Page 178 - Introduction to Petroleum Engineering

LITHOLOGY LOGS 165

Sedimentary Illustrative
rock stratigraphic column

Clastics Carbonates Shale
Sandstone
Sandstone Limestone Shale
Conglomerate Dolomite
Shaly sand Evaporite Limestone
Shale

Shale

Shaly sand

FIGuRE 9.3 Common reservoir rock types and an illustrative stratigraphic column.

Natural radioactivity includes alpha, beta, and gamma rays. Alpha rays are helium
nuclei, beta rays are electrons, and gamma rays are photons. Alpha and beta rays are
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low energy. Gamma rays emitted by nuclei have energies in the range from 10 eV
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(electron volts) to 10 eV and can penetrate several feet of rock. Naturally occurring
radioactive materials (NORM) are minerals that contain radioactive isotopes of the
elements uranium, thorium, potassium, radium, and radon. The decay of naturally
occurring radioactive isotopes produces gamma rays.
Coals and evaporites, such as NaCl salt and anhydrites, usually emit low levels of
gamma radiation. Anhydrites are composed of anhydrous (water‐free) calcium sulfate
and are usually formed as an evaporite when a body of confined seawater evaporates.
Some of the highest levels of gamma radiation are observed in shales and potash
(potassium chloride). A segment of a gamma‐ray log is illustrated in Figure 9.4.

9.2.2 Spontaneous Potential Logs
The SP log is one of the oldest logging methods. The SP log is an electric log that
records the direct current (DC) voltage difference, or electrical potential, between
two electrodes. One electrode is grounded at the surface, and the other electrode
on the logging tool moves along the face of formations in the wellbore.
The range of the voltage on an SP logging track is typically up to 200 mV. The SP
voltage for shales in a particular well is fairly constant and forms a “shale baseline”
for interpretation of the SP log for that well. The SP for shale‐free formations is
negative by 50–100 mV relative to the shale baseline and forms a “sand baseline.”
These two boundaries provide a rule for interpolating the amount of clay or shale in
a formation based on its SP.
The SP results from diffusion processes in porous rock driven by differences in
the ionic composition of aqueous mud filtrate and in situ brine. As a result, the SP log
cannot function with air‐ or oil‐based drilling mud. In porous formations with less