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Deep holes may have more than two. For each logging depth the BHT* can
be obtained, and so geothermal gradients determined (including interval
geothermal gradients between two such determinations). These will be of
considerable interest to geologists and geochemists, and the interval geother-
mal gradient between two logging depths may well be more interesting than
those from the surface for extrapolating to greater depths.
The necessary data are acquired at virtually no extra cost: they should be
acquired at every opportunity, and steps should be taken to ensure their
reliability. Subsequent reservoir tests may yield a better temperature, but
these estimates can be obtained before the logging programme is complete,
and so may be used to assist in the decision whether to drill deeper or not.
But, as always, extrapolations are made at one’s peril. There are numerous
examples in Kenyon and Beddoes (1977) of reservoir temperatures measured
during tests that show a deeper reservoir to be cooler than a shallower, or at
much the same temperature.

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