The Northern Arafura shelf area is frontier in nature but is on trend with a number of large fields, the largest of which are the multi-Tcf Abadi and Tangguh gas fields. It is relatively unexplored, with the two dry wells (Koba-1 in‘84 and Barakan-1 in ’95) drilled offshore on highs in and around the Aru Ridge, and only one on land (Kai Basar-1 in‘96) which was drilled on the island and encountered thick Mesozoic rocks. The new data was acquired under the jurisdiction of the Indonesian Ministry of Energy and Mineral Resources (MIGAS). Its eastern and south-eastern portion covers part of the Arafura Shelf and Basin whilst its west and north-western portion covers the Tanimbar and Aru Troughs and the eastern part of the Banda Arc collision zone.
Known Petroleum Systems
The major discoveries in the region lie both to the north and south-west of the survey area. The Jurassic Plover sandstone petroleum system is proved to the north in the Tangguh field in the Bintuni Basin and to the south-west in the Abadi field, and also further westwards in fields such as Sunrise, Evans Shoal, Troubadour and Bayu Undan. In this petroleum system, the gas/condensate fields are thought to be charged from equivalent-aged deeper-lying Early-Middle Jurassic Plover formation source rocks.
Jurassic clastics have been sampled on the islands of Tanimbar and Kai (Charlton, 2004, 2008) and it has been suggested by a number of authors that reservoir quality sandstones could be present over a large area along the Jurassic shelf margin, which is expected to run northwards from the Bonaparte Basin/Sahul Platform, through the survey area to New Guinea and the Bintuni Basin/Tangguh area.
The Jurassic reservoirs are expected to occur both on the western margin of the Arafura Shelf (west of the island of Aru and the Aru Ridge, where the Mesozioc section thickens rapidly) and in the western part of the survey area. The seismic data off Tanimbar and around the Kei Islands show them to be buried below a Neogene-aged melange, which is associated with collision tectonics forming the eastern margin of the islands. This situation is similar to that seen off the northern coast of Seram Island and off the southern coast of East Timor.
Reservoirs of probable Cretaceous age are also seen to the west and south-west of Aru, where the Jurassic soles out as it rises from west to east. Here a number of four-way dip closures, ranging in size from 25 to 250 km2, have been mapped from the new seismic data.
In addition to this, the presence of deeply buried Early Cretaceous source rocks cannot be discounted, with Barber et al (2003), suggesting that these could be present along the flanks of the Tanimbar Trough. They also suggest that these could possess similar attributes to the excellent quality Echuca Shoals Formation source rocks, which have sourced oil discoveries in the Bonaparte Basin, for example in the Elang Field in the Australia – East Timor Joint Petroleum Development Area (JPDA).
It is interesting to note that Cornee et al (1997) report encountering black laminated marlstones and shales of Early to Late Cretaceous age in samples dredged from the eastern side of Kai Besar Island, which is on the northern margin of the survey area. The oldest of these were dated as Late Albian, which sits above the Echuca Shoals Formation.
Oil and gas seeps are evident on some of the islands of the Banda Forearc, such as Timor, Kei Besar and Seram. However, to date, the only established petroleum producing province in this collision complex is on the island of Seram. Results from an onshore well drilled on the island of Kai Besar (Kai Besar 1) have been reported (Widodo 2006) to show the presence of mature source rocks of Mid-Late Jurassic Plover Formation and Late Cretaceous Ekmai Formation with potential reservoir rocks at several levels.
Charlton (2004) proposed that the tectonism in the outer island arc has involved the inversion of graben basins which were filled with Permian to Jurassic sediment and that these could have potential for hydrocarbon trapping. In 2001 Charlton also reported that rock fragments of Triassic and Jurassic age, including Jurassic coals, have been ejected from mud volcanoes on Tanimbar Island.
Potential in Paleozoic?
In addition to the Mesozoic petroleum system, we see thick Paleozoic sediments on the Arafura Shelf. These are particularly well developed south of the island of Aru in the South Aru Graben (Livsey et al 1992) which is possibly filled with several thousand metres of Cambro-Ordovician to late Devonian/Early Permian sediments. The Graben is characterised by a thin (< 1 sec TWT) cover of often faulted Cretaceous and younger sediments, unconformably overlying rocks of Paleozoic age which demonstrate a very high (> 5000 m/s from 1 sec TWT) stacking velocity (Adhyaksawan et al 2010). The Graben attains a depth greater than 10,000m in the easternmost part of the survey area. Its petroleum generating potential is evidenced by the presence of gas chimneys and bright reflections, interpreted as possible gas sands. This Paleozoic petroleum system is believed by a number of authors to be analogous to that seen in parts of the Bonaparte Basin and Goulburn Graben.
A number of authors have reported on the nature and petroleum potential of the undrilled Northern (Australian) Arafura Basin and contrasted it to that of the Goulburn Graben to its south, which contains a number of unsuccessful wells. The contention is that since the platform area to the north is less deformed than the Goulburn Graben, this area could be less risky as far as the timing and expulsion of hydrocarbons is concerned; and since the Paleozoic rocks are not so deeply buried, the reservoirs in the north could have improved qualities.
The petroleum potential of the Arufura Shelf is borne out by the recognition (on the Australian side to the north of the Goulburn Graben) of seeps both on satellite altimeter data and on sub-bottom profiles from a marine seepage survey; and by amplitude brightening on seismic data (Kennard et al 2007). The primary source rocks are thought to be near the base of the Cambrian but other source rocks could occur in the Devonian to Permian section. With the platform running north/north-eastwards from the Goulburn Graben to New Guinea, a similar scenario could be expected in the south and south-eastern part of our survey area.
Multi Tcf Potential
The new long offset data has allowed us to recognise a number of different plays in the survey area, which are defined by their relationship to the main structural/stratigraphic provinces outlined briefly above and summarised on the cross section.
In conclusion, it is interesting to note that there are a number of potential petroleum systems in the area, rather than just the Cenozoic one which is generally the case in the Far East (USGS 2010), and that the geology has much in common with that of the North West Shelf of Australia. Discoveries could well be in the multi-Tcf range and include oil as well as gas.
We can also note that the long offset data, together with the use of Pre-Stack Time Migration, has been particularly good in assisting the imaging of the data below the Melange, which was formed by uplift and collision tectonics in this part of the Banda Arc and which runs along the eastern margin of the Tanimbar and Kei Islands. It is also useful in helping extract primary data from multiples on the shelf, thus illuminating the data within the underlying Paleozoic grabens such as the South Aru Graben. It will also be valuable in future work which may involve AVO analysis.
This data is being used to assess the petroleum potential of the area, which is currently unlicensed. It will be used by the authorities to help them decide the number and size of the exploration blocks to be put up in the next bid round, which could be announced as early as May of this year.