Page 132 - Petroleum Geology

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with great precision from these samples, and this precision beguiles us into
accepting them as valid in situ. Nevertheless, they are very valuable samples
to the geoIogist because they reved the lithology and environment of the
sedimentary rocks cored, and provide a valuable check on porosity and per-
meability data obtained by other means.

PRINCIPLES OF ELECTRICAL LOGGING

Electrical logging of boreholes was developed in the late 1920s by the
French engineer, Schlumberger, and the techniques grew with the demands
until it is now an essential service using a wide range of sophisticated tools
for subsurface logging and interpretation. Techniques have developed so fast
that no attempt will be made here to discuss the details of the tools available.
These details, and the description of interpretive methods, can be obtained
from the handbooks of the companies performing these services. The petro-
leum geologist - indeed, any geologist today - must acquire a knowledge
of the principles of borehole logging with wire-line devices if he is to read his
logs intelligently.
The basic log used by the petroleum geologist consists of a recording of
the resistivity of the rocks and the spontaneous potential in the borehole
against depth (Fig. 6-1). This log is obtained in a single run in the borehole.
The device is contained in a sonde that is lowered down the hole on a cable
within which electric cables pass. Most logs are run from the bottom of the
hole to the top because this direction, with a taut cable, gives more positive
depth control on the log, which is measured from the cable movement at the
surface. Temperature logs are run while running in; and other logs should
be run when running in if the condition of the hole is such that there is
danger of losing the device.
The readings from the sonde are converted to a signal at the surface that
is recorded on photographic film that is wound past the signal at a speed
scaled to the speed of the sonde in the borehole. The data can also be stored
on tape for subsequent computer processing, but a visible record is usually
required at the well site at the time of logging.

Resistivity

Nearly all the common rock-forming minerals are non-conductors of elec-
tricity. Dry porous rocks are non-conductors because the fluid in the pore
spaces (air) is also a non-conductor. In the ground, however, porous rocks
contain water, and the water is usually saline. Saline water, an electrolyte,
conducts electricity by the movement of charged ions that result from the
dissociation of salts in solution in water.
The conductivity, or capacity of ground water to conduct electricity, is