The petroleum potential of inversion anticlines in the Banda Arc

The Banda Arc in eastern Indonesia and newly independent Timor-Leste (East Timor) is the zone of collision between the northern margin of the Australian continent and an oceanic island arc system bordering the Eurasian plate. Structurally, the Banda forearc is a fold and thrust belt, consisting of the imbricated outer edge of the Australian continent, overlain locally by fragments of the precollisional oceanic forearc. The Banda forearc is an established petroleum province in the island of Seram, but the remainder of the forearc has so far yielded no further exploration success despite close geological similarity to Seram. A major disincentive to exploration in the Banda forearc has been a common perception of structural complexity. This complexity may, however, have been overstated, and this paper presents a simple structural model for the evolution of the Banda forearc. Key to this model is the recognition of basement-involved inversion structures in the deeper parts of the collision complex. The inverted graben basins are filled with Permian-Jurassic continental margin sequences, including high-quality source rocks of Late Triassic-Early Jurassic age, and contemporaneous potential reservoir sequences, sealed by Middle-Late Jurassic shales. These Jurassic shales also act as an important decollement horizon, separating shallow-level structural complexity from a simpler structural style beneath, characterized by large inversion anticlines. The Oseil field in Seram. may be located in such an inversion anticline, and comparable prospective structures are identified elsewhere in the Banda forearc, particularly in Timor and the Tanimbar islands.