Identifying depositional sequences in seismic sections

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Critical elements of the petroleum system
Chapter Sedimentary basin analysis
Author John M. Armentrout
Link Web page
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Analyzing seismic sections

We identify depositional sequences in seismic sections by finding repetitive patterns of seismic reflections. To test the validity of the sequences identified from seismic reflection profiles, we compare the seismic sequences with sequences identified from biostratigraphic and well log data to see if they make geologic sense. Identifying depositional sequences can be complicated by postdepositional erosion and deformation. It is often helpful to begin a seismic sequence analysis using a grid of relatively few profiles with an area of relatively undeformed rocks.

GOM basin example

Figure 1 Seismic reflection profile from the East Breaks field area. Modified. Copyright: Armentrout;[1] courtesy Gulf Coast SEPM.

In the shelf-margin facies of the East Breaks study area of the GOM basin, a depositional sequence in its simplest form is identified in seismic sections as a couplet consisting of two patterns:

  • Sigmoidal clinoform packages
  • Regionally extensive parallel reflections

Each clinoform package defines a locally thick progradational unit interpreted as a relative sea level lowstand delta.[2] They are lateral to other clinoform packages and are bounded above and below by regionally extensive, parallel, often uniformly high-amplitude seismic reflections. The regionally extensive parallel reflections correlate across faults and have the same relative thickness on both sides of most outershelf and upper-slope faults.

The seismic reflection profile of Figure 1, from the East Breaks field area, illustrates both clinoform and parallel reflection patterns in late Pleistocene sediments immediately below the sea floor (between two sets of bold arrows). Three listric growth faults (down arrows) cut through the clinoforms. These growth faults are part of the regional fault system bounding the shelf edge and upper slope salt-withdrawal basins in the High Island and East Breaks areas.

The arrow at the far left edge of Figure 1 marks the trough (white) between parallel, high-amplitude, continuous reflections (black) that underlie the clinoforms (best expressed toward the left side of the figure). Two up arrows show the correlation of this trough across the faults. The clinoforms toplap to the right (north) against the sea floor reflection, defining the overlying transgressive surface above the clinoform tops and below the regionally extensive sea floor reflection. Additionally, the clinoform downlaps basinward, defining a downlap surface. In this case, the downlap surface coincides with the underlying sequence boundary.[3]

Depositional cycle

The data from the High Island–East Breaks shelf-margin delta suggest that regionally extensive and uniform layers of mud occur above and below locally shingled clinoform packages. Couplets of these two depositional facies constitute a sequence of one depositional cycle. The position of the sequence at the shelf edge suggests that it is composed of a shelf margin systems tract and a condensed section. For criteria for recognizing depositional cycles in other settings, see Loucks and Sarg,[4] Steel et al.,[5] Van Wagoner and Bertram,[6] and Weimer and Posamentier.[7]

Type 1 vs. type 2 sequences

The sequence stratigraphic model includes type 1 and type 2 sequences.[8][9] A type 1 sequence boundary is interpreted to form when the rate of eustatic fall exceeds the rate of subsidence at the depositional break, producing a relative fall in sea level at that position. This usually results in an extensive erosional surface with stream incision landward of the depositional break. In contrast, a type 2 sequence boundary forms when the rate of eustatic fall is slightly less than or equal to the rate of basin subsidence at the depositional break. There is no relative fall in sea level at the depositional break, and erosion and stream incision is less than at type 1 boundaries.

Depositional break

In early publications the depositional break is referred to as the shelf break, often uniquely imaged on seismic reflection profiles.[10][11] More recently, the depositional break was referred to as the shoreline break, a position coincident with the seaward end of a stream-mouth bar in a delta or the upper shoreface in a beach environment.[12] Shoreline breaks are well imaged on highresolution seismic profiles and in well-exposed outcrop belts but are usually below resolution scale of most industry seismic reflection profiles.

Nomenclature problems

These scale differences result in nomenclature problems. The High Island–East Breaks shelf-margin delta (Figures 4-19, 4-21) fits a type 2 sequence criterion of Vail and Todd[10]) because it represents a lowstand prograding complex at the same position as preceding shelf-edge depositional breaks. As such, this lowstand is part of a type 2 depositional sequence and would be called a shelf-margin systems tract. If the criteria of Van Wagoner et al.[12] are used, the High Island–East Breaks shelf-margin delta would be called a type 1 lowstand prograding complex because the preceding shoreline break is tens of miles further north on the Texas shelf. Clarification of such scale-dependent reference points is critical to effective communication through the careful selection of precise labels for elements of depositional sequences.

See also

References

  1. ↑ Armentrout, J. M., 1993, Relative seal-level variations and fault-salt response: offshore Texas examples: Proceedings, Gulf Coast Section SEPM 14th Annual Research Conference, p. 1–7.
  2. ↑ Sutter, J. S., and H. L. Berryhill, Jr., 1985, Late Quaternary shelf-margin deltas, northwest Gulf of Mexico: AAPG Bulletin, vol. 69, p. 77–91.
  3. ↑ Armentrout, J. M., 1991, Paleontological constraints on depositional modeling: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene–Pleistocene, Gulf of Mexico, in P. Weimer, and M. H. Link, eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 137–170.
  4. ↑ Loucks, R. G., and J. F. Sarg, eds., 1993, Carbonate Sequence Stratigraphy: AAPG Memoir 57, 545 p.
  5. ↑ Steel, R. J., V. L. Felt, E. P. Johannessen, and C. Mathiew, eds., 1995, Sequence Stratigraphy on the Northwest European Margin: Elsevier, 608 p.
  6. ↑ Van Wagoner, J. C., and G. T. Bertram, eds., 1995, Sequence Stratigraphy of Foreland Basin Deposits: AAPG Memoir 64, 487 p.
  7. ↑ Posamentier, H. W., and P. Weimer, 1993, Siliciclastic sequence stratigraphy and petroleum geology: where to from here?: AAPG Bulletin, vol. 77, no. 5, p. 731–742.
  8. ↑ Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in A. W. Bally, ed., Atlas of Seismic Stratigraphy: AAPG Studies in Geology No. 27, p. 1–10.
  9. ↑ Posamentier, H. W., and P. R. Vail, 1988, Eustatic controls on clastic deposition II—sequence and systems tract models: SEPM Special Publication 42, p. 125–154.
  10. ↑ 10.0 10.1 Vail, P. R., and R. G. Todd, 1981, North Sea Jurassic unconformities, chronostratigraphy and seal-level changes from seismic stratigraphy: Proceedings, Petroleum Geology of the Continental Shelf, Northwest Europe, p. 216–235.
  11. ↑ Vail, P. R., J. Hardenbol, and R. G. Todd, 1984, Jurassic unconformities, chronostratigraphy and sea-level changes from seismic stratigraphy and biostratigraphy in J. S. Schlee, ed., Interregional Unconformities and Hydrocarbon Accumulation: AAPG Memoir 36, p. 129–144.
  12. ↑ 12.0 12.1 Mitchum, R. M., Jr., and J. C. Van Wagoner, 1990, High-frequency sequences and eustatic cycles in the Gulf of Mexico basin: Proceedings, Gulf Coast Section SEPM 11th Annual Research conference, p. 257–267.

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