Tahbilk

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Tahbilk gas discovery, Bonaparte Basin

Field: Tahbilk gas discovery
Location: Southern Vulcan Sub-basin, Timor Sea
Top of HRDZ: 700 ms
Pull-up: Strong
Intensity: High
Amplitude Anomalies: Moderate

Several high intensity, large and linear HRDZs are present near the Tahbilk gas accumulation, located in the southern Vulcan Sub-basin, and show diagnostic pull-up and degradation of the stack response (Fig. 5). The HRDZ over Tahbilk displays an amplitude anomaly near 700 ms, about 100 ms of pull-up and stack response degradation beneath the anomaly. The reduced amplitudes of the reflectors below 1,000 ms under the HRDZs may be related to the combined effects of gas in the formations and reduced stacking effectiveness.

The HRDZs over Tahbilk show that this field has leaked considerably, in contrast to the nearby Montara oil-gas accumulation. It appears that the HRDZs associated with Tahbilk run along low displacement, Mio-Pliocene faults that provide leakage paths from the Mesozoic reservoirs to the Eocene Grebe Formation, in a similar fashion to that described at Skua (O’Brien and Woods, 1995). Exactly why Tahbilk has leaked and Montara has not remains uncertain, though it could be related to minor differences in the age and lithology (plasticity) of the sealing facies, the extent of fault reactivation and its relationship to the regional stress field, or a combination of these factors. Another field in the region that is similar to Tahbilk in terms of its HRDZs is Keeling, which contains a sub-commercial gas accumulation. This accumulation previously held a 33 m oil column, was breached at about 5 Ma, and then accumulated a 33m gas column in the Pliocene (O’Brien et al, 1996). Whether or not Tahbilk ever had a significant oil leg is at this stage unknown.

Figure 5. HRDZs near the Tahbilk gas accumulation, southern Vulcan Sub-basin.