Geological interpretation - 3-D Seismic Interpretation - page 107

latter task is aided by the concepts of sequence stratigraphy, which uses unconformity
surfaces to define boundaries of packages of rocks that are of similar age and deposited
within a related family of depositional systems.
Galloway (1998) gives several examples of the way in which it may be possible to
infer sand distribution from detailed seismic mapping. For example, channel systems
are often seen in submarine fan, fluvial, and deltaic environments. Channels scour their
beds and banks during periods of high-volume flow, and deposit sediment within and
around the channel during periods of lower flow. Channels may be straight to highly
sinuous in plan view, broad to narrow, and shallow to deep. They may be largely
erosional (depositing little sediment beyond their banks) or depositional, building large
levees. In general, muddy systems tend to have narrow, deep sinuous channels with
prominent levees; in such a system, sands are often narrow isolated lenticular bodies.
Sand-rich systems, on the other hand, tend to have broad, low-sinuosity channels that
do not have well-developed levees. Another type of sand deposition is the result of
unconfined fluid flow. This is most obviously found in a marine shelf or aeolian setting,
but also occurs within other environments, such as crevasse splays along rivers and
turbidite lobes in submarine fans.
Within a depositional system, sandy reservoir and muddy seal associations show pre-
dictable patterns. For example, in a fluvial system the best sands are found as channel-fill
deposits (e.g. point bars). Crevasse splays along the channel banks may contain sands
deposited in small branching flood channels that are poorly connected to the sands of
the channel fill. Muddy deposits in abandoned channels may segment the top of the
channel-fill sand with shale plugs; levees on opposite sides of a channel may not be in
pressure communication if the channel is mud-filled. Vertical as well as lateral facies
changes may be predicted. For instance, in map view a delta consists of the delta plain
with a network of distributaries, the delta front with beaches, tidal flats and channel
mouths, and the submarine delta shoreface and muddy prodelta. As the delta builds out
across the shelf, a corresponding vertical succession is formed: a basal muddy prodelta
facies is overlain by delta front sands, capped by lenticular distributary channel fill units
and sealed by mudstones deposited when delta lobes are abandoned and transgressed
by the sea. The distinctive contribution of 3-D seismic is that mapping of these indi-
vidual units will be much more reliable than can be achieved with a 2-D grid, and so
inferences based on the shape of the bodies (e.g. channel sinuosity) will be much more
reliable.
The overall depositional setting can be elucidated using the concepts of sequence
stratigraphy. A useful summary of current thinking on this topic has been given by Read-
ing & Levell (1996) . As originally published, there was considerable emphasis on cycles
of sea-level change as the cause of sequence development and the main control on strati-
graphic facies; charts were published purportedly showing global sea-level behaviour
over geological time (Vail et al. , 1977) . The concept of a universally valid, global, sea-
level curve has been questioned by many authors (see, for example, Underhill, 1991) ,