Geoscience Reference
In-Depth Information
up interpretation projects to throw away all the useless horizons should be part of the reporting process
at the end of a project; if this is not done, it is very difficult for another interpreter to come to grips with
the interpretation and to use it as a basis for further work. To make such re-use easier, there also need
to be fairly rigid naming conventions for horizons, linked to the stratigraphic column. Over a period
of time, a given seismic volume will be interpreted by a number of people, and different versions may
exist of the same stratigraphic horizon (depending, for example, on the view taken about correlating
horizons into undrilled fault blocks). In addition, a given geographical area may be covered by several
different interpretation projects, corresponding to different seismic surveys or different reprocessed
versions of a single survey. The interpreter may need to review all of them, and carry forward several
of them into the next stage of study (e.g. reservoir modelling), in order fully to reflect the degree of
uncertainty about the subsurface (Herron,
2001
)
. It may be hard to find out what interpretations are
available, let alone deciding which is the best for any particular purpose. Ideally, a database is required
showing available interpretation data by geographic area, stratigraphic level, etc.; it is not difficult to
devise a simple scheme, but maintaining such a database will have a significant manpower cost.
Finally, the interpreter needs to be able to access well data to tie the wells to the seismic survey.
Sonic and density logs, at least, need to be available in digital form, so that well synthetics can be
created. To understand seismic response and its lateral variation, it is helpful to have other wireline logs
as well; for example, the caliper log, which measures borehole diameter, is a useful indicator of log
reliability, because where the borehole diameter is locally much larger than expected the logging tools
may not be able to record correct formation parameters. The interpreter also needs stratigraphic and
lithological information. In general, management of well data is not easy. Stratigraphic information
in particular can be complicated, with repeated or missing sequences in any particular well and lateral
changes in nomenclature. A system designed primarily for use by geologists may be over-complicated
for the needs of the seismic interpreter. For the creation of well synthetics, it may be useful to set up
a separate database (or partition of a larger database) containing sonic and density curves specially
edited for synthetic creation, together with checkshot information, major formation tops and perhaps
the caliper log and the gamma-ray log for correlation with other displays and for identification of
sand/shale trends. The issue here is that, as we saw in
chapter 3
,
the well synthetic can be sensitive
to noise in the sonic and density logs. Where wireline logs are being used primarily for formation
evaluation, defective readings in the shales will probably not get paid much attention; therefore, there
is a need to edit logs specifically for synthetic generation, and once edited they might as well be
retained in a simple database structure.
Reference
Herron, D. A. (2001). Problems with too much data.
The Leading Edge,
20
, 1124-6.
