- Copyright © 2001 Society of Exploration Geophysicists
Seismic sedimentology is the use of seismic data to study sedimentary rocks and the processes by which they form. It differs from classic seismic stratigraphy in that it uses mainly the horizontal, instead of the vertical, character of seismic data to provide high-resolution images of seismic attribute patterns that can be related to geomorphology and depositional models. A display of seismic attributes on geologic time surfaces is the basic tool of seismic sedimentology. However, there are strict limitations and conditions under which reservoir geometries can be optimally delineated on time and/or horizon slices. Specifically, the slice must be picked at or parallel to a geologic time-equivalent seismic event. This condition is seldom satisfied in practice because of structural dipping and lateral thickness variations of depositional sequences. Stratal slicing of 3-D seismic data overcomes some shortcomings of time slicing and horizon slicing by proportionally slicing between geologic time-equivalent seismic reference events. This facilitates the seismic mapping of sedimentary features on depositional surfaces.
Miocene deposits in Mioceno Norte, Lake Maracaibo, Venezuela, provide an excellent example of this seismic sedimentological approach. A high-resolution stratigraphic framework, based on wireline logs, defines approximately 30 genetic stratigraphic units (GSU) separated by marine flooding surfaces, lacustrine flooding surfaces, paleosols, and floodplain shales (Figure 1). These GSUs, each 50–150 ft thick, were correlated by tracing low-resistivity shale markers on logs. The origin, scale, and hierarchy of GSUs in Mioceno Norte are similar to those of depositional episodes and genetic depositional sequences defined by Galloway (1989). A GSU, typically bounded by flooding surfaces or paleosols, represents a cycle of coastal progradation commonly capped by transgressive facies and a condensed section. Detailed isopach and facies mapping based on conventional core and SP/GR log-facies patterns showed that the GSU origins are shoreface, deltaic, and fluvial, respectively.