Discoveries, offshore oil attract firms to northern Perth basin

Feb. 10, 2003
A multiwell drilling program is under way to appraise the first offshore discovery in the Perth basin off Western Australia.

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 that cover a combined 7 million acres within 125 km of shore in the Indian Ocean (Fig. 1). The blocks stretch 350 km along the coast from Cerrantes to north of Kalbarri.

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The discovery well, Cliff Head-1, is in 16 m of water 11 km south of Dongara on WA-286-P. Roc Oil called Cliff Head "the first potentially commercial, live oil discovery in the northern part of the offshore Perth basin." It has said Cliff Head could contain 80-100 million bbl of oil in place plus 20-30 million bbl in a satellite structure.

Meanwhile, various groups of Australian independents led by Arc Energy NL, West Perth, and Origin Energy Resources Ltd., Sydney, are appraising several recent onshore discoveries.

The Perth basin has about 21 hydrocarbon discoveries, of which eight are considered commercial. Six fields are on production.

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 sandstone just below the Triassic Kockatea shale.

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 January 2003. Cliff Head-3, the first well in the new program, is 2.4 to 3 km southeast of Cliff Head-1 and 2. A sidetracked core hole, CH-3-ch1, flowed 33.3° gravity oil at a maximum stabilized 3,000 b/d on a downhole electrical submersible pump with a GOR of 100 cf/bbl in late January 2003. Roc Oil said the encouraging rate was constrained by the capacity of surface facilities.

If Cliff Head were developed, it would be with horizontal wells. The Ensco 53 jackup was to be released in early February to drill the Twin Lions-1 exploration well on a smaller structure on nearby TP/15, which Roc Oil operates with 20% interest. Northwest along the coast, Roc Oil holds 55% of WA-325-P and WA-327-P and operates both.

Holdings in WA-286-P are Roc Oil 30%, AWE Oil (Western Australia) Pty. Ltd. 27.5%, Wandoo Petroleum Pty. Ltd. 25%, Arc 7.5%, and 5% each for Voyager Energy Ltd. and the Westranch Holdings Pty. Ltd. subsidiary of Norwest Energy NL.

Meanwhile, Apache Northwest Pty. Ltd. drilled the Morangie-1 wildcat and a sidetrack on behalf of a multicompany group in late 2002 and found the Permian Wagina formation wet. TD is 2,188 m MD in the main hole and 2,165 m MD in the sidetrack.

Morangie is on WA-226-P, operated by Origin 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.

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During 2002 the government awarded WA-326-P to Agip Australia and WA-328-P to AGIP Australia 67% and Santos Offshore 33% (Fig. 2).

Perth onshore

Operators reported success at several onshore Perth operations in 2001-02.

Origin plans production tests 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 Permian Dongara at 2,407-19 m, while the Early Permian 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 1/2 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 to Beharra Springs facilities last July.

Arc and Origin 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 1/2 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 some 200 km to BP PLC's refinery at Kwinana. Arc said consulting engineers assigned 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.

Industry sources have discussed preliminary in place figures of 30 million bbl for Hovea, 5 million bbl for Jingemia, and 46 bcf of gas for Beharra North.

Hardman Resources NL, West Perth, operates Woodada gas field and three onshore blocks in the vicinity of the producing gas fields.

The most recent Woodada well confirmed gas in the main Late Permian Carynginia limestone reservoir and had oil shows in deeper zones drilled to test the potential indicated at Cliff Head. Further drilling is planned to maintain Woodada output and explore the blocks.