Sequence stratigraphy methods: from base level to Wheeler diagrams

Every stratigraphic package is the product of two competing rates: the creation or destruction of space available for sediment (accommodation, A) and the amount of sediment delivered to that space (supply, S). Accommodation itself combines eustatic sea level, tectonic subsidence or uplift, and compaction. What an interpreter can observe is not the individual controls but their sum — the base-level cycle — and the shoreline trajectory it produces.

When A > S the shoreline moves landward and the succession is transgressive; when A < S it moves basinward and the succession is regressive. Systems tracts are simply the named segments of that cycle, and the surfaces between them are the turnaround points: the onset of transgression (TS), maximum flooding (MFS), the onset and end of relative fall (sequence boundary and its correlative conformity).

In GeoSequence: Load LAS curves, run QC, and set the stratigraphic framework before any tract is computed. open the module →

The original Exxon model (Vail et al., 1977; Van Wagoner et al., 1988) defines a depositional sequence as the interval between two unconformities and their correlative conformities. Two boundary types are distinguished:

• SB1 — a sharp, erosional boundary formed when relative fall exceeds the shelf break; incised valleys, abrupt basinward shift of facies.
• SB2 — a weak, non-erosional boundary formed when fall stops above the shelf break; recognisable as a subtle facies offset rather than a truncation.
• TS — the transgressive surface, first significant landward shift above the boundary.
• MFS — maximum flooding surface, the most landward position of the shoreline; usually the cleanest and most laterally continuous marker in a well.
• DLS — the downlap surface at the base of prograding clinoforms of the highstand wedge.

In practice the Vail scheme is fast and works well on data with clear erosional boundaries — an incised, sand-prone shelf setting being the classic case. Its weakness is that SB1/SB2 depend on where the shelf break is, which is often unknown in a single-well interpretation.

In GeoSequence: The Vail panel auto-detects SB1/SB2 from the selected curve with adjustable thresholds, and accepts manual boundaries. open the module →

Catuneanu (2006; and the later syntheses) removed the dependence on a specific boundary definition by anchoring the framework in the shoreline trajectory. Systems tracts are defined by the type of shoreline shift they record, and each is bounded by the surface that marks the corresponding turnaround. The result is model-independent: the same four tracts can be built in a siliciclastic or carbonate setting, and the interpreter chooses which surface is used as the sequence boundary rather than being forced into one.

Siliciclastic versus carbonate

In siliciclastic systems supply is largely independent of water depth, so regressive tracts thicken basinward and log trends are strongly progradational. Carbonate factories, by contrast, produce most sediment in shallow, illuminated water — supply collapses during rapid rise (give-up) and peaks during slow rise (keep-up). The same tract labels apply, but the expected thickness and facies patterns are different, which is why the facies model must be chosen before running an automatic tract detector.

In GeoSequence: The Catuneanu panel supports siliciclastic and carbonate facies models and three surface sources: from cycle markers, pure curve detector, or hybrid. open the module →

The three are not rivals so much as different bounding conventions applied to the same base-level cycle. A robust interpretation picks the surfaces once, then re-labels the sequence under whichever convention the project requires — which is exactly why an interpretation tool should store surfaces and tracts separately.

With logs only, the workflow is: (1) pick a supply-sensitive curve — usually gamma-ray or a shale-volume derivative; (2) separate the long-wavelength trend from bed-scale noise; (3) mark trend turnarounds as candidate surfaces; (4) assign tracts to the intervals between them; (5) cross-check against curve shape and core.

Curve shape carries facies information independently of the tract logic (Muromtsev, 1984): a funnel (upward-cleaning) shape indicates progradation, a bell (upward-shaling) shape indicates retrogradation or channel fill abandonment, a cylinder indicates aggradation at steady supply, and an irregular shape usually flags a condensed or reworked interval. Agreement between shape and tract is the first quality check on any automatic interpretation.

In GeoSequence: The correlation board displays tracts from several methods side by side across wells, with editable tops. open the module →

Sequences are hierarchical: a first-order sequence contains parasequence sets, which contain parasequences. Picking turnarounds by eye mixes those orders. A continuous wavelet transform of the log separates them explicitly — power at long periods traces the sequence-scale cycle, power at short periods traces bed sets.

The practical procedure is to compute a scalogram, choose the ridge whose period matches the order you want to map, and convert the ridge extrema into cycle markers. Because the choice of ridge is explicit, the hierarchical order of the resulting interpretation is documented rather than implied.

In GeoSequence: The wavelet panel computes the scalogram and converts ridge maxima into cycle markers feeding the tract detectors. open the module →

A depth section shows rock; a Wheeler diagram shows time. By remapping each genetic package to constant thickness between its bounding horizons, the vertical axis becomes stratigraphic time. Intervals with no preserved rock — hiatus, erosion, non-deposition — then appear as blank gaps instead of disappearing into a thin or missing bed. That is the fastest way to see where a sequence is truly absent versus merely condensed.

The Railway diagram is the one-horizon-at-a-time variant: the volume is flattened on each horizon in turn, so stratigraphic windows open one after another and onlap or truncation geometries can be traced surface by surface.

In GeoSequence: 2D and 3D Wheeler, Railway mode, stratal (sedimentation) slicing and a volumetric Wheeler cube are all available in the seismic module. open the module →

On seismic data the same framework is built from reflection geometry. Reflections approximate timelines, so terminations — onlap, toplap, truncation, downlap — identify the surfaces directly. A workable order of operations:

1. Condition the display: AGC, band-pass, instantaneous-attribute or semblance volumes improve pickability without altering the stored amplitudes.
2. Pick seed points on a strong loop and auto-track — 2D on the section, region-growing in 3D with a waveform-correlation threshold.
3. Type each horizon lithostratigraphically: erosional surface, sequence boundary, intrusion contact.
4. Define zones between horizon pairs and label them as systems tracts; cross-check against wells projected onto the section.
5. Flatten to Wheeler / stratal slices to inspect depositional patterns in plan view.

In GeoSequence: SEG-Y import (2D profiles and 3D volumes), attributes, interactive auto-correlation with a confidence slider, zones, minimap and arbitrary lines. open the module →

Before you trust an interpretation

• Is the hierarchical order stated? A surface only means something relative to the cycle order it bounds.
• Does the curve-shape facies agree with the assigned tract? Persistent disagreement usually means the trend window is wrong.
• Do MFS picks correlate between wells before the other surfaces do? If not, revisit the datum.
• In carbonates, was the carbonate facies model used rather than the siliciclastic default?
• Does core porosity/permeability separate by tract? If tracts carry no petrophysical signal, they carry no predictive value either.
• Does the Wheeler view show hiatus where the depth section shows a thin package, or is the package genuinely condensed?

Key references

• Vail, P.R., Mitchum, R.M., Thompson, S. (1977). Seismic stratigraphy and global changes of sea level. AAPG Memoir 26.
• Van Wagoner, J.C. et al. (1988). An overview of the fundamentals of sequence stratigraphy and key definitions. SEPM Special Publication 42.
• Galloway, W.E. (1989). Genetic stratigraphic sequences in basin analysis. AAPG Bulletin 73.
• Catuneanu, O. (2006). Principles of Sequence Stratigraphy. Elsevier — plus the later standardisation papers.
• Muromtsev, V.S. (1984). Electrometric geology of sand bodies — lithological traps of oil and gas (in Russian).
• Wheeler, H.E. (1958). Time-stratigraphy. AAPG Bulletin 42.

Every method described here is implemented in GeoSequence, a browser-based sequence-stratigraphy workbench with a desktop build for large seismic volumes.