Trident Energy Limited
Trident Energy Limited is exploring offshore Otway Basin permit Vic/P62 (see Fig. 1); the permit covers 4630 km2. Trident Energy has interpreted a number of significant anticlinal closures, and is preparing to improve their structural definition with 2D seismic acquisition in 2008.
Only three wells have been drilled on a relatively sparse grid of seismic within the permit, Nerita-1 (1967), Snail-1 (1972) and Wild Dog-1 (1992-93). None of these wells is interpreted to have tested a valid structural closure. The primary objectives in all three wells were sandstone reservoirs in the Eocene Boonah Formation and the Late Cretaceous-Eocene Eastern View Group. Water depths are shallow, with a maximum depth of approximately 90 m.
The Torquay Sub-basin was part of the larger Otway Basin tectonostratigraphic system until the mid- Cretaceous. It is therefore interpreted that source rocks from elsewhere in the Otway Basin would have been deposited in the Sub-basin, namely the Casterton Formation, the Crayfish Subgroup and the Eumeralla Formation. Evaluation of source rocks onshore to the northwest of Vic/P62 indicates very good source rock potential in these sequences. This part of the basin has evidence of petroleum seepage at the seabed, with thermogenic hydrocarbons in vibrocore samples confirmed by geochemical analysis in 1992, and interpreted DHI’s on seismic. Marginally mature source rocks were intersected in the well Nerita-1; these are inferred to be mature for oil and gas in deeper kitchens for defined leads. Non-productive gas shows were recorded from Nerita-1 within this interval (up to 18% C1 recorded at 1653 mKB and up to 20% C1 from 1926 mKB to TD.
The Boonah Formation encountered in Wild Dog-1 comprised unconsolidated shallow marine sands with excellent reservoir properties (net:gross of 99.6% and an average log-derived porosity of 32.1%). The Eastern View Group comprises a sequence of Campanian to Early Eocene interbedded nonmarine sands, carbonaceous silts, clay and coal, deposited in a dominantly fluvial environment. The group is up to 450 m thick in Vic/P62. All three wells encountered fair to excellent reservoir properties there with average log derived porosities of 29-31% where measured in Wild Dog-1 and Snail-1.
Although the overlying Demons Bluff Group is anticipated to have excellent sealing potential, based on mercury injection capillary pressure measurements in Wild Dog-1, Trident Energy believes the sealing potential of the intra-formational fluvial plain to deltaic claystones and mudstones within the Eastern View Group to be more significant, by analogy with discovery wells in the Bass Basin such as Yolla. Well data from Nerita-1 indicates there are several massive shale sequences, each of 20-30 m thickness.
Seismic interpretation by Trident Energy has identified several anticlinal closures at top Eocene Boonah Formation and top Late Cretaceous-Eocene Eastern View Group. These structures will be the focus of further 2D seismic delineation. Two periods of structuration are identified within the Tertiary. The first are Miocene aged anticlines, similar in age to fields found in the Gippsland Basin. The second are Oligocene anticlines; Figure 2 shows an area of focus of the depth mapping at top Eastern View Group, in the north of Vic/P62, showing significant anticlinal closures identified.
Trident Energy is completing interpretation and preparing a prospect and leads inventory in preparation for 2D seismic acquisition of 420 km.
For more information contact:
Trident Energy Limited
60 Kooyong Koot Road, Hawthorn,
Vic 3122, Australia
Tel +419 894147
Fax +61 3 98185948