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Pick the right key surfaces

Seismic stratigraphy is an approach that allows the geoscientist interpreter to extract stratigraphic and depositional environment information and insight from seismic data. One of the critical steps in any rigorous seismic stratigraphic workflow is the definition of key surfaces, which form the basis and the framework for any further qualitative or quantitative analysis.

Key surfaces are seismic reflectors of regional extent that can be defined and mapped on the basis of seismic reflection terminations, internal reflection configuration, and rigorous integration with any available well data. Five principal types of reflection terminations are associated with key seismic stratigraphic surfaces: onlap, downlap, toplap, truncation, and concordant. The internal reflection configuration is the seismic proxy for sedimentary stratal patterns; recording and documenting it gives valuable insight into depositional processes and products. Commonly six types of reflection configurations are used for seismic key surface definition: parallel, subparallel, divergent, prograding, chaotic, and reflection-free.

Types of seismic key surfaces

Seismic stratigraphic key surfaces are of two types: sequence boundary and maximum flooding surface (MFS); several criteria are used to properly define them.

Sequence boundary — probably the most abused key seismic surface. Inexperienced seismic stratigraphic interpreters may call any strong and/or continuous reflector a sequence boundary. A correct definition is not only an academic issue; it has significant implications for depositional history and stratigraphic and depositional environment predictions. We use the following criteria to define a sequence boundary:

1. onlap of coastal deposits onto the boundary;
2. erosion and incision on the shelf;
3. toplap or apparent toplap below the surface;
4. a major basinward shift in sedimentary facies across it.

Genetically, a sequence boundary is the result of an abrupt relative sea-level lowering, resulting in subaerial exposure, erosion, and fluvial incision of the shelf. During this time, sediment generally bypasses the shelf area and is deposited on the slope or basin floor via gravity-flow processes.

Maximum flooding surface — a continuous draping event resulting from regionally extensive and significant water deepening. It is commonly recognized on the seismic because of downlap reflection terminations onto the surface. Maximum flooding surfaces are commonly overlain by a regionally extensive marine mudstone or shale and, as a result, abrupt changes in incremental overpressure are present across them. Genetically, maximum flooding is the result of a rapid relative sea-level rise resulting in the shoreline migrating to its maximum landward position.

Maximum flooding surfaces and sequence boundaries are the key bounding surfaces in a depositional sequence. They are used to define the systems tracts which, together with the depositional environment and sedimentary facies, are used to interpret the sedimentary evolution of a basin or area. If the sequence stratigraphic analysis is used in the context of a petroleum system evaluation, the proper key surface definition and interpretation plays a major role in the prediction of reservoir, seal, and source rock.

For lots more on seismic stratigraphic key surfaces, or seismic sequence stratigraphy in general, visit the SEPM Sequence Stratigraphy Web at sepmstrata.org.

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