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

Ian Deighton*, M. Ma’ruf Mukti**, Satish Singh**, Tom Travis*, Anthony Hardwick*, Katie Hernon*

* TGS – Surrey and Bedford, UK
** IPGP – Laboratoire de Géosciences Marines, Institut de Physique du Globe de Paris, France



The Nias Basin is located in the Sumatran Forearc, between the island of Nias and the Sumatran mainland to the NE, and the Batu Islands to the SE. 1868 km of long offset (8km cable), 2D seismic data, acquired in 2009 by TGS, closes the data gap in long offset seismic data between the deeper water NW-SE oriented Simeulue and Mentawai forearc basins to the NW and SE respectively. Unlike most of the Sumatran Forearc, a large part of the Nias Basin is oriented N-S. The main Nias basin (~500m water depth), is much shallower than the other larger basins to the NW (750-1000m water depth), and SE (~1500m water depth), reflecting much more carbonate platform growth and forearc sediment fill. Most of the fold and thrust structures observed in the seismic data can be related to those in the Mentawai segment to the SE. However, a significant change in direction (coincident with the major indentation in the Sumatran trench), along the Nias segment, has resulted in increased uplift of Paleogene forearc sediments that are exposed on Nias Island. A novel seismic processing flow has been implemented for the first time in TGS forearc seismic data, and has successfully imaged thick sediments (down to 4.5 seconds TWT sub seabed), having a half-graben/syn-rift character, beneath the main forearc package, which by analogy with nearby Sumatran geology, we interpret as Paleogene. The thickest syn-rift section is interpreted in the southern part of the basin under thick forearc sediments. Well data in the basin is limited to the NW and SE margins, but depth conversion using migration velocities indicates that, even using the low geothermal gradients observed in wells in the area, the temperature in the bottom half of the Paleogene section is sufficient (>>110ºC), to expel hydrocarbons from lacustrine or coaly sediments. Reefal prospects are located updip from the Paleogene kitchen, on the eastern margin of the deep basin, but the sparse 2-D seismic currently prohibits quantitative assessment of migration pathways and reservoir prospect volumetrics. Viable updip migration pathways from the kitchen to reefs are probably present based on structural dip.


Thirty-Eighth Annual Convention & Exhibition, May 2014


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