IPA99-G-150

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATIONTwenty Seventh Annual Convention & Exhibition, October 1999

TECTONIC ELEMENTS OF THE NORTH IRIAN BASIN

R.L. McAdoo*J.C. Haebig*

ABSTRACT

Between the Central New Guinea Mountain Rangesand the New Guinea Trench lies a terrane consistingof a hybrid, forearc basin and at least one, andpossibly two, subduction-related accretionary prisms.Some authors refer to a sedimentary basin in thisregion as the Waropen Basin but we prefer tocharacterize a larger area as the "North Irian Basin".The prism we refer to as the "Niengo BasementPlatform" appears to have formed coeval with thebasin as a consequence of backthrust-subductionunderplating of the Indo-Australian plate by a down-going slab of the Caroline-Pacific Plate. Subductionstopped and relative plate motion is now oblique. Theplate boundary appears now to be marked by theYapen Fault Zone, a sinistral wrench fault whosetrace is marked on the mainland by a line of activemud volcanoes and extends westward along thenorthern coast of Yapen Island and may continuewestward to connect to the Sorong Fault system.

The North Irian Basin contains over 25,000 feet ofTertiary-aged clastic and carbonate sediments inseveral localized depocenters or sub-basins, mostnotably the Waropen, Teer River and Waipogadeeps and an intermontane sub-basin known as theMeervlakte. Rapid subsidence has created asymmetricbasin fills dominated by turbidites. Potential reservoirdistribution across the depocenters appears to beproblematical with good quality turbidite reservoirsbeing concentrated near depocenter margins. Largereef complexes are evident. There is evidence ofample terrigenous-derived kerogens to serve as apotential petroleum source. An apparent low thermalgradient of 1.67o F per 100 ft in the basin extends

___________________________________________* Apex (Yapen) Ltd.

source maturity to considerable depth. Since the1950's, 12 petroleum exploration wells, have beendrilled including one stratigraphic test and resulted infour dry holes, two gas discoveries and one gas/oildiscovery. Four were abandoned before reachingtarget depth due to encountering severe overpressure.

INTRODUCTION

Location

The "North Irian" area as referred to herein occupiesthe northern coast of the western half of the island ofNew Guinea in the Indonesian province of Irian Jaya.The North Irian area is bounded to the south by thetopographic divide of the east-west trending CentralNew Guinea Mountain Ranges and extends offshoreto the north into the Caroline Sea/Pacific Ocean toinclude the Yapen and Biak islands. To the west, thearea is bounded by the Bird's Head peninsula and tothe east, by the Papua New Guinea political border.Geologically, the North Irian region is a complexassemblage of tectonic and physiographic terranesoccupying the northern margin of the Indian-Australian continental plate at its convergence withthe oceanic Pacific Plate (Figure-1).

The North Irian region is dominated by threeprominent physiographic features, i.e. CendrawasihBay, mainland Irian Jaya, and the offshore CarolineSea surrounding the Yapen and Biak Islands.Cendrawasih Bay (formerly known as Geelvink Bayor Teluk Serera or Teluk Irian as it is alternativelycalled in Bahasa Indonesia) lies between mainlandIrian Jaya to the south and east, Kepala Burung to thewest and Yapen Island to the north. Water depthsexceed 1,000 meters in its centre. The easternmostportion of Cendrawasih Bay narrows and is oftenreferred to separately as Waropen Bay. Mainland

IPA, 2006 - 27th Annual Convention Proceedings, 2000

northern Irian Jaya is dominated by the MamberamoRiver system, the largest major river system ofnorthern Irian Jaya. It drains the east-west orientedaxis of an intermontane basin in the Central NewGuinea Mountain Ranges before turning and flowinggenerally northward across the North Irian area toempty into the Caroline Sea at Cape D'urville at themouth of an extensive delta.

