Figure 5. Top Triassic map of Belgravia and Palmerston prospects within WA-155-P.
Figure 6. Top Triassic model of the “JBS structure”

The Outtrim project is in the Barrow sub-basin, within the Carnarvon Basin of the North West Shelf of Australia. Carnarvon recently increased its equity to 70% and secured operatorship of the WA-155-P permit, which forms the Outtrim project.

The Outtrim project contains the Palmerston, Belgravia and Belgravia-East prospects, all Triassic Mungaroo gas targets. Carnarvon believes there to be significant gas potential in this area of the Carnarvon Basin, as outlined in figure 5.

The Triassic play system is the most successful petroleum play within the North West Shelf creating a heartland of LNG scale gas and condensate discoveries. Triassic reservoirs have underpinned fields such as Gorgon, Rankin and Wheatstone. The petroleum traps within this play tend to be simple fault block structures with excellent reservoir quality, dependant on depth of burial.

The Palmerston prospect, which is a fault bounded Triassic structure, sits on the eastern side of the Alpha Arch. While this Triassic play has been successful in the Gorgon field further outboard to the Outtrim area, there are several sub-economic discoveries on the nearby Alpha Arch which have proved a working petroleum system in the region. Palmerston has the potential to have over 500 metres of Jurassic shales over the top of the structure, which differentiates the prospect from those on the Alpha Arch to the west. The thickness of these sealing shales could indicate the potential to trap a larger hydrocarbon column, meaning any discoveries could be significantly larger than those on the Alpha Arch.

This north-east block of the WA-155-P permit was only recently amalgamated back into the rest of the project and Carnarvon is still evaluating the prospective resource, including the potential size, at Palmerston. Given the surrounding infrastructure, a discovery could be used as backfill gas for existing LNG projects, or alternatively tied into the domestic gas system either as a standalone project or via already existing gas plants. In the western graticular block of the permit lies the Belgravia prospect, which has been upgraded following the Swell-1 well previously drilled by Woodside Petroleum. The Swell-1 well, whilst being a tight gas discovery, is encouraging because it confirms the presence of a working petroleum system, over a large hydrocarbon column, only some 20 kilometres from the Belgravia prospect.

Carnarvon’s mapping indicated the crest of the Belgravia structure is up-dip of the Swell-1 gas discovery and around 650 metres shallower than the Swell structure. Typically, reservoir quality improves at shallower depths. Accordingly, on face value, Belgravia could expect to contain better reservoir quality compared to that reported at Swell. During the year, Carnarvon progressed its regional Triassic mapping, through which Carnarvon has also observed that the Belgravia prospect could be part of a much larger structure that Carnarvon refers to as the Jurabi-Belgravia-Swell structure (“JBS structure”), as indicated in figure 6.

The JBS structure has been proven to contain gas based on the down-dip Swell-1 well (Top Triassic is around 4,500 metres below seabed) and the up dip Jurabi well (Top Triassic is around 3,600 metres below seabed). Both wells were reported as relatively low porosity Triassic gas discoveries.

Carnarvon undertook an analysis to determine the reason for the low porosity sands, noting that comparable Triassic gas fields on the North West Shelf of Australia have higher porosities. This analysis included cutting based thin section petrography work which indicated that the Swell-1 and Jurabi-1 wells are likely cemented due to hydrothermal fluids. This type of cementation is usually quite aerially limited and as such would indicate the porosity reduction is a local affect around the wells within proximity to faults.

The consequence is that the porosity and reservoir quality within the Belgravia and Belgravia-East prospects, and potentially within the larger JBS structure, could potentially be similar to the other nearby commercial Triassic gas fields. If the average porosity across the JBS structure was even marginally better than that observed in the Swell-1 and Jurabi-1 wells, this would have a significant beneficial effect on permeability, potentially reclassifying a tight gas discovery into a significant gas field discovery.

Given the significant scale of the JBS structure, which is similar to other fields on the North West Shelf, there is considerable potential from what could already be a significant discovery. On this basis, Carnarvon plans to mature this project through a future farm-out to progress towards further testing the highly prospective Belgravia and Palmerston opportunities.

Figure 5. Top Triassic map of Belgravia and Palmerston prospects within WA-155-P.
Figure 6. Top Triassic model of the “JBS structure”