Stratigraphic interpretation

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Problem

Interpret the seismic section shown in Figure 10.15a.

Solution

The most striking feature of this section is probably the progradational pattern indicating a source to the right of the section. These prodelta reflections (seaward of a river delta) show both toplap and downlap terminations. Note that these prograding reflections are more continuous at the right and left sides of the section than in the region under the surface channel. There may also be a slight sag in reflections under the channel, and they suffer similar quality deterioration. An interpreter would not interpret these quality changes as having stratigraphic significance.

The thin wedge above that pinches out about midway on the section shows downlap from the left, suggesting a shift in the source of the sediments from the time when the deeper prodelta sediments were being deposited. Downlap is seen onto and reflections below truncate against this reflection; it is probably an unconformity and possibly a sequence boundary.

Figure 10.15a.  Seismic section (courtesy Chevron Oil Company).

Note the change in overall character at reflection ; it is known from well control that separates marine sediments below from nonmarine sediments above. However, the decrease in multiplicity on this CMP section as one goes shallower (because of the mute applied in processing) might also produce a character change.

Dip is gently to the left on this section so the right dip at and below constitutes a dip reversal that may have structural significance.

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