Previous Work

Hamilton (1978), Hutchison (1989), Hall (1997) andLongley (1997) have all recently written on the broadsubject of the tectonics of the entire South-East Asiaregion. Dickinson (1995) and Condie (1997) havetaken a more global view of tectonic mechanisms andprocesses. Dow and Hartono (1982) and (1984),Hegarty et.al. (1983) and Katili and Hartono (1983)have focussed on eastern Indonesian tectonics.Waschsmuth and Kunst (1986), Williams andAmiruddin (1983) and Fearne (1985) have publishedstudies on the North Irian Jaya area itself, particularlythe Mamberamo delta. Last but not least a series ofgeological maps with descriptive text published bythe GRDC, most notably those by Atmawinata et.al(1989), Dow et. al. (1986), Masria et. al. (1981),Surono et. al. (1995) and Hakim et. al. (1994) and(1995) have contributed greatly to the knowledge ofthe geology of the North Irian region. We haveincorporated many of the concepts and ideasexpressed by the foregoing authors and theircontribution to our understanding of this complexregion is recognized and respected.

Tectonic Setting

The tectonic history of the island of New Guinea isextremely complex and many details of it are poorlyunderstood. The generally accepted "big picture"theory is that the island of New Guinea formed at theconvergent margin which was present between theIndian-Australian continental plate (our "Indo-Australian Plate") and an oceanic plate called byvarious authors either the Pacific Plate or the CarolinePlate, the latter of which is a poorly defined butdistinctly possible, separate plate or perhaps sub-plateof the Pacific Plate (Hegarty et. al., 1983). We call itthe "Caroline-Pacific Plate" and avoid the issue andimplications of whether the Caroline and Pacificplates move independently with respect to oneanother. Some tectonic elements are shown withstructural features in Figure 2.

Convergence of the Indo-Australian Plate and theCaroline-Pacific Plate is continuing. Recentmeasurements indicate the convergence direction is35o south of west. This orientation together with thecoastline of northern New Guinea and the roughlyeast-west traces of the New Guinea Trench andYapen Fault zone would indicate an obliqueconvergence capable of imparting a strong sinistralstrike slip component to local tectonic movementsalong the northern New Guinea coast. Whether actualsubduction of the Caroline-Pacific Plate beneath theIndo-Australian Plate has stopped, or is continuing, isunclear. Lack of recent earthquake seismicity alongthe projected New Guinea Trench Benioff Zonewould indicate that major subduction has stopped.Volcanism apparently ceased in the mid-Miocene.Most recent seismicity has occurred on the generallyeast-west trending, sinistral, strike-slip faults of theYapen Fault Zone.

The east-west trending mountain ranges, (with peaksto 18,000 feet) which dominate the topography ofCentral New Guinea, represents a collisional orogenor suture created upon convergence and collision ofan island arc with the Indo-Australian Plate. Closureof any intervening oceanic basin and convergenceoccurred in a north-south relative direction. Theisland arc itself was formed previously as a result ofsouth dipping subduction beneath the New GuineaTrench. At the time, the New Guinea Trench markedthe trace of subduction of the oceanic Caroline-Pacific Plate beneath another oceanic sub-plate whichhas now been all but consumed by the processes ofsubduction and accretion beneath and onto the Indo-Australian continental plate.

The collision of the magmatic arc complex and theIndo-Australian Plate may have begun as early as theLate Cretaceous but it clearly continued into the earlyTertiary and the suture of the arc massif itself mayhave occurred as late as the Late Miocene. Thenorthern slopes of the Central New Guinea MountainRanges contain an outcropping, ophiolite assemblageof rocks which are believed to be derived from theCaroline-Pacific Plate. These are now emplaced andare upthrust over and against the arc as a result ofuplift accompanying the collision. The magmatic arcitself is now volcanically inactive. Seismic activityand subduction at the New Guinea Trench ended soonafter the collision and the New Guinea Trench cannow be considered to mark the boundary between theCaroline-Pacific Plate to the north and the Indo-Australian continental plate to the south.

