Geoscience Reference
In-Depth Information
and will usually have been drilled near the crests of anticlines; the interpreter, however,
may need a reasonably accurate depth conversion of the intervening synclines, either
to map the spill points of the anticlinal structures or to assess maturity of hydrocarbon
source rocks. At first sight the (
v
0
+
kz
) model gives the required help, but this is
not always the case. The simplest case is where the
k
factor reflects the effects of
compaction, as unconsolidated sediments become more deeply buried over time. In
such a case, the value found for
v
0
may vary little from one well to another. This might
be the case if all the rocks are currently at their greatest depth of burial. However, when
rocks that once were deeply buried are later found near the surface after a period of
uplift and erosion, the velocities usually remain close to what they were at the time
of deepest burial. If there has been variation of the uplift from one well to another,
then
0
between the wells is valid
only if uplift values can be similarly interpolated. Another possible complication is that
the
k
factor may represent a change in velocity due to lithological effects, for example
a consistent coarsening or fining upwards of a clastic sequence;
k
may then be quite
similar from one well to another, but give no clue about the effect of depth on velocity.
Rather than deriving
k
from the sonic log, it may therefore be preferable, where several
wells are available, to determine a compaction trend by plotting the average velocity
in each formation against midpoint depth. The gradient of this line (
κ
) is not the same
thing as the
k
value for instantaneous velocity unless the interval is thin (time thickness
much less than 1
v
0
values will also vary. Simple interpolation of
v
/
k
). For thick intervals, with the notation of the previous section,
we would find:
(
z
2
−
z
1
)
(
t
2
−
t
1
)
/
2
=
v
0
+
κ
z
2
+
z
1
2
so that
z
2
(1
−
κ
(
t
2
−
t
1
)
/
4)
=
z
1
+
(
v
+
κ
z
1
/
2)(
t
2
−
t
1
)
/
2
0
and
4
z
1
+
(2
v
0
+
κ
z
1
)(
t
2
−
t
1
)
4
−
κ
(
t
2
−
t
1
)
z
2
=
.
However the velocity maps are calculated, it is usually a requirement that the final
depth map should match the formation tops in the wells. This will always be the case
if the velocity derivation methodology honours the well data exactly, as can easily be
done if maps are being made of the velocity in each layer. However, if some or all layers
are depth-converted using constant parameter values (e.g. the average for the velocity in
the layer, across all the wells), then there will be discrepancies between the depth map
and the true well depths. If they are large, the method of depth conversion used needs
to be revisited; if they are small, the usual practice is to grid up the mistie values and
apply them as a correction across the whole map. The gridding algorithm needs to be
chosen so that it will not produce unreasonable values outside the area of well control,
