Geology Reference
In-Depth Information
a)
flat spot, discordant relationship to the fault block
stratigraphy, clear lateral terminations at the fault
and top reservoir consistent with a fluid change in a
dipping fault block, as well as dimming (and possible
phase change) at top reservoir and lowering of fre-
quency beneath the flat spot.
A good example of amplitudes conforming to
structure is shown in Fig. 7.22 . Such conformance
often increases confidence in a hydrocarbon inter-
pretation. However, there may be good geological
reasons why the seismic representation of the con-
tactisnotflatintwo-waytime.Thismaybedueto
the effects of overburden velocities varying laterally
(e.g. Blow and Hardman, 1997 ). On the other
hand, it is quite possible that the contacts are not
actually flat in depth but inclined in response to
pressure gradients within the sedimentary basin
( Fig. 7.23 ).
Unfortunately, a flat spot related to the presence
of hydrocarbon may not always be a guarantee of
reservoir quality or productivity. For example, gas
water contacts have been observed in shale and silt
dominated sequences. Figure 7.24 is an example of a
bright spot and flat spot related to oil in laminated
sands where the permeability of the sands is such that
the formation is not producible.
The character of hydrocarbon contacts depends
on a number of factors including
Fluid Projection
b)
Lithology Projection
formation thicknesses,
stratigraphic dip,
dry rock frame stiffness of the reservoir rocks,
difference between hydrocarbon and water elastic
moduli,
Figure 7.14 Fluid and Lithology Projections in a Class IIp oil sand
scenario (after Whitcombe et al., 2002 ). Note how the fluid related
AVO signature (red in (a)) follows the field oil water contact (blue
line) (except for in the far west where there is a problem with data
quality).
effective angle of the seismic stack.
When the reservoir comprises a series of thin bedded
sequences it is usual to see contacts as a series of
separate segments each of which is related to interfer-
ence effects between the fluid contact and the tops and
bases of the sands ( Fig. 7.25 ).
In carbonates and relatively stiff sandstone rocks
interpretable contact effects are relatively rare, owing
to the low reflection coefficients associated with the
contacts and the presence of strong lithological con-
trasts. In these situations, contact effects are usually
related to subtle tuning effects between top and bases
of porous units and the fluid contact. Figure 7.26
shows a modelled effect in a relatively homogenous
carbonate grainstone reservoir with porosity varying
between 15% and 20%.
signatures show all these effects so it is important that
seismic observations are integrated with all available
geological information. Non-hydrocarbon explan-
ations for the flat event should be investigated, for
example presence of strong multiple energy or diage-
netic changes such as temperature-related quartz
transformations.
A dramatic flat spot in Jurassic sediments from the
Troll gas field in offshore Norway is shown in
Fig. 7.21 . This example shows a number of DHI
effects, including the clear positive reflection of the
137
 
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