The Geology of the Central and South Sumatra Basins

PROCEEDINGSINDONESIAN PETROLEUMASSOCIATION ThirdAnnualConvention,June1974. 77 THEGEOLOGYOFTHECENTRALANDSOUTHSUMATRABASI NS G.L.DE COSTER*) BASIN DESCRIPTION TheCentralandSout hSumat rabasinsare i mport ant oilproducingareasontheislandof SumatraintheRepublicofIndonesia(Figs.1 and2).TheyareTertiarystructuraland depositionalbasinscomposedofaTertiary sedi ment arysectionlyingonanunconf or mi t y surfaceofpre-Tertiarymet amorphi cand igneousrocks.Thesearetwoof thethreebasins locatedonSumatra( t hethirdistheNort h Sumatrabasin)alignednorthwest-southeast betweentheBarisanMount ai nstothesouth- westandtheMalaccaandKarimataStraits and theJavaSeatothenort heast andeast.Therocksexposedinthebasinsconsistalmost whollyofTertiaryst rat at houghseveral upliftedblocksi nthebasins(includingthe TigapuluhandDuabelasmount ai ns) doexpose pre-Tertiaryrockatthesurface.Therocks exposedi n theBarisan Mountains arecomposed ofPaleozoicandMesozoicmet amorphi cand igneousrocksandofyoung Tertiaryt oRecentvolcanics. Analysis oftheCentral andSouth. Su- matra basins show thatthey hadvery similar and relatedhistoriesandcoul d beconsidered as one largebasinwi t hmany troughs and grabens. The Nort hSumat rabasin,ontheot herhand,appearstohavebeenseparatedfromthe CentralSumat raareat hroughout mostofits hi st or y bytheAsahanarchandcanbetreated asaseparatebasin.ForthisreporttheCentral andSout hSumat raareaswillbediscussedand described as separatebasins. TheCentralandSout hSumat rabasins (see Fig.2)areasymmet ri cbasins boundedonthe southwestbyfaultsandupliftedexposuresof pre-Tert i ary rocksalongthemount ai n front of theBarisanMountains;. onthenortheastbythe sedi ment aryordepositionalboundariesofthe Sundashelf(site-oftheancestralSunda landmass);tothesouthandeastbythe Lampunghighandbyan archthatparallels the eastcoastofSumatra;andtothenort hand nort hwest bytheAsahan archandtheout crops ofpre-Tertiaryrocksnort hwest ofPekanbaru.Bot hTheAsahanandLampungarcheswere positiveelementst hroughout muchofTertiary time,separatingtheCentral andSout hSumatra basinsfromtheadjoiningNort hSumat raand Sundabasins, respect i vel y. The twoarcheswere coveredbyshaUowmarineseasonl yduring t he .EarlyandearlyMiddleMiocenetimetoform t emporaryconnect i onstotheneighboring basins. Thenort heast ern andeastern boundari es ofthebasinsalongtheSundashelfandthe Lampunghigharedifficultt odefineprecisely, but areusuallyplacedwherethesedimentary sectionis less t han1500feet (460meters)thick andcomposedofPlio-Pleistocene andYounger stratalyingonLowerTert i aryorpre-Tertiary rocks.Theboundarybet weenthecentraland southbasinsisalsoindefinitedueto l ackof majorstructuralfeaturesseparatingthem.This factoris a not he r argument forconsidering the twoareast obeonebasin rathert han two.The boundarybetweenthetwobasinsisusually drawnasanortheast-southwestbandt hrough thenort hernpartoftheTigapuluhMountains,j oi ni ngtheaxisofabroadarchext endi ng southwestfromt he Sundalandmasstoa istructurally complexareain themount ai n front*)P.T. Stanvac Indonesia Jakarta,Indonesia. The writerwishes to acknowledge his indebtedness tothemany geologists and geophysicists presently andfomaerlyassociatedwithP.T.Stanvac Indonesiaformuchofthegeologic information addinterpretationincorporatedinthispaper. Informationwasdrawnfreelyfromdiscussions with colleaguesin P.T.S.I. and fromthe f'des in the Exploration DepartmenLIn addition, he is grateful to.personnelintheexplorationandgeology departments of Pertaminaand" of othereomp/mies operatinginIndonesiafortheircontributions in generaldiscussionsabouttheregionalgeologyof the Sumatraarea. IPA, 2006 - 3rd Annual Convention Proceedings, 1974Disc ContentsContentsSearch78 I|IIIIIlSe. i o . g o a 9 1 ~ e i o o oI , , I ,0&I4DAMAN o 0 NICOeAR ~*ISLANDS \ XN OR T H ' % C E N T R A L A NDSOUTH . ' C ' : - ~ - ' ~ S U . A T ~ S l - R U C T U ~ LBASI NS S C A L E I:15, OO0, OOO 0wO3004 5 0 6 0 0 - 1t'50 KtL,DMi~Tf , l ~z 10~o i. . . . . . . . . . . ~ ( " . . . . :,I$ ~,C A MB OD I A " ~I ETN AM~ MA L A Y AI10e IN~ o J0 #, 4W,4S E A0 , ; o . , ~ . , ; o .- i oFI G. II NDE X MA P C E N T R A L A NDSOUT HS U MA T R A B A S I NS79 |! .oo g, , . ~ $. . q# - - <