NIAS BASIN, NW SUMATRA – NEW INSIGHTS INTO FOREARC STRUCTURE AND HYDROCARBON PROSPECTIVITY FROM LONG-OFFSET 2D SEISMIC DATA

摘要

The Nias Basin is located in the Sumatran Forearc,rnbetween the island of Nias and the Sumatranrnmainland to the NE, and the Batu Islands to the SE.rn1868 km of long offset (8km cable), 2D seismicrndata, acquired in 2009 by TGS, closes the data gaprnin long offset seismic data between the deeper waterrnNW-SE oriented Simeulue and Mentawai forearcrnbasins to the NW and SE respectively.rnUnlike most of the Sumatran Forearc, a large part ofrnthe Nias Basin is oriented N-S. The main Nias basinrn(~500m water depth), is much shallower than thernother larger basins to the NW (750-1000m waterrndepth), and SE (~1500m water depth), reflectingrnmuch more carbonate platform growth and forearcrnsediment fill. Most of the fold and thrust structuresrnobserved in the seismic data can be related to thosernin the Mentawai segment to the SE. However, arnsignificant change in direction (coincident with thernmajor indentation in the Sumatran trench), along thernNias segment, has resulted in increased uplift ofrnPaleogene forearc sediments that are exposed onrnNias Island.rnA novel seismic processing flow has beenrnimplemented for the first time in TGS forearcrnseismic data, and has successfully imaged thickrnsediments (down to 4.5 seconds TWT sub seabed),rnhaving a half-graben/syn-rift character, beneath thernmain forearc package, which by analogy withrnnearby Sumatran geology, we interpret asrnPaleogene. The thickest syn-rift section isrninterpreted in the southern part of the basin underrnthick forearc sediments Well data in the basin is limited to the NW and SErnmargins, but depth conversion using migrationrnvelocities indicates that, even using the lowrngeothermal gradients observed in wells in the area,rnthe temperature in the bottom half of the Paleogenernsection is sufficient (110℃), to expelrnhydrocarbons from lacustrine or coaly sedimentsrnReefal prospects are located updip from thernPaleogene kitchen, on the eastern margin of therndeep basin, but the sparse 2-D seismic currentlyrnprohibits quantitative assessment of migrationrnpathways and reservoir prospect volumetrics.rnViable updip migration pathways from the kitchenrnto reefs are probably present based on structural dip.