THE PRE-TERTIARY PETROLEUM SYSTEM IN NORTH SUMATRA BASIN: AN INTEGRATED STUDY FROM ONSHORE NORTH SUMATRA OUTCROPS AND SUBSURFACE DATA FROM OFFSHORE WEST GLAGAH KAMBUNA

摘要

The offshore area of North Sumatra Basin isrnbelieved to have good hydrocarbon accumulationrnpotential and is considered challenging forrnhydrocarbon exploration. Several exploratory wellsrnhad been drilled in this area with major discoveriesrnmainly in Tertiary age reservoirs such as the Baongrnand Belumai Formations. The Pre-Tertiaryrnsedimentary deposit of North Sumatra Basin is stillrnunderexplored and is believed to have an importantrnrole in future hydrocarbon exploration in the area.rnThe Pre-Tertiary play is the main exploration targetrnin West Glagah Kambuna in offshore NorthrnSumatra Basin. Previous regional studies suggestedrnthat the basements in this area are mainly thernTampur Formation of Eocene - Early Oligocenernage. Based on present study however, the basementrnof the West Glagah Kambuna area mainly consistsrnof the Batumilmil, Kaloi and Kualu Formations ofrnPermian - Triassic age that are parts of thernPeusangan Group.rnOffshore seismic data clearly indicate angularrnunconformity contact between Pre-Tertiary andrnTertiary stratigraphy. In addition to the clearlyrndefined Tertiary sedimentary-fills section, thernseismic also shows good images of Pre-Tertiaryrnpackages which can be differentiated based on theirrninternal-external reflection characteristics andrnreflectivity. By correlating the pre-rift seismic section withrnonshore geology which is based on outcrop data, itrnis interpreted that the Pre-Rift stratigraphy found inrnoffshore area belongs to Mesozoic PeusanganrnGroup. The Peusangan Group in the offshore arearncan be divided into three different formations; fromrnthe oldest to the youngest are the Batumilmil, Kaloi,rnand the Kualu Formations. The BatumilmilrnFormation is represented by chaotic seismicrncharacter; the Kaloi Formation is represented byrnhigh amplitude seismic reflection, while the lowrnamplitude seismic reflection above the KaloirnFormation is interpreted as the Kualu Formation.rnThe good match between seismic and well data inrnWGK Block and surrounding area strongly supportrnthe interpretation.rnOutcrop study in Aceh and North Sumatra Provincernhas confirmed that the pre-Bampo/Bruksah sectionrnof the North Sumatera Basin comprising not onlyrnthe Dolomite Tampur Formation, but also severalrnformations from Meureudu and Peusangan Groupsrnwhich are Pre-Tertiary in age. The study alsornsuggested that the pre-rift dolomitic carbonate is notrnjust belong to the Tampur Formation since thernPeusangan Group also shown similar lithology,rnparticularly in Batumilmil and Kaloi Formations.rnExtensive tectonism and subaerial exposure havernpromoted fractures and caving in those formations.rnBeside the dolomitic carbonate, the study also foundrnclastic rock within the Pre-Tertiary stratigraphy.rnThe clastic rocks are black shale of Batumimil andrnmudstone of Kualu Formations. Several fossilsrncould still be found in Batumilmil carbonate andrnKualu mudstone. The outcrop sedimentologyrncharacteristics are still showing sedimentary rock character where bedding and layering is clear andrncommon.rnThe Pre-Tertiary formations that developed inrnWGK Block and surrounding area are parts of thernPeusangan Group, which based on field observationrnand laboratory analysis of Kaloi, Kualu, andrnBatumilmil Formations, they show good reservoirrnproperties and high carbonaceous content in shalesrnin several localities. It indicates the possibility forrnthe Pre-Tertiary formations to have its ownrnpetroleum system, separated from the Tertiaryrnformations. This promotes new explorationrnopportunity in the southern part of North SumaterarnBasin, particularly in WGK Block.