Page 271 - The Geological Interpretation of Well Logs

16

CONCLUDING REMARKS

16.1 The book of revelations way a geologist defines lithology in the field was the
correct way and that any work should be referred to it.
Sentences end with a full stop: scientific books end with
But the new logging tools describe formations, albeit
a conclusion. But a book on wireline logging can have
differently, in far greater detail and with more precision
no conclusion. The technique is dramatically active and
than a geologist is able to do with traditional outcrop
rapidly evolving. There are, nevertheless, strands to be
descriptions. New detection methods must surely push
pulled together and comments to be made. As an author’s
the outcrop-bound geologist into thinking, not in terms of
privilege, this last chapter is personal and idiosyncratic.
eye and touch, but in terms of the senses extended by
In it are comments on what is happening at present and
technology. In many sciences this is a conscious move. It
some ideas on what may happen in the near future. It is
wil] become more and more the necessity for oilfield
good to know where you are going but it is essential to
outcrop analogues.
know, in the first place, where you are!
Using portable gamma ray detectors to collect outcrop
I have just returned from the 36th annual SPWLA
information comparable to subsurface data is a beginning
(Society of Professiona] Well Log Analysts) logging
to this process. The technique has been illustrated for
symposium in Paris. It was very pleasant. However, the
lithological use and lithostatigraphic correlation of out-
conference keynote speaker complained that while most
crop to the subsurface (Chapters 7 and 15). The example
oilfield costs were dropping, logging costs were climb-
illustrated here is more subtle (Davies and Elliott, 1996).
ing, to reach even 20% of total well budget. Does this
It shows spectral gamma ray togs acquired specifically
mean that we should cut down on logging? Any article
to recognise important (key) sequence stratigraphic
bought should be assessed from two viewpoints: is it
surfaces (Figure 16.1). There is an implicit realisation
needed and is it worth the money? Are logs needed?
that geophysical sensing must be used to extend the
Certainty, yes. Are they really worth the money? The
unaided senses. Geologists are often amazed to find that
answer must be no. They cost too much for rhe way in
distinctive subsurface log markers, and this includes
which they are presently used, We should not cut down on
gamma ray spikes, are not immediately identifiable when
the logs run, we should use them better, and demonstrably
seen in cores. The same occurs at outcrop: the eye alone
so. Many times I find that the geological use of the formi-
is not sufficient.
dable amount of data that logs provide is not properly
Taking a subsurface tool to measure the outcrop is the
employing new technology. It is falling further and
first step. What must also be done is to make geological-
further behind other disciplines. The data are not being
ly significant analyses, chemical, geochemical, physical,
used, the techniques are not being used. The water of
petrophysical, and compare these to the geophysical log
progress is rishing past the geologist to make its own,
measurements. Such direct sample analysis is routine
confident course. This is a disappointment.
with core. What can be done with core can be done at
This final chapter is not written to express disappoint-
outcrop: and better. But, as the case below illustrates, the
ment. Quite the reverse. [t is written with the active future
best examples of this are stil) with core data.
in mind. How to harness the new technologies: how to
Detailed analyses of carbon isotope ratios (8'°C), made
use all this formidable data. Those old ideas, those dusty
routinely in DSDP/ODP cores from very widely spread,
shelves of old books, those inherited habits, must be
deep oceanic sites, show remarkable similarities over
by-passed. Discoveries lie in the new technologies. Let
the Palaeocene-Eocene boundary interval, between
me illustrate in practice what I mean by looking at four
50Ma-65Ma (Corfield and Norris, 1996) (Figure 16.2).
things: how we look at outcrops; how new tools are
These and other isotope changes can be interpreted as
changing ideas; how image logs are developing; and the
indicating long term, world-wide, oceanic water temper-
software glut.
ature changes. Superimposed on the slower changes, is a
well documented, very short term 8°C effect which, it is
16.2 Outcrop bound
suggested, indicates that surface water temperatures rose
As the sophistication of logging tools and software by up to 4°— 6°, and then cooled, over a period of only
advances, we are having more and more difficulty relat- thousands of years (<50 kyr) (especially in mid-high lat-
ing what the tools detect with what we observe in core itudes). This excursion is associated with a very unusual
and at outcrop. We have always assumed that the outcrop extinction of deep sea benthic foraminifera (Thomas and
was the norm, the standard to which we refer: that the Shackleton, 1996).
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