistocene depositional facies in the offshore Louisiana south additions: Gulf Coast Assoc. of Geological Societies Transactions, vol. 40, p. 1–18.</ref> are illustrated.
The composite of all the local studies appears under the column Sum of Sequences, three of which occur in only one or two studies and are considered to be local and possibly autocyclic events (locally forced redistribution of sediments). The youngest six cycles of the chart occur between the ''Pseudoemiliani lacunosa'' bioevent (0.8 Ma) and the sea floor (0.0 Ma) and average 130,000 years in duration. The ten older cycles were deposited between ''Globigerinoides mitra'' (4.15 Ma) and ''P. lacunosa'' (0.8 Ma) bioevents and average 330,000 years duration. These 16 cycles are interpreted as regionally significant and allocyclic (forced by changes external to the sedimentary unit). They are probably glacioeustatic cycles. (See Figure 4-25 and accompanying discussion.)
Using this age model to calibrate each depositional cycle helps us calculate geologic rates, such as rates of rock accumulation and burial and thermal heating rates of the stratigraphic section.
The following figure shows a rock thickness vs. time plot for nine key wells south of eastern Louisiana within the area of the 6-4 Ma depocenter (Figure 4-13; see also <ref name=ch04r31>Fiduk, J., C., Behrens, E., W., 1993, A comparison of Plio-Pleistocene to Recent sediment accumulation rates in the East Breaks area, northwestern Gulf of Mexico: Proceedings, Gulf Coast Section SEPM 14th Annual Research conference, p. 41–55.</ref> Each major depositional interval is characterized by changes in depositional rates from oldest to youngest, in large part due to the geographic shifting of depositional centers. Dating within the wells is based on key biostratigraphic marker species for the deep-water environments of the GOM basin. Interval B is characterized by high rates of sedimentation associated with abundant gravity-flow sand deposition. It is followed by interval C, characterized by slow sedimentation and deposition of regionally effective top seal.
==Example of [[modeling]] oil generation==
The figure on the following page shows the rock accumulation rates for the Green Canyon 166 No. 1 well as a histogram (lower graph) and as a set of burial history curves (upper graph). Using temperature data from exploration wells, Piggott and Pulham.<ref name=ch04r75>Piggott, N., Pulham, A., 1993, Sedimentation rate as the control on hydrocarbon sourcing, generation, and [[migration]] in the deepwater Gulf of Mexico: Proceedings, Gulf Coast Section SEPM 14th Annual Research conference, p. 179–191.</ref> calculated temperature thresholds for the accumulated stratigraphic section. Burial of potential marine [[source rock]] above a temperature of approximately [[temperature::100°C]] could initiate generation of oil.
The dominant hydrocarbon type in the Green Canyon area is associated with hydrocarbon family 6 (Figure 4-5), suggesting a Jurassic source rock. This source rock is indicated by the diamond labeled S and the shaded stratigraphic intervals. Based on the calculation of Piggott and Pulham<ref name=ch04r75 />), using BP Exploration's Theta [[Modeling]], generation of significant oil from a Jurassic source rock may have begun approximately 6 Ma in the Green Canyon 166 No. 1 well area when the Jurassic source rock was buried below [[depth::5000 m]] and above a temperature of [[temperature::120°C]], the threshold for significant oil generation (see “[[Petroleum system]]s”).
These calculations of rock accumulation and source rock maturation rates are dependent on good age models. Biostratigraphic data are the primary correlation tools in the GOM basin, as in most basins. Considerable care must be used in correlating basin bioevents to the global geologic time scale. The methodology for and problem of such correlations are discussed in Armentrout and Clement<ref name=ch04r10>Armentrout, J., M., Clement, J., F., 1990, Biostratigraphic calibration of depositional cycles: a case study in High Island–Galveston–East Breaks areas, offshore Texas: Proceedings, Gulf Coast Section SEPM 11th Annual Research Conference, p. 21–51.</ref> and Armentrout<ref name=ch04r7>Armentrout, J., M., 1991, Paleontological constraints on depositional modeling: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene–Pleistocene, Gulf of Mexico, in Weimer, P., Link, M., H., eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 137–170.</ref><ref name=ch04r9>Armentrout, J., M., 1996, High-resolution sequence biostratigraphy: examples from the Gulf of Mexico Plio–Pleistocene, in Howell, J., Aiken, J., eds., High Resolution [[Sequence stratigraphy]]: Innovations and Applications: The Geological Society of London Special Publication 104, p. 65–86.</ref>
==See also==
* [[Sea level cycle phase]]
* [[Determining sea level cycle order]]
* [[Sea level cycle phase and systems tracts]]
* [[Identifying systems tracts]]
* [[Systems tracts and trap types]]
* [[Identifying sea level cycle phase with biostratigraphy]]
* [[Biofacies and changing sea level]]
* [[Superimposed sea level cycles]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.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:Sedimentary basin analysis]]
| 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 = Sedimentary basin analysis
| frompg = 4-1
| topg = 4-123
| author = John M. Armentrout
| link = http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
==What is an age model?==
Detailed correlation of depositional sequences and the calculation of [[maturation]] and timing of generation vs. trap formation requires an age model for the stratigraphy of a study area. An age model is a chart showing the chronostratigraphic relationship of different depositional sequences and associated formations within a study area. Integration of biostratigraphy and depositional sequences and their correlation to a global geologic time scale provides such an age model. Using this age model to calibrate each depositional sequence lets us calculate geologic rates, such as rates of rock accumulation and burial and thermal heating rates of the stratigraphic section.
