GCFuture Trends in 3-D Seismic Analysis: The Integration of Seismic Stratigraphy and Seismic Geomorphology*
Henry W. Posamentier1
Search and Discovery Article #40127 (2004)
*Adapted from the Geophysical Corner column in AAPG Explorer, February, 2004, entitled “3-D Yields Strat Geologic Insights” and prepared by the author. Appreciation is expressed to the author, to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Manager, geoscience and technology, Anadarko Canada Corporation., Calgary, Canada ([email protected])
The 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.
Most of 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 and space. Such direct imaging of geology has resulted in refinement of depositional models, especially within the context of sequence stratigraphy.
Geologic 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.
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.
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.