By OGJ Staff
HOUSTON, Jan. 23 -- A multiwell 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 Ltd., Sydney, believes enough in the basin's potential that it has acquired 20-55% equity in and operatorship of four contiguous permits covering 7 million 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 m 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 Ltd., 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 m 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° gravity oil.
The Cliff Head-2 sidetrack encountered a 28.5-m gross vertical oil column at a bottomhole location 1.1 km from Cliff Head-1.
No further drilling occurred until this month. 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 m of that in CH-1 and 2. A minimal sidetrack is planned to core the reservoir and run liner and a production test, perhaps within a week.
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) Pty. Ltd. 27.5%, Wandoo Petroleum Pty. Ltd. 25%, ARC Energy NL 7.5%, and 5% each for Voyager Energy Ltd. and the Westranch Holdings Pty. Ltd. subsidiary of Norwest Energy NL.
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 Pty. Ltd. drilled the Morangie-1 wildcat and a sidetrack on behalf of a multicompany group in late 2000 and found the Wagina formation wet. TD is 2,188 m MD in the main hole and 2,165 m MD in the sidetrack.
The wells are on WA-226-P, operated by Origin Energy Resources Ltd., 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 bbl of 37° gravity oil from Dongara at 2,407-19 m, while the High Cliff sandstone was wet.
Jingemia-1 cut 34 m of Dongara sandstone of which 32 m is interpreted as being of good to excellent reservoir quality. This overlies 43 m of poorer quality Wagina formation.
The well averaged 1,850 b/d 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 MMcfd of gas at 2,158 psig on a ¾ in. choke from Wagina at 3,370-87 m and 3,392-95 m 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 NL 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 b/d of 41.6° gravity oil (950 b/d calculated rate) with 100 Mcfd of associated gas on a drillstem test in the Dongara sandstone at 1,995-2,002 m.
Hovea-2 drillstem tested 16.5 MMcfd of gas on a ¾ in. choke with 1,220 psi flowing tubing pressure from High Cliff sandstone at 2,370-2,419 m. 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 b/d, 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 million bbl.
Hovea-5 was to spud in February 2003, followed by the Eremia-1 exploration well west of the northern part of the Hovea structure
