Area NT07-1

Troubadour Terrace, Bonaparte Basin

Exploration History

Bonaparte Basin Exploration Overview

Offshore exploration of the Bonaparte Basin commenced in 1965 when regional aeromagnetic data were acquired. This was supplemented by regional seismic coverage acquired between 1965 and 1974. The first offshore exploration wells, Ashmore Reef 1 and Sahul Shoals 1, located on the Ashmore Platform, were drilled as stratigraphic tests. Although these wells failed to encounter hydrocarbons, they indicated that the Jurassic section is either thin or absent and that Triassic sandstones form potential petroleum reservoirs over much of the Ashmore Platform.

Between 1969 and 1971, seven wells were drilled in the offshore Petrel Sub-basin. This drilling campaign resulted in the discovery of the Petrel and Tern gas accumulations reservoired within the Late Permian Hyland Bay Formation, which constitutes a primary exploration target in the outboard Petrel Sub-basin.

In the early 1970s, exploration expanded beyond the limits of the Petrel Basin, into the Vulcan Sub-basin and onto the Londonderry High and Sahul Platform. Between 1971 and 1975, 24 wells were drilled—a further nine in the Petrel Sub-basin, four on the Sahul Platform, six in the Vulcan Sub-basin, two on the Londonderry High, and two on the Ashmore Platform. Several significant petroleum discoveries were made during this period including Puffin, Troubadour and Sunrise.

Between 1975 and 1982 relatively low levels of exploration drilling were recorded in the offshore Bonaparte Basin (a total of eight wells) due to disputation between Indonesia and Australia over sovereignty of the sea-bed boundary.

The discovery in 1983 of economic oil and gas in Jabiru 1A, which tested a Jurassic horst block in the Vulcan Sub-basin, stimulated further exploration in the offshore part of the Bonaparte Basin, and 21 exploration wells were drilled in the next three years (1984 to 1986). Of these wells, 12 were located in the Vulcan Sub-basin and on the western flank of the Londonderry High. This phase of exploration resulted in the discovery of a further three commercial oil accumulations in the Vulcan Sub-basin (Cassini, Challis and Skua). Oil production from the Jabiru Challis and Cassini fields is via Floating Production Storage and Offloading Vessels (FPSOs). Production ceased at Skua in 1997.

During the mid 1980s, two non-commercial discoveries of oil were made in stacked reservoirs within the Early Carboniferous Milligans Formation and Late Carboniferous to Early Permian Kuriyippi Formation at Turtle 1 (1984) and Barnett 1 (1985) in the inshore Petrel Sub-basin.

After a brief downturn in 1987, levels of offshore exploration drilling in the Bonaparte Basin accelerated. Between 1988 and 1990, 31 exploration wells were drilled in the Vulcan Sub-basin. Drilling results from these wells proved disappointing, although several other oil and gas discoveries were made. Further to the north on the Troubadour Terrace, Evans Shoal 1 (1988) identified a significant gas accumulation within the Jurassic Plover Formation, with the appraisal well, Evans Shoal 2, being drilled later in the same year.

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Resolution of the territorial dispute between Indonesia and Australia in 1991 established of the Zone of Cooperation (ZOC) and allowed exploration on the Sahul Platform to resume. Between 1992 and 1998, the focus of exploration in the offshore Bonaparte Basin shifted to this area. Of the 73 exploration wells drilled here during this period, 43 were located either on or adjacent to the Sahul Platform, Laminaria High and Flamingo High. The first commercial petroleum success in the area resulting from this phase of exploration occurred in 1994, with the discovery of oil at Elang 1 and the identification of a new oil play on the Flamingo and Laminaria highs.

In 1999, Timor-Leste was granted independence by Indonesia. This created a climate of uncertainty with regard to petroleum exploration in the Zone of Cooperation. In that year, only one exploration well (Jura 1) was drilled in the former ZOC Area A. Since that time, Coleraine 1 (2000) and Kuda Tasi 1 (2001) have been drilled within ZOC Area A. These wells were followed by oil recoveries at Kuda Tasi 2 (2003) in what is now known as the Joint Petroleum Development Area (JPDA).

Exploration drilling on the Londonderry High in 2000 identified numerous gas accumulations within the Hyland Bay Formation at Prometheus 1, Rubicon 1, Ascalon 1A and Saratoga 1.

In the Petrel Sub-basin, two wells (Sandbar 1 and Blacktip 1) were drilled during 2001 in the inshore portion of the basin. No hydrocarbons were encountered in Sandbar 1, but Blacktip 1 was completed as a gas discovery. This well encountered a 20 m gross gas column within the Triassic Mount Goodwin Formation, and a 339 m gross gas column from several high quality, stacked reservoir zones within the Early Permian Keyling Formation. Further drilling in the Petrel Sub-basin met with limited success, with Shakespeare 1 and Weasel 1 having minor oil and gas indications. Polkadot 1 (2004) encountered non-commercial gas in the Hyland Bay Formation, but the recently drilled well Blacktip North 1 (2006), which also targeted gas in this formation, was dry.

