Sequence Boundary

Key Takeaways

• The original Vail-Posamentier sequence boundary classification distinguished Type 1 and Type 2 boundaries based on the magnitude of sea level fall relative to the shelf edge: a Type 1 boundary (the more dramatic and petroleum-geologically significant type) occurs when relative sea level falls below the shelf break, causing stream incision across the entire exposed shelf, canyon cutting at the shelf edge, and direct delivery of fluvial and shallow marine sediment to the basin floor as turbidite fans; the entire shelf is subaerially exposed during Type 1 conditions, creating a regional unconformity mappable across the shelf, and the base-level fall causes rivers to incise valleys up to 50 to 200 meters deep into the pre-existing shelf topography; a Type 2 boundary occurs when relative sea level falls only within the inner shelf (not below the shelf break), causing minor coastal plain exposure and progradation of the shoreline without shelf-wide erosion or direct turbidite shedding; later refinements of the sequence stratigraphic model by Plint (1988) and Catuneanu et al. (2009, 2011) introduced the concept of the falling-stage systems tract (FSST), which recognizes that sediment delivery to the slope and basin begins during sea level fall (before the lowstand proper) rather than only at the lowstand, modifying the original two-type classification into a more continuous spectrum of sea level fall rates and magnitudes that produce different geometries and facies distributions in the sequence boundary region.
• Incised valleys on sequence boundaries are among the most important stratigraphic petroleum traps in the geological record: when sea level falls below the fluvial equilibrium profile (the long profile of the river from headwaters to base level), the river responds by incising downward into the pre-existing sediment to re-establish its graded profile at the new base level; the incision depth depends on the magnitude of sea level fall (large fall produces deep incision), the erodibility of the shelf sediment (unconsolidated sand is incised more easily than cemented carbonate), and the drainage area and discharge of the river system (large rivers incise deeper because they carry more sediment-transporting discharge); the resulting incised valley (a topographic depression 10 to 200 meters deep and 1 to 50 kilometers wide on the ancient shelf) is subsequently filled with fluvial, estuarine, and marginal marine sediment during the subsequent sea level rise (the transgressive systems tract), with the valley fill sandstones encased laterally by the erosional valley walls (which provide lateral sealing) and sealed above by the overlying marine transgressive shale; the incised valley fill is therefore a three-dimensional stratigraphic trap that can contain significant oil and gas accumulations even where the structure is flat, as exemplified by the Viking Formation valley fills in the Western Canada Sedimentary Basin, the Woodbine Formation valley fills in East Texas, and multiple incised valley targets in the Cretaceous section of the US Rocky Mountain region.
• Carbonate sequence boundaries with subaerial exposure develop dramatically enhanced reservoir quality through meteoric diagenesis, dissolution, and karstification: when a carbonate platform is exposed at a Type 1 sequence boundary, rainwater (meteoric water) percolates through the carbonate, dissolving calcium carbonate in the vadose zone (above the water table) and in the phreatic mixing zone (where fresh meteoric water mixes with underlying saline formation water); the dissolution creates secondary porosity (vugs, molds, karst channels, and cave systems) that can increase the porosity of a tight micritic carbonate from 2 to 3 percent to 15 to 30 percent over geological time; the resulting karst porosity at the unconformity surface is the primary drilling target in many Middle East carbonate reservoirs (the karst horizons at the tops of the Jurassic Arab Zone carbonates beneath evaporite seals), in the Ordovician carbonates of the Williston Basin (Midale and Nisku formations), in the Permian carbonates of the Permian Basin (San Andres, Yates, and Grayburg formations), and in the Silurian pinnacle reefs of Michigan and the Illinois Basin; the presence of a regional unconformity above a carbonate sequence is therefore a strong predictor of reservoir-quality enhancement at the unconformity surface, making unconformity identification from seismic stratigraphy (using reflection termination patterns, amplitude character, and velocity inversions) a high-value exploration technique in carbonate basins.
• Seismic identification of sequence boundaries uses the characteristic patterns of seismic reflection termination defined by Mitchum et al. (1977): at the base of the unconformity (looking upward from below), onlap (reflections terminating landward against the unconformity surface) indicates that younger sediment has progressively filled in topographic relief created by erosion during the sequence boundary forming event; at the top of the sequence below the unconformity (looking downward from above), erosional truncation (reflections terminating abruptly by erosional removal) indicates that the upper part of the older sequence has been removed, with the erosion surface often showing irregular relief that corresponds to paleovalleys, fault scarps, or differential compaction features on the exposed shelf; the correlative conformity in the deep basin (where no erosion occurs) is identified by a change in reflection character (from prograding clinoform geometry below to aggrading or backstepping geometry above) and often by a thin, high-amplitude condensed section reflector at the correlative conformity position (because the condensed section deposits the minimum sediment volume and concentrates organic matter and authigenic minerals that contrast with the flanking coarser deposits); in salt basins, sequence boundaries may be additionally marked by changes in salt deformation style (from passive diapirism during highstand to reactive diapirism during lowstand) that alter the local structural geometry and the trap geometry for hydrocarbons in the sequence boundary region.
• Global correlation of sequence boundaries using the Vail sea level curve (and its successors, the Haq et al. (1987) Mesozoic-Cenozoic cycle chart and the Hardenbol et al. (1998) Paleogene-Neogene chart) provides a temporal framework for correlating sequence boundaries between widely separated basins without requiring direct physical continuity of stratigraphic sections: if a sequence boundary of known age (dated by biostratigraphy in wells penetrating the unconformity) correlates in age with a major eustatic sea level fall on the global cycle chart, it is inferred to reflect global eustasy rather than local tectonics; sequences that correlate globally (appearing at similar ages in basins on different continents) are more likely to be driven by eustasy, while sequence boundaries that do not correlate with the global chart are more likely to be driven by local or regional tectonics (faulting, volcanism, differential subsidence); the global correlation of sequence boundaries has practical exploration value because it allows sequence boundaries and their associated petroleum system elements (lowstand reservoirs, maximum flooding source rocks, highstand shoreface reservoirs) to be predicted in frontier basins based on the global sea level record, before wells have been drilled to calibrate the local sequence stratigraphy; this predictive application of sequence boundary correlation from global charts to regional basin analysis is a routine component of frontier basin petroleum system assessment by oil companies and national geological surveys.