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Basement is a term used in exploration that refers to the deepest geology that can be recorded using the seismic technique. The earth is generally layered, the basement is the start of the igneous, metamorphic or crystalline rock that lies below the sedimentary or "clastic" layers and no energy is returned after passing through this boundary.
On a seismic section, the basement is easily identified by the lack of any horizons or rock interface boundaries.
Hydrocarbon bearing formations are in the sedimentary layers. Knowing the base of sediment can be useful to limit the depth of drilling for example. The shape of the interface between the basement crystalline rock and the overlaying sedimentary layers can also be used to help reconstruct the geologic history of the area.