Nias basin, NW Sumatra – new insights into forearc structure and hydrocarbon prospectivity from long-offset 2D seismic data



Last week, my colleague from TGS, Ian presented a talk in the 38th Indonesian petroleum Association annual meeting. He talked about the structures and hydrocarbon prospectivity of Nias forearc basin, which is situated in the central part of the offshore Sumatra forearc area. 1868 km of newly acquired long offset 2D seismic, covers the area in between Nias and Sumatra mainland. Compare to the other offshore Sumatra forearc basins, the water depth in Nias basin is much shallower (~500m),  than the other basins (up to ~1500m), reflecting much more carbonate platform growth and forearc sediment fill. The new data set show thick sediments with half-graben/syn-rift character beneath the Neogene sequences that interpreted as Paleogene. By using using the low geothermal gradients observed in wells in the area, the temperature in the bottom half of the Paleogene section is calculated to be sufficient to expel hydrocarbons from lacustrine or coaly sediments.

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Neotectonics of the southern Sumatran Forearc

Mukti_2012_neotec ipa

This article notes the structural features in the Mentawai forearc that can be interpreted as products of compression of the accretionary wedge and forearc basin sediments. Compressional phases since the Late Miocene initiated (a) the landward vergent foldthrust belt in the Mentawai Fault Zone (MFZ), (b) reactivation of seaward vergent imbricated thrusts in the retro-accretionary wedge, and (c) uplift of the accretionary wedge and some parts of the forearc basin. The authors interpret that the northwest part of the accretionary wedge underwent higher compression and tilting. The compression of the forearc is suggested to be controlled by the combination of the geometry and position of the continental backstop, and the subducting bathymetric high in the oceanic plate.

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New insight on the deformation along the southern Java forearc

This note is taken from an abstract presented at American Geophysical Union, Fall Meeting 2012. The new seismic data set in south Java resolved detailed structures of the southern Java forearc. The plate interface expressed as irregular surface in the western part and rather a continuous smooth boundary in the eastern area. This observation coincides with the higher uplift of the western part of the forearc high. Cluster of shallow earthquakes can be observed near the western part of Java trench. Deformation of the forearc high is more intense in the west compared to that in the east. This coincides with higher uplift of the western part of the forearc high, thus complicated the structures.

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