Geology Reference
In-Depth Information
Coal: V
p
/V
s
can vary between 1.9 (typical of
anthracite) to 2.1 (typical of bituminous coals).
Morcote et al.(
2010
) have published a V
p
-
4500
4000
V
s
relation appropriate for effective pressures above
5MPa:
3500
3000
V
s
¼
0
:
4811V
p
0
:
0038
:
2500
Volcaniclastics: relatively high V
p
/V
s
, often lying
on the Greenberg
2000
Castagna limestone trend
(Klarner and Klarner
2012
).
-
Sand
Shale
Limestone
Dolomite
1500
1000
High-velocity volcanic rocks: V
s
¼
0.51V
p
+ 0.148
(Klarner and Klarner
2012
).
Using empirical relations for V
s
prediction has been
shown to be as accurate if not better than using
effective medium models (such as the Xu
500
1000
2000
3000
4000
5000
6000
7000
White
(
1995
) model) for V
s
prediction (Avseth et al.,
2005
;
Jorstad et al.,
1999
). However, care should be taken in
using the Greenberg
-
V
p
(m/s)
Figure 8.9
Crossplot showing Greenberg-Castagna V
p
-V
s
relations.
Castagna V
s
prediction model
without local validation from shear sonic logs. Experi-
ence has shown that there can be significant vari-
ations away from these trends.
-
and shear velocity (Castagna et al.,
1985
).
Figure 8.8
shows an example of a North Sea sandstone dataset
compared to the sandstone relation published by
Castagna et al.(
1985
), based on a variety of data
types from the Gulf Coast and onshore United
States. It is evident that the sandstone relation is only
valid for the brine fill case. The effect of replacing
water with gas moves the points up and to the left
so that they lie above the Castagna sandstone trend
(
Fig. 8.8
).
Greenberg and Castagna (
1992
) defined four
trends for commonly occurring (brine bearing) lithol-
ogies (
Fig. 8.9
):
Unconsolidated and partially consolidated
mudrocks can have a slightly different trend to the
shale relation. Castagna
'
s mudrock relation is
defined as:
V
s
¼
0
:
862V
p
1
:
172
ð
Castagna etal.
;
1985
Þ:
Various sandstone lithologies can have V
s
higher
than that predicted by the sandline (e.g. Smith,
2011
), including clean (quartz prone) sands and
glauconitic sands. A clean sand line (referred to
by the authors as
) can be
derived from the data presented by Murphy et al.
(
1993
):
'
the quartz line
'
sandstone: V
s
¼
0.8042V
p
0.8559,
0551V
p
+1
limestones: V
s
¼
0
:
:
016V
p
1
:
0305,
dolomite V
s
¼
0.58321V
p
0.07775,
V
s
¼
0
:
802V
p
0
:
75
:
shale V
s
¼
0.7697V
p
0.86735,
Hossain et al.(
2012
) describe a V
p
-
V
s
relation for
glauconitic greensand that gives slightly higher V
s
than Castagna
where V is in km/s.
Combining these trends to predict V
s
for mixed
lithologies effectively follows the Voigt
'
s sandline:
V
s
¼
Hill
average approach of averaging the arithmetic and
harmonic averages (Greenberg and Castagna,
1992
).
Other useful data on V
p
-
-
Reuss
-
0
:
86V
p
0
:
96
:
In some areas, brine sands may fall below
Castagna
V
s
relations include the
s sandline.
Figure 8.10
shows shear log
data in which the sands fall significantly below
Castagna
'
following.
Gas sands: V
p
/V
s
~1.4
1.8 (an average V
p
/V
s
ratio
of 1.6 for gas sands is a good rule of thumb).
-
V
s
trend
would be a reasonable model for predicting V
s
for
both brine bearing sands and shales.
'
s sandline and a single V
p
-
Halite: V
p
/V
s
¼
1.74 (Simmons and Wang,
1971
).
In the shallowest, most unconsolidated, part of the
section it is possible that the V
p
/V
s
of sands is
Anhydrite: V
p
/V
s
¼
1.8 (Simmons and Wang,
156
1971
).
