Geoscience Reference
In-Depth Information
3
Structural interpretation
This chapter is mainly about the most fundamental interpretation activity: making maps
of horizons. Historically, it was the need for better maps of complex structural traps
that was a key driver in the early adoption of 3-D seismic. Usually, however, it is not
enough just to map the top of the reservoir. To understand how structures were formed
and when, it is usually necessary to map a range of marker horizons above and below
the target. Also, depth conversion will, in most cases, require the mapping of several
horizons above the target level.
This chapter begins by considering how stratigraphic horizons encountered in wells
can be tied to particular reflections on a seismic survey; this is an issue for all seismic
interpretation, and is often easier on a 3-D survey because of the more nearly correct
positioning of subsurface features. Having decided what to map, the interpreter is faced
with the daunting problem of working with a huge number of seismic traces, all of which
should ideally be taken into account. Happily, the power of computer workstations
has increased faster than the quantity of traces waiting to be interpreted; the chapter
continues with an explanation of how the tracking of reflecting horizons through a
3-D volume can be partly automated. This is not a matter only of mapping the horizon
itself; in an area of even moderate structural complexity, it is the mapping of fault
systems that will consume much of the interpretation effort. Semi-automated methods
can help here too. For completeness, the chapter concludes with a brief discussion of
how to convert a reflection-time horizon map into a depth map; the issues involved are
the same as those with 2-D seismic, but the greater density of data in 3-D surveys may
make the task easier in practice.
3.1
Well ties
One of the first steps in interpreting a seismic dataset is to establish the relationship
between seismic reflections and stratigraphy. For structural mapping, it may be suffi-
cient to establish approximate relationships (e.g. 'reflection X is near Base Cretaceous'),
although for more detailed work on attributes, as described in chapter 5 , it is usually nec-
essary to be more precise and establish exactly how (for example) the top of a reservoir
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