Systems tracts identification

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
Store	AAPG Store

Certain types of hydrocarbon traps are more commonly associated with a particular depositional systems tract. Identifying the highstand, lowstand, or transgressive systems tract and the specific depositional environments within each lets us predict possible reservoir, seal, and charge system for each potential trap.

Methods

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  Figure 1 .

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  Figure 2 .

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  Figure 3 .

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  Figure 4 .

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  Figure 5 .

Identifying a depositional systems tract can be achieved by analyzing seismic geometries (Figures 1 and 3), wireline logs motif (Figure 4), and biostratigraphic data (Figures 2 and 5). Carefully integrating multiple data sets increases the probability of a correct interpretation.

Stratal pattern simulation

Figure 6 After Vail. and Loutit et al.

Figure 7 Modified. Copyright: Haq et al. (1988); courtesy SEPM.

Figure 6 shows the computer-simulated stacking pattern of stratal units within an unconformity-bound depositional sequence. [For computer modeling, see Jervey.] The simulation forces all sediment to be deposited within the 2-D plain of the diagram. In the natural world, the depositional thicks associated with each systems tract are likely to occur lateral to each other, and their recognition requires a 3-D data set. Additionally, postdepositional deformation and erosion significantly modify the idealized geometry shown in Figures 6 and 7.

Parasequences

For transgressive and regressive shallow-water facies, each of the depositional layers is called a parasequence; they stack into parasequence sets. More basinal facies deposited well below wave influence reflect gravity-flow processes and are not called parasequences. The depositional sequence lithofacies diagram is presented in Posamentier and Vail for siliciclastics and in Sarg for carbonate rocks.

Interpreting parasequence sets

Stratal geometries that show parasequences stacked into sets that forestep progressively toward the basin center reflect progradation from the sediment supply exceeding the accommodation space; those that stack into sets that backstep progressively toward the basin margin reflect transgression from an increase in accommodation space that exceeds the sediment supply (Figure 4-22). Progradation of parasequence sets basinward of their age-equivalent shelf edge are, by definition, the lowstand prograding complex; parasequence sets prograding from the basin margin to the age-equivalent shelf margin may be either highstand prograding complexes or shelf margin systems tracts. The absence of a well-defined shelf/slope break complicates recognition of highstand vs. lowstand systems tracts.

Interpretation of stratal patterns example

Relative changes in sea level can also be inferred from detailed analysis of local depositional geometries on seismic reflection profiles. On the seismic reflection profile schematic in Figure 8 clinoforms 1-5 pinch out with toplap against a common horizon, suggesting oblique clinoforms. These oblique clinoforms can be interpreted as forming when sediment supply exceeds the accommodation space and causes shelf-margin progradation; sea level falls at the same rate as subsidence, completely bypassing the shelf with no accumulation of seismic-scale topset beds. Clinoforms 6 and 7 are sigmoidal These can be interpreted as sediment supply exceeding accommodation space, forcing progradation but with subsidence exceeding the relative change in sea level and consequent accumulation of topset beds. The change from no topset beds to aggradational topset beds indicates a turnaround from apparent still-stand to apparent rise in sea level at the site of deposition.

Time significance of seismic reflections

Figure 8 . Copyright: Armentrout (1987); courtesy Gulf Coast SEPM.

Using seismic reflection geometries to suggest relative sea level phase requires confidence in the coeval character of seismic reflections. The first downhole occurrence of Glob alt (Globoquadrina altispira, bold arrows) in Figure 4-27 suggests a correlation cross-cutting the seismically imaged clinoforms. If the Glob alt occurrences are coeval, the seismic reflections are time transgressive.

Note that the first downhole well-cutting sample occurrence of the bioevent Glob alt is at the interface of outer neritic and upper bathyal biofacies, except in the two southern wells, A446-1 and A267-1, where the first occurrences occur within stratigraphic intervals containing bathyal biofacies. Glob alt is a planktonic foraminifer normally found associated with open marine faunas and floras interpreted as upper bathyal assemblages. The occurrences of Glob alt coincident with the first upper bathyal biofacies assemblage suggests a facies-controlled top, depressed below the true extinction top by environmental factors. The two occurrences within upper bathyal biofacies are interpreted as true extinction events. These true extinction events correlate with a seismic reflection, suggesting that specific reflection approximates a time line and can be used to extend the Glob alt extinction event datum (2.8 Ma) northward toward the basin margin (see .

This type of bioevent analysis is essential when identifying chronostratigraphically useful bioevents and demonstrating that seismic reflections approximate time lines.

See also

• Sea level cycle phase
• Determining sea level cycle order
• Sea level cycle phase and systems tracts
• Systems tracts and trap types
• Identifying sea level cycle phase with biostratigraphy
• Biofacies and changing sea level
• Constructing age model charts
• Superimposed sea level cycles

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