Geoscience Reference
In-Depth Information
Stack Response
Brine Oil Gas
Sand B
Stack Response
Brine Oil Gas
brine
oil
G
gas
Sand 3
Stack Response
Brine Oil Gas
Sand A
Sand C
R
0
Fig. 5.14
Schematic sand responses on the
R
0
-
G
crossplot.
sand this is a red loop; the reflection coefficient decreases with increasing offset, but
remains positive so that the summed response over all offsets is positive. The oil sand
starts off with a small positive reflection coefficient at zero-offset, and reverses sign
as it becomes more negative with increasing offset. Summing over all offsets results
in partial cancellation of the positive and negative amplitudes, leaving a weak blue
loop. The gas case reflection coefficient starts off negative and becomes more so with
increasing offset, so the summed response is a strong blue loop. Sand B has negative
R
0
with positive
G
, i.e. a class IV AVO response. The stacked top sand amplitude is a
soft loop that increases in amplitude from brine to oil to gas, owing to the change in
R
0
.
Sand C has a positive
R
0
and negative gradient, so exhibits class I behaviour. The top
sand reflector is a red loop that decreases in amplitude from brine to oil to gas, because
of the decrease in
R
0
.
A crossplot for a top sand reflector is shown in
fig. 5.15(a)
.
The brine and oil sand
cases form separate trends. Within each trend, it is porosity (increasing to the left) that
causes the points to be strung out along the trend line. The oil trend is systematically
displaced away from the brine trend, towards more negative values of
R
0
and
G
. If there
was reasonable seismic coverage (as is likely with 3-D data), then it would be possible
to fit a trend line to the brine sand and use distance from this trend as a predictor for
pay sand
(fig. 5.15(b))
. In this example, the points are colour-coded according to TWT,
which allows us to see that the points furthest toward the bottom left (green ellipse) are
slightly shallower than the points (red ellipse) nearer to the main trend. In this case, the
data come from a drilled anticlinal structure that contains both gas and oil. The points
in the green ellipse are from the top of the gas reservoir, the points in the red ellipse
from the top of the oil reservoir. Projecting data onto the perpendicular to the trend
line, as shown, is equivalent to making a weighted sum of intercept and gradient. From