Although drilling continued in the Vulcan Sub-basin throughout 2002–2003, no new commercial discoveries were made. However, from June 2005 to mid-2006, there has been a revival in exploration success with the wildcat wells Katandra 1, 1A and Vesta 1 discovering both oil and gas. The development wells Puffin 7 and 9 recovered oil, and the extension/appraisal wells to the Montara field by Coogee Resources successfully discovered more oil at Swift North 1, ST1 and Swallow 1.
During 2002–2003, drilling on the Laminaria High and Flamingo High has been focussed on the development of the accumulations at Buffalo, Kuda Tasi, Laminaria and Bayu/Undan. Of the few recent exploration wells that have been drilled in this area, Firebird 1 (2005) discovered gas.

Drilling has been active in the Malita and Calder graben area since mid-2005, with gas discoveries been made at Caldita 1 (2005), Evans Shoal South 1 (2006) and Barossa 1, ST1 (2006), with appraisal drilling continuing at Caldita 2 (2007).


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Troubadour Terrace Exploration History

Exploration commenced in this general area in 1965 and has continued through to the present day, with the exception of an exploration hiatus between 1978 and 1985, due to a border dispute between Australia and Indonesia. The study area was first explored on a regional basis in the 1960s and early 1970s when Shell, BOC and ARCO acquired regional aeromagnetic surveys providing a regional grid of 10–20 km, and a semi detailed grid of 3–10 km. Shell drilled Lynedoch 1 in 1973 and gas shows were encountered in the Bathurst Island Group and Plover Formation, but the well was not tested.

A second phase of exploration took place in the late 1980’s to early 1990’s when there was acquisition of more detailed seismic data and two wells were drilled in the area (Evans Shoal 1 and Beluga 1). Between 1985 and 1987, WMC acquired 2500 km of regional and semi detailed 2D seismic data, prior to an aeromagnetic survey carried out by BHP in 1988. Later BHP recorded over 5000 km of 2D seismic data, largely over the Malita and Calder graben, with a grid spacing of between 1x3 km and 1x5 km. This resulted in the drilling of Evans Shoal 1 (BHP, 1988) that tested a tilted horst block. The well encountered gas in the Plover Formation, proving gas charge from the graben. BP conducted an airborne laser fluorescence survey (ALF) over part of the area in 1989. Beluga 1 (BHP, 1991) tested a stratigraphic play on the southern margin of the Malita Graben, but, despite some minor gas shows, the well was plugged and abandoned.

The most recent exploration phase occurred in the late 1990s and continues to present day. Acquisition of new seismic, and reprocessing of existing seismic data, preceded the drilling of Evans Shoal 2 (Shell, March 1998). Evans Shoal 2 was a successful gas appraisal well, proving favourable deliverability from Plover Formation reservoirs that flowed gas at 25 MMscf/d (Figure 8). Exploration continues in proximity to this field with Evans Shoal South 1 being drilled in July 2006 by Santos. Lynedoch 2, which was drilled in 1998 by Shell, encountered a tight gas column in the Plover Formation that had originally been suspected from the results of Lynedoch 1. ConocoPhillips has recently further appraised this field with the drilling of Barossa 1, ST1 in October 2006. Gas production of 30 MMcf/d from Barossa has greatly improved assessments of reservoir deliverability; however, knowledge of the areal extent of productive reservoir facies awaits further appraisal drilling. Tyche 1 was drilled by Shell in 2000, and tested a shallow stratigraphic play; the well was plugged and abandoned without encountering significant shows. The Abadi field, drilled in Indonesian waters, is a 5 TCF gas field reservoired within the Plover Formation (Yui, 2003). Recent drilling by ConocoPhillips in NT/P61, resulted in the gas discovery at Caldita 1 (August 2005), which flowed gas at a rate of 33 MMscf/d from reservoirs within the Plover Formation (Ottoman, 2005). This potentially large gas field is being evaluated by the appraisal well Caldita 2 (February 2007).

Relevant wells

No wells have been drilled in Area NT07-1. A number of wells have been drilled in permits adjacent to this release area; Lynedoch 1, 2, Barossa 1, ST1, Sunrise 1 and Greater Sunrise wells, Evans Shoal 1, 2, Evans Shoal South 1, and Caldita 1 and 2.

Lynedoch 1 (1973) was drilled by Shell Development (Australia) in permit NT/P19 and was the first test of the Calder Graben, targeting the Bathurst Island group in a seismically defined, low relief, anticlinal structure. The well reached 3967 mRT with a thin hydrocarbon bearing zone (9.7 m), which is probably gas, in the Bathurst Island Group. Due to the thin Bathurst Island Group reservoir and tight Jurassic sandstones the area was interpreted as not prospective.

