Carnarvon wins Rowley block

CARNARVON Petroleum has been granted WA-521-P in the Rowley Sub-basin of the Roebuck Basin, adjacent to where it has successfully drilled its Roc-1 and Phoenix South-1 wells with its partners.

Haydn Black

Reporter

The 100%-owned Rowley Sub-basin permit covers 5035sqkm directly north of the Roc and Phoenix South-1 oil and gas discoveries in the Bedout Sub-basin.

The block, originally released as W15-5 has water depths of 250-1500m, and a single well penetration at Whitetail-1.

It is south of where Woodside Petroleum and Shell drilled a number of dusters in 2014.

Woodside operated three wells in its Outer Canning Campaign - Hannover South-1 in WA-466-P, Steel Dragon-1 in WA-464-P and Anhalt-1 in WA-462-P - but wrapped up the work without a result.

It axed the program after the dry holes, finding little promise and it assigned the Deepwater Millennium rig to drill the Pyxis-1 well, which was later recognised as a discovery.

Carnarvon's new blocks are closer to shore, about 200km from the north-west coast.

Past drilling has been minimal, with Woodside's Whitetail-1 the only well drilled in WA-521, then known as WA-296-P.

Whitetail-1 spudded in January 2003, reaching a total depth of 2504m in sandstones belonging to the Middle Jurassic Legendre Formation. The objective Legendre Formation reservoir sandstones were encountered water wet at 2362m.

No hydrocarbons were encountered and Whitetail-1 was plugged and abandoned as a dry hole, as was a follow-up well, 2006's Huntsman-1, which set a new water depth record of 1468m at the time.

Huntman-1 was drilled outside WA-521-P.

Woodside CEO Peter Coleman had been bullish about the potential of the Rowley Sub-basin for multi-trillion cubic feet gas accumulations and with Shell offered to drill up eight wells with encouragement, but the failure of the costly Outer Canning program means companies such as Woodside and BHP are now looking at the Beagle Sub-basin instead.

Of the 10 wells drilled in the Rowley Sub-based to date, four were attempts to test the East Mermaid structure in 1973, which the last owner, private oiler Pathfinder Energy has been reworking to take account of the emerging Triassic play.

Pathfinder says East Mermaid was never properly tested as the well did not hit the target, and it says its new 250MMbbl Pina Colada target sits along the same Lower Triassic fairway along the inboard margin of the Roebuck Basin.

Shell drilled a well in the Rowley Sub-basin in 1991, with Trochus-1 penetrating the Tertiary to Lower Cretaceous stratigraphic sequence to a total depth of 1122m.

Quality sandstone reservoirs were intersected in the Jurassic section of the well, however, only very minor fluorescence was recorded and no net hydrocarbon zones were found.

Petrophysical evaluation found the Jurassic reservoir sequence to be fully water-bearing.

The well was plugged and abandoned as a dry hole.

The Rowley Sub-basin is an underexplored area covering some 66,000 sq.km of the North West Shelf, with water depths ranging from 250m on the shelf to greater than 5000m in the Argo Abyssal Plain.

It is adjacent to the proven Northern Carnarvon Basin hydrocarbon province and contains late Paleozoic through Mesozoic strata, with the first evidence of a petroleum system suggested by an interpreted gas column at BHP Billiton's Phoenix-1 in the 1970s, a well followed up on by Carnarvon in the 2010s after the Bedout Sub-basin was largely ignored after the disappointments of the 1970s and 80s.

Despite decades of being ignored, Geoscience Australia says modelling of potential Rowley Sub-basin source rocks indicates that liquid hydrocarbons are currently being generated.

With success at Phoenix South-1 in 2014 the Rowley is being reassessed for its Triassic potential.

The main reason for the lack of discoveries in this area is thought to be the lack of valid traps and not a lack of oil charge.

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