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 Visualization of Channel
Systems
Figure 1 illustrates a Pleistocene deep-water depositional environment
on the basin floor of the Gulf of Mexico. Both seismic stratigraphy and
seismic geomorphology are employed in the analysis of the stratigraphic
succession and the prediction of geologic facies distribution.
In
addition, more far-reaching sequence stratigraphic insights can be
derived as well.
The
stratigraphic architecture, as shown by the seismic section, reveals a
condensed section as suggested by the high-amplitude reflection that can
be correlated over a large area. Immediately overlying this is a subtly
mounded moderate-to-high-amplitude reflection package.
Seismic
geomorphological analysis of this stratigraphic unit reveals that it is
composed of a leveed channel feeding a frontal splay or turbidite fan
lobe, composed of multiple bifurcating channels. The geological
interpretation of this map pattern is that of a turbidite system
consisting of numerous shallow channel-levee deposits, likely resulting
in a near sheet-like deposit of sand. Detailed slicing of the seismic
volume reveals that the system gradually evolves from a distributary
channel complex (i.e., the frontal splay) to a single-leveed channel
crossing the study area (Figure 1).
The
interpretation of the succession shown in
Figure 1 suggests that this
deep-water environment was a site of low rate of deposition, resulting
in deposition of a widespread condensed section. Presumably at that
time, river systems on the shelf were not capable of delivering
significant volumes of sediment to the slope or basin beyond. This
situation must have abruptly changed, as evidenced by the deposition of
deep-water turbidites in the form of a channel feeding a frontal splay
deposited directly over the condensed section.
The
interpreter could surmise that shelf fluvial systems were now delivering
their sediment load directly to the upper slope and ultimately to the
basin floor, possibly as a result of sea-level fall, which would have
had the effect of shifting depocenters from the inner/middle shelf to
the outer shelf. Subsequently, the gradual change from splay complex to
isolated leveed channel within the deep-water study area suggests a
progressive shutdown of the sediment supplied from the shelf.
Specifically, the interpreter could suggest that the sand:mud ratio
delivered to the deepwater was progressively diminishing, possibly as a
result of sea-level rise and backstepping of depocenters on the shelf.
Figure 2
also illustrates the value of integrating seismic stratigraphy and
seismic geomorphology. Shown are the stratigraphy and geomorphology of a
shelf-edge environment. The stratigraphic section shows the presence of
a shelf-edge, prograding system, likely a shelf-edge delta.
The base
of these prograding deposits is characterized by a gullied surface;
these gullies are most densely distributed in the area nearest the
thickest part of the prograding system.
Ultimately, one of these gullies dominates and captures the bulk of the
flow from the associated fluvial system, as expressed by the large
single slope channel shown in section view.
Visualization of Channel
Systems
The power
of visualization is illustrated in Figure 3, which shows a basin-floor leveed channel in perspective view. The channel is apparently
sand-filled, as suggested by the raised core of the channel caused by
differential compaction effect.
Two
avulsion nodes can be observed. These are locations where flows have cut
through the levee walls and established new channels in the overbank
area. Note that the channel is not sand-filled upstream of the avulsion
nodes, but rather is incised there.
Each of
these examples is that of a Pleistocene shallow-buried system. These
shallow-buried examples are very well imaged and provide the interpreter
with information that can be exported to similar deposits more deeply
buried but more poorly imaged.
Such
near-seafloor analogs have proven invaluable in the understanding of
deep-water depositional processes and, consequently, in our ability to
predict geologic relationships in advance of drilling. The integration
of seismic stratigraphy and seismic geomorphology is rapidly becoming a
mainstream style of analysis, necessarily involving both geologists and
geophysicists. This approach promises to further mitigate risk
associated with geologic prediction, as ever more stratigraphic/geologic
insights are extracted from 3-D seismic data.
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