Definitions of depositional system elements
|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|
Depositional cycle vs. sequence
The term depositional cycle refers to time through which one complete cycle of relative sea level change occurs. The sediments deposited during one such cycle are called a depositional sequence.
A depositional sequence is bounded by unconformities or the correlative conformities and is subdivided by internal surfaces of transgression and maximum flooding. Each of these surfaces is chronostratigraphically significant, separating consistently older strata from younger strata.
An alternative concept of defining a depositional sequence is that of Galloway. Galloway uses the maximum flooding surface and correlative condensed section as the bounding surface of the “genetic” depositional sequence. Both sequence concepts use the erosional unconformities, maximum flooding surface, and transgressive surface as interpretation horizons for partitioning each sequence. Sequence surfaces are often best recognized on seismic reflection profiles by stratal terminations called lapouts, such as downlap and onlap.
Maximum flooding surface
The maximum flooding surface represents the greatest transgression of shallow marine facies within a sequence. This is typically associated with a downlap surface formed by the progradation of the overlying highstand systems tract. Not all downlap surfaces are associated with maximum flooding surfaces.
The transgressive surface is the first significant marine flooding surface across the shelf. Above this surface, shallow marine facies shift landward dramatically.
Systems tracts are composed of all deposits accumulating during one phase of relative sea level cycle, such as lowstand systems tract or highstand systems tract.
An age model is the chronostratigraphic relationship of different depositional sequences.
A biofacies is an assemblage of organisms (living or fossil) found together because they responded to similar environmental conditions.
Microfossil abundance patterns
Microfossil abundance patterns are relative high and low peaks in the number of microfossils found in a sample or set of samples. They most often indicate sedimentation rates. Intervals with slow rates of sediment accumulation have consequent concentrations of abundant fossils and are associated with maximum flooding and transgressive surfaces. Intervals with high rates of sedimentation usually have low fossil abundances due to dilution and are often associated with sequence boundaries.
Illustration of sequence boundaries
The following figure illustrates the bounding surfaces for sequences. The GOM basin analysis example in this chapter is based primarily on well log and seismic data interpretation using this passive margin sequence stratigraphic model. Different models are necessary for different settings, such as a foreland basin or a rift basin
- Identifying depositional sequences
- Identifying depositional sequences in seismic sections
- Identifying depositional sequences from biostratigraphic data
- Recognizing stacked depositional sequences in seismic profiles
- Recognizing stacked depositional sequences from well data
- Mitchum, R. M., Jr., 1977, Seismic stratigraphy and global changes in sea level, 11: Glossary of terms used in seismic stratigraphy, in Seismic Stratigraphy—Applications in Hydrocarbon Exploration: AAPG Memoir 26, p. 205–212.
- 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.
- 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.
- 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.
- Galloway, W. E., 1989a, Genetic stratigraphic sequences in basin analysis I: architecture and genesis of flooding-surface bounded depositional units: AAPG Bulletin, vol. 73, p. 125–142.
- Galloway, W. E., 1989b, Genetic stratigraphic sequences in basin analysis II: application to northwest Gulf of Mexico Cenozoic basin: AAPG Bulletin, vol. 73, p. 143–154.
- Armentrout, J. M., and J. F Clement, 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.
- Van Wagoner, J. C., and G. T. Bertram, eds., 1995, Sequence stratigraphy of Foreland Basin Deposits: AAPG Memoir 64, 487 p.
- Prosser, S., 1993, Rift-related linked depositional systems and their seismic expression, in C. D. Williams, and A. Dobb, eds., Tectonic and Seismic Sequence Stratigraphy: Geological Society Special Publication 71, p. 35–66.