The Tanimbar islands are situated in the southeastern sector of the Banda arc-continent collision zone (Figure 11). Structurally the Tanimbar area comprises a fold and thrust belt, consisting of the imbricated outer edge of the Australian continent, overlain by minor fragments of the pre-collisional oceanic forearc complex. In terms of both stratigraphy and structural setting, the Tanimbar islands are closely analogous to the island of Seram in the northern sector of the Banda forearc, which is the location of several producing oil fields. The Tanimbar islands also lie immediately north of the Abadi gas field in INPEX’s Masela PSC block.
Figure 11: Setting of the Tanimbar islands and the Calder Graben
The Tanimbar islands expose a stratigraphic succession ranging in age from Permian to Recent (Figure 12). The Permian-Early Cretaceous succession was deposited within, and on the western flank of, a large graben basin, the northward continuation of the Calder Graben on the Australian continental margin (Figure 11). The mid-Cretaceous to Miocene succession is a deepwater sequence that accumulated on the Australian margin after continental breakup.
Figure 12: Stratigraphy of the Tanimbar islands
The hydrocarbon prospectivity of the Tanimbar islands is treated in greater detail in an oil industry consultancy study (see my Indonesian Banda Arc report). Source rocks are interpreted particularly within the Late Triassic-Early Jurassic succession, based on close stratigraphic similarity with proven source rocks of this age in Seram and Timor. In Tanimbar these are likely to include coastal plain fluviatile and restricted marine carbonate successions. Potential reservoir sandstones occur at several levels in the Tanimbar stratigraphy, but particularly in a fluviodeltaic succession of Late Triassic-Early Jurassic age (the Maru Formation). This is contemporaneous with the inferred source rocks, and is directly analogous to the Kanikeh Formation of Seram, which forms the reservoir sequence in the Bula Tenggara and Belien oil fields. The Maru Formation also forms part of the same Plover fluviodeltaic system that reservoirs (in slightly younger sequences) the Sunrise, Troubadour and Abadi gas fields on the Australian Northwest Shelf, to the south and southwest of Tanimbar. The Maru Formation is succeeded by Middle Jurassic transgressive shales, which are interpreted as forming a regional seal. These shales also form an important structural décollement level, separating near-surface structural complexity from a simpler structural style beneath.
The most likely trap type in the Tanimbar area is inversion anticlines, formed by reactivation in compression of pre-collisional graben structures (Figure 13). Listric normal faults on the eastern margin of the pre-collisional Calder graben system are interpreted as having been reactivated in compression to form inversion anticlines beneath the complex surficial fold and thrust/mélange belt. Based on surface geology, gravity data and offshore seismic, these inversion anticlines are likely to be similar in size to, or larger than, the inversion anticline hosting the Oseil oilfield in Seram.
Figure 13: Tanimbar cross-section