Geoscience Reference
In-Depth Information
4
Geological interpretation
All seismic interpretation is of course directed toward geological understanding of the
subsurface. In the previous chapter, the objective of the interpreter was to make maps of
surfaces, mainly in order to delineate traps by mapping the top of a reservoir. However,
how does he or she recognise where the reservoirs are likely to be in an undrilled area?
What reflectors are most likely to be the top of a reservoir body? If there are some
well data available, perhaps reservoirs have already been encountered, but what is their
lateral extent likely to be? What lateral changes in reservoir quality are likely, and
how should they be related to changes in seismic appearance? These questions are of
course just as relevant for 2-D seismic as for 3-D, but the dense data provided by 3-D
seismic offers more scope for defining the external geometry and internal architecture
of reservoir bodies. The detailed map view derived from 3-D seismic is often more
instructive than an individual section can be.
Before embarking on a more detailed discussion, it is important to understand the lim-
itations on achievable seismic resolution; this is discussed in
section 4.1
. The principles
of seismic stratigraphy are briefly explained in
section 4.2
, including the recognition of
seismic facies. Some tools to allow the interpreter to look for the expression of differ-
ent sedimentary facies are described in
section 4.3
,
and some examples of the results
presented in
section 4.4
.
The structural geologist also has of course an input to make to 3-D interpretation.
The need for validation of fault patterns is less than in the case of 2-D surveys, where
aliasing of fault patterns can be a major issue. However, understanding of fault systems
may be critical to understanding whether faults will form effective lateral seals. These
topics are discussed in
section 4.5
.
4.1
Seismic resolution
Both vertical and horizontal resolution of seismic data are limited, and this imposes
limits on what geologically significant features can actually be recognised on seismic
data. Vertical resolution is determined by the seismic source signal and the way it is
filtered by the earth. For example, the signature of a typical marine air-gun array has
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