==Procedure==
The chart below outlines the procedure for constructing an age model.
{| class = "wikitable"
|-
! Step
! Action
|-
| 1
| Construct a depositional sequence chart for the study area. Use all available depositional sequence and biostratigraphic data.
|-
| 2
| Normalize all available sequence charts for the basin, including the study area sequence chart, to the same time scale using the bioevent marker taxa or zonal assemblages.
|-
| 3
| Make a sum of sequences curve by integrating the depositional sequence chart for the study area with the other sequence charts for the basin.
|-
| 4
| Calibrate the sum of sequences curve to a global time scale using global biostratigraphic zones, magnetostratigraphic polarity scales, oxygen isotope chronology, and global sea level cycle charts.
|}
==Example==
Constructing depositional cycle charts for the GOM basin extends back to at least Kolb and Van Lopik<ref name=ch04r55>Kolb, C., R., Van Lopik, J., R., 1958, Geology of the Mississippi River deltaic plain, southeastern Louisiana: U., S. Army Engineer Waterway Experiment Station, Corps of Engineers, Vicksburg, MS, Technical Report 3-483, 120 p.</ref> and Frasier<ref name=ch04r33>Frasier, D., E., 1974, Depositional episodes: their relationship to the Quaternary stratigraphic framework in the north-western portion of the Gulf basin: University of Texas at Austin, Bureau of Economic Geology Circular 74-1.</ref> with Beard et al..<ref name=ch04r18>Beard, J., H., Sangree, J., B., Smith, L., A., 1982, Quaternary chronology, paleoclimate, depositional sequences, and eustatic cycles: AAPG Bulletin, vol. 66, p. 158–169.</ref> demonstrating the link between depositional sequences and glacial eustasy. The following figure is a composite chronostratigraphic chart that serves as an age model for the GOM basin Pliocene and Pleistocene, summarizing nine studies published between 1982 and 1993. The local cycle charts from each of these studies have been calibrated to the same time scale using the same bioevent marker taxa and are in turn correlated to the global foraminiferal zones and magnetostratigraphic polarity scale as defined by Berggren et al.<ref name=ch04r19>Berggren, W., A., Kent, D., V., Van Couvering, J., A., 1985, The Neogene: part 2. Neogene geochronology and chronostratigraphy in Snelling, N., J., ed., The Chronology of the Geologic Record: Blackwell Scientific Publishing and Geological Society of London Memoir 10, p. 211–260.</ref> and the oxygen isotope chronology of Joyce et al.<ref name=ch04r51>Joyce, J., E., Tjalsma, L., R., C., Prutzman, J., M., 1990, High-resolution planktic stable isotope record and spectral analysis for the last 5., 35 myr: ODP site 625 northeast Gulf of Mexico: Paleoceanography, vol. 5, p. 507–529.</ref> The resulting sum of the depositional sequences and their associated condensed sections (Schaffer, 1987a, b, .<ref name=ch04r72>Pacht, J., A., Bowen, B., E., Bearn, J., H., Schaffer, B., L., 1990, [[Sequence stratigraphy]] of Plio–Pleistocene depositional facies in the offshore Louisiana south additions: Gulf Coast Assoc. of Geological Societies Transactions, vol. 40, p. 1–18.</ref> are illustrated.
