Tim Charlton, Banda Arc Geologist


The petroleum potential of the Kei islands

The Kei (or Kai) islands (Figure 14) are situated at the eastern curve of the Banda Arc. The Kei group differs from other islands around the arc in that the forearc deformation front bisects the island group. The easternmost island, Kei Besar, forms part of the Australian passive continental margin, although it is separated from the Arafura continental shelf to the east by young extension in the Aru Trough. The thrust front of the Banda forearc lies along the shallow marine Nerong Straits between Kei Besar and Kei Kecil.


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Figure 14: Kei islands location and simplified structure


As a result of this unusual setting straddling the Banda deformation front, several rather different structural régimes are interpreted across the Kei group, as illustrated in cross-section in Figure 15.

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Figure 15: Cross-section through the Kei islands


Due to its unique tectonic setting, several rather different hydrocarbon plays exist in the Kei islands, as described in more detail in my oil industry consultancy study (see the Indonesian Banda Arc prospectivity report). Firstly there is the potential beneath the west-central Kei islands (Tayandu group) for an inversion anticline of the type interpreted from other parts of the Banda forearc in the linked web pages. Secondly, the shallowness of the 'frontal trough' in the Nerong Straits suggests that relatively small but structurally simple thin-skinned  folds may be present in the shallow subsurface of Kei Kecil island, rather than under water depths of 1500-3000m as in other parts of the Banda forearc (e.g. south of Timor or north of Seram). Most significant, however, is the potential for prospective passive margin-type structures beneath Kei Besar.

Kei Besar

Kei Besar island is about 90km long N-S, but a maximum of 10km wide E-W. The structure of the island is dominated by tilted fault blocks elongated N-S, with typically 2-3 major fault blocks E-W across the island. This gives the northern half of the island a sawtooth topographic profile, with west-facing slopes controlled by beds dipping westward at 5-15°, and east-facing slopes forming steep scarps that mark large normal faults downthrowing to the east (Figure 16). A strong gas seep at Yamtimur (Figure 14) is located close to the trace of one such normal fault.

The outcropping stratigraphy of Kei Besar consists of a broadly shallowing-up carbonate succession ranging in age from Paleogene to Recent. Offshore dredging suggests that the deepwater Paleogene section continues down through the Late Cretaceous, and to at least the Albian (latest Early Cretaceous). This succession probably overlies a dominantly shaly Jurassic succession unconformably, and the Jurassic in turn unconformably overlies Late Carboniferous-Permian shallow marine clastics (Cornée et al., 1997). An exploration well, Kai Besar-1, was drilled by Union Texas in northern Kei Besar island (Figure 14). The details of this well were not available to this study, but it is known to have reached TD (at 3193m/10,500ft) in Early Jurassic section. The target was reportedly Jurassic-Cretaceous and possibly Tertiary sandstones in tilted fault block traps (P.T. Patra Nusa Data website).

An offshore commercial seismic line located immediately north of Kei Besar (in Cornée et al., 1997) shows pronounced normal faulting at shallow structural levels, similar in style to that observed onshore in Kei Besar. At deeper structural levels, however, the interpreted base-Jurassic unconformity appears much less affected by the normal faulting, and the shallow faulting probably detaches within the Jurassic succession, as do thrust structures within the Banda forearc proper (see the linked pages). The base-Jurassic horizon defines a broad anticlinal structure.

Kei Besar island is interpreted as the surface expression of a large stuctural dome (or perhaps two en echelon domes beneath the northern and southern halves of the island) developed particularly within the pre-Jurassic succession. The normal faulting seen at the surface is a brittle response to more ductile folding at depth. The domal structure has a vertical relief in excess of 800m. As with potential trapping structures interpreted in the Banda forearc, the Jurassic shales that form the structural décollement under Kei Besar might also form the seal to a deeper anticlinal trap. Reservoir and source sequences are speculatively interpreted in the underlying Carboniferous-Permian succession, with offshore dredge samples (Cornée et al., 1997) suggesting a late Palaeozoic succession comparable to the Aifam Group of the Bird's Head, which has been identified as a probable source sequence for hydrocarbons in the Tangguh gas fields. The Kei Besar structure (or structures) is potentially large enough to contain a giant hydrocarbon accumulation.



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Figure 16: East and west coasts of Kei Besar (looking north), showing respectively dip and scarp slopes



© Tim Charlton
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