BARRAMUNDI–1: A CASE HISTORY IN FRACTURE ANALYSIS AND SEAL INTEGRITY IN THE BASS BASIN

Abstract

Barramundi–1 was drilled to test a faulted rollover structure with thick potential reservoir sands in the top Eocene Eastern View Coal Measures. A structure-wide AVO anomaly was mapped corresponding to these sands and this suggested the presence of a valid hydrocarbon trap.

Barramundi–1 reached a total depth of 2,100 m on October 2 1999 after penetrating a thick reservoir sand sequence in the Eastern View Coal Measures within the T27/P permit operated by GLOBEX Far East.

No significant hydrocarbons were encountered. Log analysis showed that large borehole breakouts occur below 1,400 m in thick Eastern View sands and background gas was observed to decrease. Dipole Sonic Imager (DSI) logs also showed that the amplitude of the fast shear wave event was attenuated from 1,250–1,450 m and was erratic in amplitude below this depth.

The Compressional Velocity (Vp) to Shear Wave Velocity (Vs) ratios showed large variations over short intervals in the break out zones.

It was concluded that Barramundi–1 failed to contain commercial hydrocarbons due to the seal failure caused by late Tertiary movement which has manifested itself both as fractures and faults, with a principal stress axis that trends northeast–southwest.

An AVO anomaly mapped across the Barramundi structure has probably been caused by a combination of lithological and stress anisotropy in thick Eastern View sands at depths below 1,500 m, as observed by ovalisation occurring on the Caliban log analysis.

It was concluded that the stresses would add to anisotropic effects in the rocks and consequently direct hydrocarbon detection methods such as AVO have to be used with caution.

The central Bass Basin has generated both oil and gas. More wells need to be analysed using seal integrity methods in order to fully determine the reason for the failed trapping mechanism at Barramundi. This could eventually lead to a fault seal model that would allow exploration to successfully continue.