Depositional sequence analysis of plays

	Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Exploring for stratigraphic traps
Author	John C. Dolson, Mike S. Bahorich, Rick C. Tobin, Edward A. Beaumont, Louis J. Terlikoski, Michael L. Hendricks
Link	Web page
Store	AAPG Store

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.^[1]^[2] Exxon workers^[3]^[4] 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

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 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

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.

Lowstand systems tracts

Figure 1 Diagrammatic cross section showing six potential trap types associated with the lowstand systems tract. From Vail;^[5] 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%.^[5] Reservoir sands can be thick because they tend to aggrade as well as prograde.

Lowstand systems tract traps

Figure 2 Play types for shelf-edge and ramp margins. From Van Wagoner et al. 1990;^[3] courtesy AAPG

The diagrammatic cross section (Figure 1) and the corresponding table describe six potential trap types associated with the lowstand systems tract.

No.	Facies	Trap description
1	Incised valley sands	Excellent reservoirs. Traps form where valley incises underlying coastal plain shales.
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.
3	Channel/overbank channel sands	Excellent reservoirs. Seal provided by toes of overlying low-stand wedge.
4	Overbank sands	Poor reservoirs. Seal provided by toes of overlying lowstand wedge.
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.
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.

Plays in different margin types

Figure 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. Figure 2 shows play types for shelf-edge and ramp margins.

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. 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.

References

↑ Handford, C. R., and R. G. Loucks, 1993, 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: AAPG Memoir 57, p. 3–42.
↑ Weimer, P., and H. W. Posamentier, eds., 1993, Siliciclastic Sequence Stratigraphy, Recent Developments and Applications: AAPG Memoir 58, 492 p.
↑ ^3.0 ^3.1 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: AAPG Methods in Exploration Series No. 7, 55 p.
↑ 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.
↑ ^5.0 ^5.1 Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in A. W. Bally, ed., Atlas of Seismic Stratigraphy: AAPG Studies in Geology No. 27, p. 2.

External links

find literature about
Depositional sequence analysis of plays

[ch21r24-1] Handford, C. R., and R. G. Loucks, 1993, 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: AAPG Memoir 57, p. 3–42.

[ch21r49-2] Weimer, P., and H. W. Posamentier, eds., 1993, Siliciclastic Sequence Stratigraphy, Recent Developments and Applications: AAPG Memoir 58, 492 p.

[ch21r47-3] 3.0 ^3.1 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: AAPG Methods in Exploration Series No. 7, 55 p.

[ch21r39-4] 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.

[ch21r44-5] 5.0 ^5.1 Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in A. W. Bally, ed., Atlas of Seismic Stratigraphy: AAPG Studies in Geology No. 27, p. 2.

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Depositional sequence analysis of plays

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