Tim Charlton, Banda Arc Geologist

reports

Prospectivity of the Indonesian Banda Arc

An oil industry multiclient prospectivity study

The Banda forearc from west of Timor in the south, through the Tanimbar and Kei islands in the east, and Seram and Buru in the north (Figure 17) is a Neogene fold and thrust belt with established commercial oil production in Seram and excellent prospectivity indicators around the rest of the arc. Near-surface geology is structurally complex and unprospective, but large and structurally simple subthrust inversion anticlines (see the linked pages on the structure of the Banda forearc)  are cored by Mesozoic carbonates which form established commercial reservoirs in Seram, and by sandstone successions analogous to commercial reservoirs on the Australian Northwest Shelf to the south, and the Tangguh fields of New Guinea to the east. Triassic restricted marine calcareous shales, the probable source for commercial oils in Seram and for abundant oils seeps in Timor, have been described as 'world class' sources for oil, and these occur laterally adjacent to, and downdip from, the potential reservoir successions located at the crests of the inversion anticlines. The source and reservoir successions are overlain by thick Jurassic shales that provide both a top seal and a structural decollement level, separating the simple structuring at depth from near-surface structural complexity.

figure 1-4 banda study area map 

Figure 17: Coverage of the Indonesian Banda Arc prospectivity study 

This report series is in eight volumes. Volume 1 considers regional aspects of the petroleum system including stratigraphy and palaeogeographic evolution, structural development, and elements of the petroleum system (source, reservoir, seal, maturation, migration). The structural section develops the model of thick-skinned inversion tectonics for the Banda foldbelt. These inversion anticlines are increasingly being imaged by seismic data offshore, and can be defined by a number of criteria onshore (in the absence so far of onshore seismic data) including outcrop geology, gravity, topographic doming, and the distribution of active oil and gas seeps and mud volcanoes.

Volume 2 summarises the petroleum geology of the northern Banda Arc, particularly the exploration history and current state of established oil production in Seram. Volumes 3-7 summarise the petroleum potential of the other sectors of the Banda forearc (Figure 17), while Volume 8 provides a summary and ranking of the exploration leads identified.

Seventeen prospects and leads are described in some detail in the study, and are compared in general volumetric terms with the Oseil oil field which is located in an inversion anticline in Seram island (estimated 200mmbbl oil-in-place). The largest structure identified in the study, beneath Babar island, is potentially thirty times the size of the Oseil structure. Other particularly large structures are found in the Savu sector of the forearc, while four good sized structures are identified in onshore West Timor, and three further structures are located offshore to the south. The exception to the inversion anticline leads is a large horst-block prospect identified on the island of Kei Besar, which geologically is not part of the Banda forearc, but is located immediately in front of the Banda forearc complex on a fragment of Australian margin crust isolated by young extension in the Aru Trough.

 

© Tim Charlton
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