Figure Captions
Figure
1. Location map of structures.
Figure
2. 3-D seismic profile across Cockroach structure showing location of
amplitude anomalies.
Figure
3. Amplitude extraction at main reservoir horizon showing OWC and
paleo-OWC.
Figure
4. Exploration wells on southern flank of Cockroach structure showing
fluid contacts and flushed zone.
Figure
5. Cartoon showing mechanism of mud volcano genesis and role in
hydrocarbon leakage.
Figure
6. 2-D seismic profile crossing Cockroach and Louse structures showing
amplitude anomalies.
Figure
7. 2-D seismic profile crossing Cockroach and Tick structures showing
amplitude anomalies.
Figure
8. Amplitude extraction at oil-bearing reservoir at Tick, showing
structurally conformable hydrocarbon-bearing anomaly.
Figure
9. 3-D seismic section across Flea, showing amplitude anomalies
corresponding to residual hydrocarbons.
Figure
10. Amplitude extraction at oil-bearing reservoir at Flea, showing
structurally conformable residual hydrocarbon anomaly.
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Case Histories of Structures with Amplitude Anomalies
Figure 1
shows four nearby structures (designated, respectively, for this
discussion as Cockroach, Louse, Tick and Flea), with seismic lines
crossing each. Geophysical modeling shows that the porous Pliocene
sandstone reservoirs in this basin tend to be characterized by lower
acoustic impedance than the surrounding shales, so they tend to show-up
as amplitude anomalies on compressional (" P-wave") seismic lines,
especially when filled with hydrocarbons.
In Figure
2 are "bright spots" in the Cockroach structure at two-way times of
1.9 seconds (gas) and 2.3 seconds (oil). Actually, all four structures
have amplitude anomalies between one and three seconds.
An amplitude extraction over a 45
milliseconds window around the main reservoir at Cockroach shows
structurally conformable high amplitudes (black), with an outer
secondary "bathtub ring" of high amplitudes (Figure
3). Several circular "no data" zones are due to mud volcanoes, which
pierce the structure.
Barely visible on this display is the
shallow gas cloud that obscures a significant percentage of the
reservoir (40% overall) by absorbing and scattering the seismic energy.
The inner amplitude anomaly, which corresponds to the present-day OWC,
has been penetrated by wells. The outer ring is asymmetric.
Note the position of the two wells on
the southern flank. The logs for these two wells (Figure
4) show that the upper well penetrated the present day OWC The lower
well penetrated a flushed zone. The outer "bathtub ring" corresponds to
the paleo-OWC.
Both regional tilting and a seal failure
have occurred, as shown schematically in
Figure 5. A thick, hydrocarbon-bearing shale underlies this
structural trend, Pliocene compression, plus sedimentary loading of the
shale on one flank of the structure, initiated argillokinesis and
uplift, which created a linear sill. This, in turn, caused further
asymmetric loading and steepening of the northern flank. Finally, the
diapir rose to a level at which explosive exsolution occurred within the
shale, and dissolved gases penetrated the overlying formations.
Hydrocarbons then leaked to the surface,
either directly through the mud volcano diatremes or via faults that
became active at this time, until the migration conduit was largely
sealed by fine clastics (satellite radar data in this area show numerous
seeps). An amplitude anomaly remains at the position of the paleo-OWC.
Figure 6 shows a seismic line that
crosses both Cockroach and Louse, which is the southernmost of the
series of structures parallel to Cockroach (Figure
1). Bright spots can be seen between 2.5 and 3 seconds at Louse
These amplitude anomalies are favorably located at the top of the
structure. However, exploration wells found that the main reservoir was
wet; there was non-commercial gas, with some questionable liquids, at a
deeper level.
In detail, Louse is seen to be highly
faulted. Structural modeling has shown that little tectonic movement
occurred throughout most of the Pliocene. The faulting occurred in
latest Pliocene time, which corresponds to the onset of mud diapirism at
Cockroach. The same structural loading that steepened the
reverse-faulted northern flanks at Cockroach also caused extensive
normal faulting at Louse, allowing hydrocarbons to leak out at this
time.
Some people have explained the
disappointing results by arguing that hydrocarbons never filled the main
reservoir at Louse; they suggest that the faults we see on the seismic
were never wide-ranging enough to act as a migration pathway. This would
make Louse somewhat unique in a basin with so many surface seeps.
Further along the trend, the Tick
structure (Figures 1 and
7) also shows structurally
conformable amplitude anomalies at deeper levels but not at the
shallower main reservoir level. Figure 8
is one such amplitude map. From the drilling results, we know that these
are hydrocarbon-related, whereas the (non-structurally conformable) high
amplitudes seen at 1.2 seconds and 2.1 seconds correspond to porous
sands. Note the lack of younger faults at Tick. The structure was
relatively unaffected by the Latest Pliocene tectonics that affected
both Cockroach and Louse; so entrapped hydrocarbons could not leak to
the surface.
The final structure to examine is Flea,
at the northern end of the trend. Figure
9 shows amplitude anomalies at 1 5 and 2.5 seconds. Both are
structurally conformable and highly faulted, with the shallower anomaly
also showing a stratigraphic component on 16-bit seismic data (not
illustrated) due to channeling. Figure 10
shows the amplitude map at the lower level. On drilling, both levels
showed only residual gas; the hydrocarbons had leaked out.
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Concluding Remarks
In retrospect, we now realize that a
typical structure in this basin could have a multiplicity of false
positive and false negative hydrocarbon indicators on conventional
compressional ("P-wave") data.
-
1. Scattering associated with a
shallow gas cloud could obscure the amplitude anomaly associated with
either the top of the reservoir or OWC (or GWC).
-
2. Velocity effects due to the shallow
gas could obscure the fact that an amplitude anomaly is structurally
conformable, even on 3-D data.
-
3. 8-bit data do not show
stratigraphic components of trapping.
-
4. Porosity, rather than fluid fill,
is the primary driver of amplitude response in this area so that
porous beds could be mistaken for pay.
-
5. Leakage of hydrocarbons due to mud
volcanism or recent faulting could leave behind a paleo-OWC that may
not be distinguishable on the seismic from the real OWC, if one still
exists.
Although the study area may be unique in
having all these effects simultaneously, there is reason to suppose that
many basins around the world could exhibit one or more of these
problems, which could mean the difference between an exploration success
or failure. At the time of this exploration campaign, the technology
needed to distinguish between residual and commercial hydrocarbons and
to image through gas clouds was not widely available Now, with
multi-component data which also utilize shear waves ("S-wave"), we have
the ability to derive density from seismic data and to make accurate
structural maps even in the presence of shallow gas.
Intelligent explorationists will
undoubtedly take advantage of these new techniques to distinguish
residual hydrocarbons from commercial accumulations. And no doubt Mother
Nature, in her turn, will find some new way to baffle us!
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