tAcknowledgments

tIntroduction

tFigure captions

tCase histories

tConcluding remarks

tAcknowledgments

tIntroduction

tFigure captions

tCase histories

tConcluding remarks

tAcknowledgments

tIntroduction

tFigure captions

tCase histories

tConcluding remarks

tAcknowledgments

tIntroduction

tFigure captions

tCase histories

tConcluding remarks

tAcknowledgments

tIntroduction

tFigure captions

tCase histories

tConcluding remarks

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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