Page 122 - Well Logging and Formation Evaluation

112 Well Logging and Formation Evaluation

7.00E-07
6.00E-07
5.00E-07 oil
Frequency 4.00E-07 gas

water
3.00E-07
2.00E-07 shale
1.00E-07
0.00E+00
0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07
Acoustic impedance

Figure 6.3.2 Comparison of Acoustic Impedance Distributions for the Same
Formation in One Well Modeled with Different Porefills

these techniques to predict, say, the presence of oil-bearing sands from
seismic.
While the compressional velocity does depend quite significantly on the
porefill, particularly on the gas saturation, the shear velocity depends
hardly at all on the fluid fill. The shear behavior of the rock may be mea-
sured with seismic to some degree by using far-offset traces. Because of
the nature of sound reflecting off a surface, at high incidence angles a P
wave will generate S waves when it is reflected/refracted, and these can
be detected. Where only the far-offset traces are used (called AVO, for
amplitude versus offset), it is possible to generate a 3-D seismic cube of
elastic impedance (EI) as well as AI. The EI cube, which is mainly
dependent on the shear velocity, is largely independent of fluid-fill effects.
In a similar way to generating AI traces using r and V p , it is also pos-
sible to generate EI traces from the logs using r, V p , V s , and knowledge
of the seismic incidence angle (q). There are various published equations
for doing this. Once such equation is:

Ÿ 2 2 2
-
s
EI = V p (1 + tan ( )) (1000*q *r ) Ÿ (1 4* (V V p ) *sin ( )*q
2
-
Ÿ (18* ) *sin 2 ( ).q (6.3.1)
s
V s (V V p