The composite of all the local studies appears under the column Sum of Sequences, three of which occur in only one or two studies and are considered to be local and possibly autocyclic events (locally forced redistribution of sediments). The youngest six cycles of the chart occur between the ''Pseudoemiliani lacunosa'' bioevent (0.8 Ma) and the sea floor (0.0 Ma) and average 130,000 years in duration. The ten older cycles were deposited between ''Globigerinoides mitra'' (4.15 Ma) and ''P. lacunosa'' (0.8 Ma) bioevents and average 330,000 years duration. These 16 cycles are interpreted as regionally significant and allocyclic (forced by changes external to the sedimentary unit). They are probably glacioeustatic cycles. (See Figure 4-25 and accompanying discussion.)
Using this age model to calibrate each depositional cycle helps us calculate geologic rates, such as rates of rock accumulation and burial and thermal heating rates of the stratigraphic section.
The following figure shows a rock thickness vs. time plot for nine key wells south of eastern Louisiana within the area of the 6-4 Ma depocenter (Figure 4-13; see also <ref name=ch04r31>Fiduk, J., C., Behrens, E., W., 1993, A comparison of Plio-Pleistocene to Recent sediment accumulation rates in the East Breaks area, northwestern Gulf of Mexico: Proceedings, Gulf Coast Section SEPM 14th Annual Research conference, p. 41–55.</ref> Each major depositional interval is characterized by changes in depositional rates from oldest to youngest, in large part due to the geographic shifting of depositional centers. Dating within the wells is based on key biostratigraphic marker species for the deep-water environments of the GOM basin. Interval B is characterized by high rates of sedimentation associated with abundant gravity-flow sand deposition. It is followed by interval C, characterized by slow sedimentation and deposition of regionally effective top seal.
==Example of [[modeling]] oil generation==
The figure on the following page shows the rock accumulation rates for the Green Canyon 166 No. 1 well as a histogram (lower graph) and as a set of burial history curves (upper graph). Using temperature data from exploration wells, Piggott and Pulham.<ref name=ch04r75>Piggott, N., Pulham, A., 1993, Sedimentation rate as the control on hydrocarbon sourcing, generation, and [[migration]] in the deepwater Gulf of Mexico: Proceedings, Gulf Coast Section SEPM 14th Annual Research conference, p. 179–191.</ref> calculated temperature thresholds for the accumulated stratigraphic section. Burial of potential marine [[source rock]] above a temperature of approximately [[temperature::100°C]] could initiate generation of oil.
The dominant hydrocarbon type in the Green Canyon area is associated with hydrocarbon family 6 (Figure 4-5), suggesting a Jurassic source rock. This source rock is indicated by the diamond labeled S and the shaded stratigraphic intervals. Based on the calculation of Piggott and Pulham<ref name=ch04r75 />), using BP Exploration's Theta [[Modeling]], generation of significant oil from a Jurassic source rock may have begun approximately 6 Ma in the Green Canyon 166 No. 1 well area when the Jurassic source rock was buried below [[depth::5000 m]] and above a temperature of [[temperature::120°C]], the threshold for significant oil generation (see “[[Petroleum system]]s”).
These calculations of rock accumulation and source rock maturation rates are dependent on good age models. Biostratigraphic data are the primary correlation tools in the GOM basin, as in most basins. Considerable care must be used in correlating basin bioevents to the global geologic time scale. The methodology for and problem of such correlations are discussed in Armentrout and Clement<ref name=ch04r10>Armentrout, J., M., Clement, J., F., 1990, Biostratigraphic calibration of depositional cycles: a case study in High Island–Galveston–East Breaks areas, offshore Texas: Proceedings, Gulf Coast Section SEPM 11th Annual Research Conference, p. 21–51.</ref> and Armentrout<ref name=ch04r7>Armentrout, J., M., 1991, Paleontological constraints on depositional modeling: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene–Pleistocene, Gulf of Mexico, in Weimer, P., Link, M., H., eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 137–170.</ref><ref name=ch04r9>Armentrout, J., M., 1996, High-resolution sequence biostratigraphy: examples from the Gulf of Mexico Plio–Pleistocene, in Howell, J., Aiken, J., eds., High Resolution [[Sequence stratigraphy]]: Innovations and Applications: The Geological Society of London Special Publication 104, p. 65–86.</ref>
==See also==
* [[Sea level cycle phase]]
* [[Determining sea level cycle order]]
* [[Sea level cycle phase and systems tracts]]
* [[Identifying systems tracts]]
* [[Systems tracts and trap types]]
* [[Identifying sea level cycle phase with biostratigraphy]]
* [[Biofacies and changing sea level]]
* [[Superimposed sea level cycles]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.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:Sedimentary basin analysis]]