Page 189 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

158 Reservoir Engineering

6000

6100

Figure 5.88. Example of a natural gamma spectroscopy log presentation.

uranium and thorium. Thus, shales typically have high radioactivity. Sandstones
(quartzose), carbonates and evaporites have strong bonds and generally do not
allow impurities. Limestones undergo rearrangement of crystal structure and
addition of magnesium to become dolomites. Impurities like uranium (which is
very soluble) may enter the crystal lattice during recrystallization. Feldspathic
sandstones contain an abundance of potassium40 and therefore show higher
radioactivity than quartzose sandstone. Some evaporite minerals (such as KC1)
contain high amounts of potassium40 and may appear as shales on the log. Serra
et al. [207] provide an excellent discussion of interpretation of the natural or
spectral gamma-ray tool.

Sonic (Acoustic) Log. The sonic (acoustic or velocity) tool measures the time
it takes for a compressional wave to travel through one vertical foot of formation.
It can be used to determine porosity (if the lithology is known) and to determine
seismic velocities for geophysical surveys when combined with a density log. The
sonic log also has numerous cased hole applications.
Theow. A 20 khz sound wave is produced by the tool and travels through the
mud into the formation. The wave travels vertically through the formation. The
first arrival of the compressional wave is picked up by a receiver about one foot
away from the transmitter. The wave continues through the formation and is
picked up by the far receiver (normally 2-ft below the ne& receiver). The time
difference between the near and far receivers is used to determine formation
travel time (At). Fractures, vugs, unconsolidated formations, gas-cut mud, lost
circulation materials, and rough boreholes can cause sharp increases in At, called
cycle skips.
Interpretation. Table 5-26 shows the velocity and travel time for several commonly
encountered oilfield materials. The tm value in the fourth column is at 0%