Page 52 - The Geological Interpretation of Well Logs

6

RESISTIVITY AND

CONDUCTIVITY LOGS

6.1 Generalities Most rock materials are essentially insulators, while
their enclosed fluids are conductors. Hydrocarbons are
The log
the exception to fluid conductivity, and on the contrary,
Of all the logging tools, those that measure resistivity are
they are infinitely resistive. When a formation ts porous
archetypal. It was with resistivity measurements that
and contains salty water the overall resistivity will be low.
Conrad Schlumberger started his company in 1919.
When this same formation contains hydrocarbons, its
The resistivily log is a measurement of a formation’s
resistivity will be very high. It is this character that is
resistivity, that is its resistance to the passage of an electric
exploited by the resistivity logs: high resistivity values
current. It is measured by resistivity tools. Conductivity
may indicate a porous, hydrocarbon-bearing formation
tools measure a formation’s conductivity or its ability to
(Figure 6.1).
conduct an electric current. It is measured by the induction
tools. Conductivity is generally converted directly and Principal uses
plotted as resistivity on log plots. The resistivity logs were developed to find hydro-
RESISTIVITY LOGS
deep
shallow
Scale: ohmatm2im(Q)
1 1060 100 1000 10 900
SHALE

GAS
POROUS —
«sanpstone OL
SALT
WATER

POROUS SALT
"SANDSTONE WATER}:

POROUS
*SANOSTONE

TIGHT SANDSTONE
* ‘QUARTZITE’

FINING UP SHALY [-
«SANDSTONE,
CLEAN
POROUS,
SALT WATER

Figure 6.1 The resistivity log: some typical responses. The resistivity log shows the effect of the formation and its contained fluids
on the passage of an electric current. *Limestone, dolomite, etc., equally applicable.

42