Potential Problems in Interpreting Amplitude Anomalies*
Search and Discovery Article #40053 (2002)
*Adapted for online presentation from the article by the author in AAPG Explorer (October, 1999), entitled “Mother Nature’s a Sly Operator.” Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
1Unocal, Sugar Land, Texas ([email protected]).
Were there any natural justice in the world, this article would be the perfect Victorian morality tale: If you do your homework, spend your money wisely and are not afraid to take prudent risks, you should be rewarded, right? Unfortunately, sometimes Mother Nature wants to play tricks on us, and our belief in the technology we use becomes shaken.
In this case we found that advanced geophysical modeling allowed us to recognize a phenomenon we had not seen before. Unfortunately--at the time--we did not have the additional technology we needed to distinguish this phenomenon from commercial hydrocarbons! Before we get to that, let us discuss something that has been widely discussed in recent years in the geological literature: the issue of paleo-oil-water contacts (paleo-OWCs). In outcrop, bitumen-stained sandstones can be easily recognized. Geologists from Norway to Abu Dhabi routinely identify residual oil or tar mats below present-day OWCs in cases where there has either been a seal failure, change in hydrodynamic regime, or subsequent regional tilting.
What is considerably less common is the identification of paleo-OWCs by geophysical means. However, as this article shows, this phenomenon of paleo-OWCs may be more widespread than previously thought and could, in fact, be responsible for some previously unexplained exploration failures.
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.
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.
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!