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borehole dates from the earliest days of drilling; and as the geological basis
for drilling increased, so did the need for accurate and informative borehole
logs. The primary need was probably for geological information from which
the shape of the structure could be determined; but with increasing know-
ledge of the technological problems of petroleum production, a parallel need
for physical data on the rocks also grew. It became necessary not only to
know at what depths the various rock types were encountered, but also to
know the nature of the rock, its porosity and permeability, its temperature,
the nature of the contained fluids (known in this context as “formation
fluids”) and the depths of any gasloil and oillwater contacts.
The process of drilling holes in the ground consists necessarily of breaking
the rock and removing the cuttings from the hole. When drilling by cable
tool, the cuttings were bailed from the bottom of the hole at intervals, and
these were reliable samples of the rock penetrated since the last bailing. Con-
tamination was largely confined to caving from higher parts of the hole
(from driving casing, or from the lash of the cable), and the only loss was a
tendency for mudstones to form mud with the water in the borehole. Fluid
samples were also obtained when bailing, but these tended to be contaminated.
Once oil or gas was encountered, this was usually quite obvious because it
flowed into the borehole, and sometimes at the surface until it could be con-
trolled. The compilation of all these data was incorporated into the driller’s
log.
The introduction of rotary drilling significantly altered the nature of the
problem of logging the borehole. The mud column confined the fluids within
the formations, and samples of these could only be obtained (intentionally
or unintentionally) by producing them into the borehole.
The rotary drilling process is so destructive of the cuttings that it has
become much more difficult to determine the nature of the rock from which
they came. As holes are drilled deeper, and ways are found of improving dril-
ling performance, so the destructive forces applied to the bottom of the hole
increase. The time taken by the cuttings in transit to the surface increases
as the depth of the borehole increases, so the sample taken at the shaleshaker
belongs not to the present depth of drilling, but some shallower depth. From
the time the cutting is broken by the bit tooth and blasted by mud that has
been accelerated through nozzles in the bit, to the time it comes onto the
shaleshaker, it has been thoroughly abused. It has been accelerated up the
annulus between the drill collars and the wall of the borehole; decelerated at
the top of the drill collars, accelerated past each tool joint at about 10 m
(30 ft) intervals, all in a spiral motion, and no doubt hammered from time to
time by the drill pipe. Clay fractions become part of the mud. The larger
fragments travel more slowly than the smaller, the round more slowly than
the angular, the denser more slowly than the less dense. The cuttings collected
at the shaleshhker can hardly be thought of as reliable samples of the rocks
drilled; and there are occasions when the well-site geologist should not be