Page 129 - Petroleum Geology

CHAPTER 6

THE LOGGING OF BOREHOLES

SUMMARY
(1) Wire-line logging devices provide borehole logs that are essential docu-
ments for the description and interpretation of the subsurface geology of
petroleum accumulations. They are run after drilling a section of the hole,
before running casing, and they are used both qualitatively and quantita-
tively. While drilling, a plot of penetration rate against depth gives a valuable
indication of the rock units penetrated, and it is commonly more reliable
than the cutting samples obtained.
(2) The basic log is an electrical log that records resistivity and spontaneous
potential (S.P.) against depth. The resistivity of a porous rock is a function
of its porosity, the pore-space geometry, the resistivity of the interstitial
fluids and their salinity, and their temperature. The S.P. log distinguishes
mudstones and shales from other lithologies. The deflexions on the log are
due to natural electrical currents circulating around the intersection of the
borehole with a lithological boundary. These currents are partly due to the
contrast in salinity between the drilling mud and the formation fluids, partly
due to mudstone or shale acting as a semi-permeable membrane, and, to a
small extent, to the movement of fluid from the borehole to the formation.
(3) Radioactivity logs record either spontaneous or induced radioactivity
of rocks around the borehole. They are used for determining lithology, poro-
sity, fluid content, and bulk density of the rocks penetrated. The gamma-ray
log, which records the natural gamma radiation, can be run in cased holes.
(4) The sonic log provides a record of the velocity of sound through the
rocks adjacent to the borehole by timing a signal over a fixed distance and
recording the transit time. It is used for integrating borehole and seismic
data, and for estimation of porosity, and also pore-fluid pressures in mud-
stones.
(5) The dipmeter is an electrical device that measures the dip of the strata
in the borehole, the attitude of the borehole, and computes the true dip. It
is used principally for the determination of structure, but can also help the
elucidation of the environment of deposition of a sequence of sedimentary
rocks.

GENERAL
The desire and need to know the succession of rock types penetrated by a