Sunrise 1 (1975) was drilled by Woodside/Burmah Oil NL in permit NT/P12 targeting a large faulted anticline. The well reached 2341 mRT with wireline log interpretations indicating gas in two Cretaceous units, 2142–2156 m and 2195–2206 m, with water saturations of 25% and 35%, respectively. Four Formation Interval Tests (FITs) were run in the well to test for the presence of hydrocarbons and to establish the level of the gas/water contact. FIT Nos. 2 and 4 were run in the interpreted gas zone and recovered gas and condensate. The recovery of significant quantities of gas and condensate from the Jurassic Plover Formation in Sunrise 1 played an important role in defining the gas/condensate reserves of the Greater Sunrise accumulation that currently stand at 8 TCF of gas and 299 MMstb of condensate (Longley et al, 2001 ). The distribution of transgressive shell beds is an important factor in controlling vertical connectivity within the field (Alsop and Ainsworth, 2006).

Evans Shoal 1 (1988) drilled by BHP Petroleum in permit NT/P40 targeting Plover Formation sandstones in a large gentle anticline. The well reached 3712 mKB with gas discovered in the Plover Formation. Repeat Formation Tests (RFTs) recovered dry gas successfully twice from three attempts. Analysis of the gas indicates it is very dry and thermally mature. Source rock analysis suggests the gas could have been derived from Jurassic or Early Cretaceous rocks in the vicinity of the well or further down-dip. High temperatures in the well confirmed that high geothermal gradients are a regional phenomenon and preclude the likely generation or preservation of liquid hydrocarbons below approximately 2.5 km (using 130°C).

Evans Shoal 2 was an appraisal well that established continuity of good quality, gas-bearing, upper Plover reservoirs in the structure. Evans Shoal South-1 tested a structure to the southwest and encountered hydrocarbons in both the primary and secondary targets, but the reservoirs were not tested. (Santos, 2006 http://www.santos.com/Archive/NewsDetail.aspx?p=121&np=78&id=947 )

Caldita 1 (2005) was drilled by ConocoPhillips Exploration Australia resulting in a gas discovery. The discovery was reported as a "significant" gas column which flowed at a rate of up to 33 million cubic feet of gas per day. The well is considered to have the potential to host over 1.5 trillion cubic feet of gas. An appraisal well is planned in the future with 3D seismic acquisition. This well is currently confidential and no further information is available.

Barossa 1 ST1 (2006) was drilled by Santos in NT/P69, south of the Lynedoch 1 and 2 wells. This well reached a total depth of 4310m and Santos reported encountering hydrocarbons. http://www.santos.com/Archive/NewsDetail.aspx?p=121&np=78&id=988
More details are unavailable as this recent well is currently confidential.


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Relevant Wells Listing

Well Operator Year Total Depth (m) Hydrocarbons
Barossa 1 ConocoPhillips Exploration Australia PtyLtd 2006 N/A Not reported
Barossa 1 ST1 ConocoPhillips Exploration Australia PtyLtd 2006 4310 Gas flowed on test
Beluga 1 BHP Petroleum Pty Ltd 1991 3100 Oil indication
Caldita 1 ConocoPhillips Exploration Australia Pty Ltd 2005 4037 Proven gas zone
Caldita 2 ConocoPhillips Exploration Australia Pty Ltd 2006 N/A Drilling at time of publication
Chuditch 1 Shell Dev (PSC 9) Pty Ltd 1998 3035 Proven gas zone, oil indications
Evans Shoal 1 BHP Petroleum Pty Ltd 1988 3712 Proven gas zone
Evans Shoal 2 Shell Development (Australia) Limited 1998 3940 Proven gas zone, oil indications
Evans Shoal South 1 Santos Offshore Pty Ltd 2006 4097 Gas indications
Heron 1 Arco Australia Ltd 1972 4209 Strong gas indications
Lynedoch 1 Shell Development (Australia) Pty Ltd 1973 3967 Strong gas indications
Lynedoch 2 Shell Development (Australia) Pty Ltd 1998 4225 Gas flowed on test
Shearwater 1 Arco Australia Limited 1974 3177 Oil indication
Sunrise 1 B.O.C. of Australia Ltd 1975 2341 Potential oil zone, oil indications
Sunrise 2 Woodside Offshore Petroleum Pty Ltd 1998 2350 Proven gas zone, oil indications
Troubadour 1 B.O.C.of Australia Ltd 1974 3459 Proven gas zone, oil indications
Tyche 1 Woodside Energy Ltd 2000 1475 No shows
Wonarah 1 Shell Development(Australia) Limited 1998 2800 No shows

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