Implications

[ Up ] [ Introduction ] [ Seismic Expressions ] [ Results ] [ Bayu-Undan ] [ Tahbilk ] [ Skua ] [ Jabiru ] [ Cornea ] [ Anomaly1 ] [ Anomaly2 ] [ Anomaly3 ] [ North Rankin ] [ Enfield ] [ Macedon ] [ Chinook ] [ Carnarvon ] [ Novara NW ] [ Zeewulf ] [ Jupiter ] [ Scarborough ] [ Otway ] [ Kingfish ] [ Exploration ] [ Skua Detail ] [ Macedon Detail ] [ Implications ] [ Summary ] [ Acknowledgements ] [ References ] [ Authors ] [ Update ]

EXPLORATION IMPLICATIONS OF SEISMIC LEAKAGE INDICATORS

Based upon the observations presented in this paper, many seemingly unrelated observations commonly made regarding oil and gas discoveries can be attributed to HRDZ formation. These include:

1. Oil fields are often located within, and masked by, zones of poor seismic data quality.
2. Drilling is often more difficult in a discovery well than in an adjacent, dry hole, perhaps because of enhanced cementation overlying the reservoir.
3. Accurate depth conversion of seismic data is more difficult over an oil or gas field than in the adjacent areas.
4. Discovery wells are often drilled on time pull-ups caused by overlying high velocity anomalies.*
5. Onshore, many oil and gas wells are located beneath (erosion-resistant) topographic highs (Saunders, 1999).

In general, hydrocarbon leakage indicators are relatively common on seismic data over the oil and gas discoveries investigated. They are also present away from known accumulations. The nature, abundance and distribution of these leakage-related seismic anomalies varies widely, however (Table 2). This variation is a direct function of the interplay between variables such as charge history, seal capacity and reactivation history. HRDZs, broadly analogous to those described in the Timor Sea by O’Brien and Woods (1995), appear to be the most common type of leakage indicator present, and are manifested by a range of seismic velocity anomalies.

Low intensity, isolated anomalies

In some basins, or over some structures investigated, leakage indicators are relatively rare, and hence their exploration significance remains poorly understood. Nevertheless, in such areas the presence of hydrocarbon-related seismic indicators does at least confirm the presence of a petroleum system, which may reduce uncertainty associated with charge. Examples of this style include subtle anomalies associated with the Chinook Oil Field in the Carnarvon Basin (Fig. 16), anomalies associated with the Griffin Oil Field (Cowley, unpublished data) and the example from the Otway Basin (Fig. 22).

These anomalies are of low seismic intensity and small lateral extent (Table 2) and, as such, their interpretation is often ambiguous, particularly if they have been mapped initially using widely spaced, regional 2D seismic data. One interpretation of these isolated, subtle anomalies is that the traps with which they are associated have high seal integrity, with attendant low rates of hydrocarbon seepage (or alternatively, they show little response to the seepage, whatever its rate). Nevertheless, subtle features such as those associated with the Chinook Field can potentially be useful for high grading prospects, and consequently, an understanding of their significance is critical. A better evaluation of these features often requires the acquisition of improved seismic data (either 3D or high resolution 2D) to accurately define the extent and nature of these features. These better quality data allow the features to be mapped more reliably, and a better appreciation of their relationship to mapped closure, reactivated faults, etc to be developed.

High intensity, isolated to laterally restricted anomalies

Several of the examples investigated consist of high intensity seismic anomalies which are either isolated or form relatively geographically restricted packets of HRDZs along the bounding fault systems. The high intensity nature of these anomalies may indicate high rates of hydrocarbon leakage (i.e. high fluxes), which is taking place essentially from point sources (O’Brien et al, 1999). Fields and structures that fall into this category include Bayu-Undan, Skua, the adjacent Spruce Prospect, and West Enfield. The intense nature of these anomalies provides compelling evidence of a volumetrically significant petroleum charge, as they suggest that a high rate of hydrocarbon seepage has taken place from relatively localised fault segments. In the case of the Jabiru and Skua fields, these anomalies actually define the palaeo- and present day extents of the fields respectively (O’Brien et al, 1996). The presence of such features over a structure is extremely positive from an exploration standpoint and can significantly upgrade the prospectivity of the trap, by indicating a high probability of charge. The key uncertainty with this type of analysis is to ensure that the inferred high rates of leakage have only taken place over a limited area, otherwise the risk of trap breach is substantial.

Again, detailed mapping of the seismic anomalies, and their relationship to the mapped closure etc, is critical if the interpretation of these seismic effects is to be soundly based. In particular, the acquisition of remote sensing seepage data, such as water column geochemical sniffer or Synthetic Aperture Radar (SAR), over these features could considerably assist in their evaluation and characterisation, particularly in regard to whether they represent oil versus gas leakage (O’Brien et al, 1999 and 2000).

Low-to-moderate intensity, laterally extensive anomalies

The Macedon Field represents an example of where subtle velocity anomalies are present over the entire prospect, and hence actually closely mirror the field size. In this way, the anomalies are similar to those mapped over the Skua Field, except that over Skua, the anomalies are seismically intense. The subtle anomalies over Macedon might suggest that leakage rates have been relatively low, but were laterally extensive, extending along the length of the faults that define the field. Leakage along the length of the fault systems may have been facilitated by the fact that the hydrocarbons at Macedon are very dry gas (94% methane; Tindale et al, 1998), which is relatively more mobile than oil, for example.

Areas of the seafloor around the Macedon Field appear to be slightly disturbed, possibly by gas seepage, which might indicate that seepage has occurred at significant rates, possibly episodically. As in the case of the high intensity/isolated anomalies, remote sensing seepage technologies might add significant value to the understanding of seepage rates and composition.

High intensity, laterally extensive anomalies

In several cases, such as at Swift, Tahbilk and Mistral, the seismic anomalies are both intense and extensive. This suggests that a high rate of hydrocarbon leakage has occurred over a large area, a scenario favouring significant to complete breaching of the accumulations. Whilst fluid inclusion data in the Swift well (Lisk et al, 1998) support this proposal, no such data are available for the Mistral and Tahbilk structures. It might be expected that analysis of these two wells would reveal either the presence of a palaeo-oil leg at the top of the trap (which has since leaked, as with the Keeling–1 well; O’Brien et al, 1996) or a significant residual gas-condensate column.

* This sentence is changed from the original published paper which stated: "Discovery wells often penetrate high velocity anomalies above the reservoir units.".