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required to follow tradition and examine and describe all samples, because
the reward cannot recompense the labour.
The sample must be taken, however, because it may contain fossils, and
these form an important part of the geological record of the borehole. It
must be remembered, though, that if Nature has been very selective in the
matter of which organisms were preserved as fossils, the drilling process is
further selective in destroying all but the micro-fossils, and even then
perhaps in destroying some of the larger, or more delicate, forms. The bio-
stratigraphic record of a borehole also suffers from uncertainties.
An essential aid to the logging of boreholes is to have installed (and proper-
ly functioning!) a penetration-rate recorder that enables one to make a plot
of penetration rate against depth. If the normal operational drilling param-
eters are kept constant (that is, weight on bit, rate of rotation, hydraulic
energy and properties of the mud) the rate of penetration is a function of
bit wear, fluid potential gradient across the bottom of the hole, and the
mechanical properties of the rock being drilled. In practice, when drilling
normally pressured rocks, the rate of penetration varies significantly with
different lithologies, and only towards the end of a bit’s run does the dulling
of the bit mask these changes. If a plot is kept of “minutes to drill 1 m”
against depth, the resulting penetration log will commonly be found to cor-
relate closely with the Spontaneous Potential or Gamma-ray log when it has
been run. The penetration log is therefore an indispensable part of the dril-
ling record. It is a valuable aid to making sense of the cuttings while drilling,
and to choosing the logging depth when wishing to case off particular sands.
Contamination of the drilling mud is also an important part of the drilling
record. Part of the fluid content of the rock drilled is retained in the cutting,
the rest is in the mud. Gas-cutting, dilution with formation water, and any
other significant changes in mud properties must be recorded, and these
changes compared with the penetration log and, eventually, with the elec-
trical logs. Modern rigs run a continuous sampling of the mud returns for hy-
drocarbons.
There is always a need to know as fully as possible the sequence of rocks,
their properties and their fluids, as they are drilled. This need can be urgent
when drilling towards a sequence with abnormal pore-fluid pressures, or
when wishing to land casing at a particular stratigraphic horizon, or below a
particular reservoir. The nature of the record is necessarily incomplete while
drilling is in progress because so much of the evidence is destroyed.
Coring is generally too slow, and so too expensive, for general use. At the
best, a core will provide a few metres of fairly reliable information, but there
is a danger here of regarding it as more reliable than it really is. The core has
been cut by drilling around it, broken off by spinning the pipe, removed
from the subsurface conditions of temperature and pressure to the surface,
and extracted from the core barrel. Samples are taken, cleaned and flushed
with solvents. Parameters such as porosity and permeability can be measured