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SUMMARY

A spectrum of anomalies is observed. The most positive, from an exploration standpoint, are either high intensity/ laterally restricted or low intensity/laterally extensive velocity anomalies. The former category includes fields and wells such as Bayu-Undan, Skua and West Enfield, whereas the latter includes Macedon. The common theme with these types of anomalies is that the total leakage from the traps appears to have been relatively small compared to charge received (i.e. total input greatly exceeds total output). In the former case, high rates of seepage may have taken place from very localised sources, whereas in the latter, low rates of seepage have probably taken place along entire fault systems. The presence of these anomalies shows that the source rocks in the region have generated significant volumes of hydrocarbons.

The least attractive traps appear to be those which have high intensity/laterally extensive anomalies associated with them. These traps have experienced high rates of seepage over a wide area, a scenario favouring substantial, if not complete breaching of the underlying accumulation (i.e. total output is large relative to total input). Unless hydrocarbon recharge rates can be demonstrated to be extremely high, these types of traps represent high-risk targets. The presence of these anomalies nevertheless demonstrates that the surrounding region, as a whole, contains a viable petroleum system. Exploration in these areas should focus on less reactivated traps, or traps with less evidence of leakage (higher seal capacity).

The fourth type comprises low intensity/laterally restricted anomalies. These anomalies, while providing some evidence of the presence of a petroleum system, are difficult to interpret. The lack of anomalies could be principally a function of high seal integrity within the traps or the presence of a weak to marginal petroleum system, or a combination of both. These anomalies can also be the artifacts of insufficient seismic coverage (too wide a line spacing) or poor seismic data quality.

None of the types of anomalies documented in this study, of themselves, provides conclusive proof of the exploration potential of a given region or structure. What does add real value to the evaluation process is the integration of these leakage-related indicators with regional and prospect-scale information regarding structural closure, fault and top seal integrity, and charge history. In some cases, the detailed mapping of the seafloor, using bathymetric data which are obtained during seismic acquisition, can define seafloor pockmarks. Similarly, seismic amplitude analysis of the seafloor can provide information on areas of seepage-related induration. Finally, if required, the concepts developed during this integration can be further tested, and ultimately confirmed, via the acquisition of focussed and appropriate remote sensing seepage data.

Table 2. Possible exploration implications of hydrocarbon-related seismic anomalies and resulting exploration strategies.