Australian firms plan appraisal of first offshore discovery in Perth basin

Jan 23, 2003 01:00 AM

A multi-well drilling program is under way to appraise the first offshore discovery in the Perth basin off Western Australia. One group of Australian independents plans three to nine exploratory and appraisal wells or sidetracks back to back on two blocks on trend with the December 2001 Cliff Head oil discovery.
Cliff Head has not been declared commercial, and its value is not known. The operator, Roc Oil, Sydney, believes enough in the basin's potential that it has acquired 20-55 % equity in and operatorship of four contiguous permits covering 7 mm acres total within 125 km of shore in the Indian Ocean. The blocks stretch 350 km along the coast from Cerrantes to north of Kalbarri.

The discovery well, Cliff Head-1, is in 16 metres of water 11 km south of Dongara on WA-286-P and a similar distance west of onshore Beharra Springs gas field. Roc Oil calls Cliff Head "the first potentially commercial, live oil discovery in the northern part of the offshore Perth basin." Onshore northern Perth has been gas prone. Cliff Head is 290 km north of Perth.
Meanwhile, various groups led by Origin Energy Resources, Sydney, are appraising several recent onshore discoveries. Drilling details Log interpretation of Cliff Head-1 indicated a 5-m oil column at 1,278.5-1,283.5 metres below rotary table in the primary objective Permian Wagina (Dongara) sandstone just below the Triassic Kockatea shale. The operator recovered a sample of 31.6 degrees gravity oil.

The Cliff Head-2 sidetrack encountered a 28.5-m gross vertical oil column at a bottom hole location 1.1 km from Cliff Head-1. No further drilling occurred until now. Cliff Head-3, the first well in the new program, is 2.4-3 km southeast of Cliff Head-1 and 2. CH-3 wireline logs indicate a "48-m gross oil column with at least 40 % net oil pay with moderate to good reservoir characteristics and the possibility of an additional 30 % potential net oil pay with poor to moderate reservoir quality," Roc Oil said.
The CH-3 oil-water contact is within 2 metres of that in CH-1 and 2. A minimal sidetrack is planned to core the reservoir and run liner and a production test. If Cliff Head were developed, it would be with horizontal wells, Roc Oil said. The Ensco-53 jackup is handling the current drilling program.

Holdings in WA-286-P are Roc Oil 30 %, AWE Oil (Western Australia) 27.5 %, Wandoo Petroleum 25 %, ARC Energy 7.5 %, and 5 % each for Voyager Energy and the Westranch Holdings Pty. subsidiary of Norwest Energy. Cliff Head is near TP/15, which Roc Oil operates with 20 % interest.
Northwest along the coast, Roc Oil holds 55 % of W01-24 and W01-22 and operates both. Meanwhile, Apache Northwest drilled the Morangie-1 wildcat and a sidetrack on behalf of a multi-company group in late 2000 and found the Wagina formation wet. TD is 2,188 metres MD in the main hole and 2,165 metres MD in the sidetrack.

The wells are on WA-226-P, operated by Origin Energy Resources, Sydney, nearly 300 km northwest of Cliff Head. Origin said it will renew WA-226-P for 6 years. It plans to acquire 500 sq km of 3D seismic data this year and might drill again in 2004. Perth onshore Origin reported success at several onshore Perth operations during 2001-02.
Production tests are planned this quarter at Jingemia-1, an apparent discovery on EP 413 at 5 km southwest of Hovea oil field. A drillstem tested in October 2002 yielded 26.5 barrel of 37 degrees gravity oil from Dongara at 2,407-19 m, while the High Cliff sandstone was wet. Jingemia-1 cut 34 metres of Dongara sandstone of which 32 metres is interpreted as being of good to excellent reservoir quality. This overlies 43 metres of poorer quality Wagina formation.

The well averaged 1,850 bpd on a 10-day flow test in December 2002 and was to be placed on a 3-month production test starting in February. Origin's Beharra Springs North-1 discovery flowed an estimated 30 mm cfpd of gas at 2,158 psi on a 3/4 inch choke from Wagina at 3,370-87 metres and 3,392-95 metres in late 2001. The rate at this well on PL L11 is similar to rates recorded at wells in Beharra Springs field 3.5 km south. Output began from Beharra Springs facilities last July.
Origin and Arc Energy 50-50 are appraising the 2001 Hovea oil discovery and exploring Production Licenses L1 and L2, excluding Dongara, Mondarra, and Yardarino fields. Hovea is the Perth basin's first commercial oil find since 1966. Hovea-1, an October 2001 discovery 14 km south of Dongara Township on L1, flowed 122 bpd of 41.6 degrees gravity oil (950 bpd calculated rate) with 100 mm cfpd of associated gas on a drillstem test in the Dongara sandstone at 1,995-2,002 metres.

Hovea-2 drillstem tested 16.5 mm cfpd of gas on a 3/4 inch choke with 1,220 psi flowing tubing pressure from High Cliff sandstone at 2,370-2,419 metres. The deposit could not immediately be declared commercial, but Origin said it is "the first discovery of gas in this particular combination of reservoir and structural setting, and so opens new exploration possibilities within our acreage in the Perth basin."
Hovea-3 is on early production of around 2,000 bpd, trucked around 200 km to BP PLC's refinery at Kwinana. Arc, of West Perth, said an independent reserves report gave Hovea field P50 reserves of 9.5 mm barrel. Hovea-5 was to spud in February 2003, followed by the Eremia-1 exploration well west of the northern part of the Hovea structure.

Source: Oil & Gas Journal
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