Geology Reference
In-Depth Information
a)
Base sand responses
and contacts
c)
Near
Top sand responses
with high gradients
b)
Inline 1215
3.2
3.6
4.0
Inline 1235
3.2
3.6
4.0
0
500m
1000m
W
E
Figure 7.13
Describing Class IIp oil sands using a near/far crossplot (after Ross,
1995
).
reservoir in which variations in porosity (related to
changes in the sedimentary environment) control the
brightening. The brightening hydrocarbon effect of
multi-layered reservoirs may be visible on vertical
seismic sections but can be enhanced by techniques
such as optical stacking (
Fig. 7.20
).
is truncated at top and base reservoir or by a fault,
shows AVO behaviour as expected for a fluid
transition (i.e. higher amplitude on far offsets than
near offsets),
is located at the base of a structurally conformable
soft reflection amplitude anomaly,
corresponds in depth to a mapped structural spill
point
7.2.7 Hydrocarbon contacts
In favourable circumstances hydrocarbon contacts
may give rise to flat events or flat spots on seismic
and the recognition of such features can have a sig-
nificant impact on the perceived chance of explor-
ation success (
Chapter 10
)
. From a general point of
view the presence of hydrocarbons in any prospect
displaying a flat event is considered very likely if the
flat event shows the following characteristics (Blom
and Bacon,
2009
):
is consistent with other direct hydrocarbon
indicators, such as
dimming or phase reversals at the top of the
reservoir,
·
lowering of dominant frequency beneath the
contact (usually when gas is present).
·
These observations, of course, should be most evident
on fluid optimised stacks. If the reservoir is fairly
continuous, lithology stacks should reveal no obvious
changes at the level of the contact. The interpreter
should be mindful of the likely relative amplitudes of
top and base reservoir and contact reflections based
on modelling. Not
displays the correct phase and polarity,
crosses the reservoir layering,
shows a difference in dip with respect to possible
multiples,
136
all
contact-related seismic
