Seismic stratigraphy is a method for studying

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

in the deep subsurface based on seismic data acquisition.

History

The term ''Seismic stratigraphy'' was introduced in 1977 by VailVail, P.R., et al. 1977 Seismic Stratigraphy and Global Changes of Sea Level in AAPG Memoir 26 () as an integrated stratigraphic and sedimentologic technique to interpret seismic reflection data for stratigraphic correlation and to predict depositional environments and lithology. This technique was initially employed for petroleum exploration and subsequently evolved into

sequence stratigraphy In mathematics, a sequence is an enumerated collection of mathematical object, objects in which repetitions are allowed and order theory, order matters. Like a Set (mathematics), set, it contains Element (mathematics), members (also called ''ele ...

by academic institutes.Sheriff, R.E., and Sheriff, M.S., 1980. Seismic Stratigraphy. ,

Basic Concept

Seismic reflection is generated at interfaces that separate media with different acoustic properties, and traditionally these interfaces have been interpreted as the lithological boundaries. Vail in 1977, however, recognized that these reflections were, in fact, parallel to the bedding surfaces, and therefore time equivalent surfaces. Interruption of reflections indicates the disappearance of bedding surfaces. Hence, onlap, down lap and top lap and other depositional features observed on surface outcrops have been demonstrated on seismic profiles. This revolutionary interpretation has been substantiated by Vail’s associated industrial drilling results and extensive multichannel seismic data. Furthermore, the most indisputable evidence comes from the progradational dipping reflection pattern associated with the advancing delta deposition in shallow marine environments. Lithological boundaries associated with delta front and slope are nearly horizontal, but are not represented by reflections. Instead, the dipping reflections are a clear indication of depositional surfaces, hence time plane equivalents.Payton, C.E., 1977 Seismic stratigraphy- — Applications to Hydrocarbon Exploration. AAPG memoir 26.

Methodology

Establishing Sequence Boundary

Sequence boundaries are defined as an erosional

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

recognized on the seismic profile as a reflection surface with reflection termination features such as truncation below and onlap above the surface, The sequence boundary, therefore, represents a

marine regression A marine regression is a geological process occurring when areas of submerged seafloor are exposed during a drop in sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously-exposed land. Descript ...

event, during which continental shelf is partially exposed to subaerial erosion processes. A seismic sequence is defined as the stratigraphic interval between two consecutive sequence boundaries, representing two marine regression events with a marine transgression event at the middle. Thus a seismic sequence is further subdivided with a basal unit of regressive systems tract, a transgressive systems tract at the middle, and a regressive systems tract at the top. The transgressive systems tract is marked at the top by a maximum flooding surface. Berg, O.R. and Woolverton, D.G., 1985.Seismic Stratigraphy II: An Integrated Approach to Hydrocarbon Exploration.AAPG Memoir 39.

Describing Seismic Facies

Within a systems tract, each seismic facies is mapped based on reflection geometry, continuity, amplitude, frequency, and interval velocity. The

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

of each facies is then predicted according to known depositional model and nearby drilling results. Ramsayer, G.R., 1979.Seismic Stratigraphy, a Fundamental Exploration Tool. Paper presented at the Offshore Technology Conference, Houston, Texas, April 1979. Paper Number: OTC-3568-MS

Estimating Relative Sea level Changes

Since onlaps on an erosional surface approximate the positions of sea level on a coastal plain, the sea level variation of a marine transgression/regression cycle could be estimated by the onlap positions on seismic profiles. The maximum sea-level rise is represented by the highest onlap position on a sequence boundary and the minimum sea-level fall by the lowest onlap position on the next younger sequence boundary. The difference in depth between the two positions represents the sea level change magnitude of the cycle.Wilgus C.K. et al ., 1988. Sea-Level Changes: An Integrated Approach SEPM special publication #42.

References

{{authority control Stratigraphy Geophysics