application of seismic data to stratigraphy and depositional systems
analysis has been widespread at least since the publication of AAPG
Memoir 26, over 27 years ago.
the early work was based on analyses of 2-D seismic — only relatively
recently has the emphasis shifted to 3-D seismic, with sometimes
astonishing results. In some instances entire depositional systems
with discrete depositional elements can be directly imaged, resulting
in highly accurate predictions of lithofacies relationships in time
imaging of geology has resulted in refinement of depositional models,
especially within the context of sequence stratigraphy.
interpretation of 3-D seismic data can take two forms:
- Analysis of cross
section views, or seismic stratigraphy. This has been the classical
approach to extracting geologic insights from seismic data, especially
when only 2-D seismic data are available.
- Analysis of plan view
images, or seismic geomorphology. This approach necessarily involves
3-D seismic data and constitutes the analysis of the geological
significance of landforms observed. Clearly the most robust geologic
interpretations involve the integration of insights derived from
stratigraphic as well as the geomorphologic analyses.
of Channel Systems
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.
more far-reaching sequence stratigraphic insights can be derived
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.
geomorphological analysis of this stratigraphic unit reveals that
it is composed of a leveed channel feeding a frontal splay or turbidite
fan lobe, comprising multiple bifurcating channels. The geological
interpretation of this map pattern is that of a turbidite system
comprising numerous shallow channel-levee deposits likely resulting
in a near sheet-like deposit of sand.
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
Turbidite Fan Deposition
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.
at this 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.
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.
the gradual change from splay complex to isolated leveed channel
within the deepwater 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.
Delta: The Staging Area
Figure 2 also illustrates the value of integrating seismic stratigraphy
and seismic geomorphology.
is 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.
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.
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.
of Channel Systems
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.
nodes can be observed. These are locations where flows have cut
through the levee walls and established new channels in the overbank
the channel is not sand-filled upstream of the avulsion nodes, but
rather is incised there.
these examples is that of a Pleistocene shallow-buried system. These
shallow-buried examples are very well imaged and provide the interpreter
with learnings that can be exported to similar deposits more deeply
buried but more poorly imaged.
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.
of seismic stratigraphy and seismic geomorphology is rapidly becoming
a mainstream style of analysis, necessarily involving both geologists
promises to further mitigate risk associated with geologic prediction,
as ever more stratigraphic/geologic insights are extracted from
3-D seismic data.