Between the Central New Guinea Mountain Rangesorogen and the New Guinea Trench lies a terraneconsisting of a hybrid or successor forearc basin(Dickinson, 1995). The hybrid forearc basin we referto as the "North Irian Basin". Although it clearlybegan its formation as a forearc basin it has, since thecollision, dramatically changed in tectonic andsedimentary style due to post collisional tectonics.Figure 4 shows a generalized cross-section through anidealized forearc basin and Figure 5 exhibits ourinterpretation of the hybridized North Irian successorbasin.

REGIONAL TERRANES OF NEW GUINEA

The regional terranes of present-day New Guineaisland and Irian Jaya province are generally reflectedin the region's geomorphology or physiography. Theisland exhibits three, parallel belts of differingterranes which trend generally east-west, in a gentlearc, with the concave side to the south.

The Southern New Guinea Terrane - A CrustalTerrane

The southern terrane occupies the southern coast ofIrian Jaya and is expressed physiographically as a flat,low-lying coastal plain. The southern terrane's floorcontinental basement consists of deformed andcompressed sediments. The southern terranerepresents the northern margin of the continental platecarrying the Australian continent and a portion of theIndian Ocean and is sometimes referred to as theAustralian Plate and sometimes as the Indo-Australian Plate. The low relief, northern edge of theIndo-Australian Plate is sometimes also referred to asthe Sahul Shelf and it extends from southern IrianJaya to the north-west coast of Australia and includesthe Arafura Sea Basin.

The Medial New Guinea Terrane - A CollisionalOrogen

The medial terrane consists of the rugged, CentralNew Guinea Mountain Ranges about 150 km widewith peaks commonly exceeding 4,000 meters and afew over 5,000 meters in elevation. These rangesappear to have begun forming during Tertiary time,perhaps as late as the Miocene, and uplift may havecontinued until as late as the Pliocene. The rangesrepresent an orogen or suture created by the collisionof at least one and possibly more, southward-facing

island arc systems with the northern margin of theIndo-Australian Plate. The island arc was most likelyactive at the time of the suture and may have been aremnant arc. No active volcanism currently exists inthe Central New Guinea Mountain Ranges and a lackof earthquake seismic activity indicates the orogen isstable. Compositionally, the orogen can also besubdivided into 3 parallel sub-terranes, thesouthernmost consisting of deformed sedimentaryrocks; the central terrane made up of heavilymetamorphosed, volcanic rocks and sediments; andthe northern sub-terrane comprising ophiolites andsubduction melange. The collision appears to havebeen oblique, commencing in the west of Irian Jayaand gradually progressing eastwards through time.

The Northern New Guinea Terrane - AnAccretionary Terrane

The northern terrane consists of the northern coastalregion of Irian Jaya to the north of the Central NewGuinea Mountain Ranges. The Northern New GuineaTerrane may be characterized as an accretionaryterrane which was once located in the forearc portionof the magmatic island arc which is now sutured ontothe Indo-Australian Plate. The southern margin of thisaccretionary terrane is marked by the northern borderof the Central New Guinea Terrane and its ownnorthern border is marked by the New Guinea Trenchlocated some 100 km offshore from the northern coastof Irian Jaya. The terrane itself consists of a deformedforearc basin and an accretionary prism located at thepresent-day, trench-slope break.

NORTHERN NEW GUINEA SUB-TERRANES

The tectonics and structural geology of the region arevividly expressed in the physiography of northernIrian Jaya and the,. Northern New Guinea Terrane canbe sub-divided into three broad physiographic "sub-terranes" evident on the mainland. Additionally, anunderstanding of the relationships of relatedphysiographic features such as the Yapen and Biakislands and Cendrawasih Bay is fundamental to anunderstanding of the tectonics and geology NorthIrian.

The Meervlakte Sub-Terrane

The southernmost Northern New Guinea sub-terrane,consists of a broad river valley known as theMeervlakte (or "Plain of Lakes"). The Meervlakte is

an enclosed, 90 by 350 km, intra-montane valleysurrounded by mountains in all directions. The lakesimplied by the name are principally abandoned andflooded, meander loops and oxbows emanating fromthe main Mamberamo River channels and numeroustributaries. Average elevation of the generally flat,valley floor is 500 feet above sea level The crustalcomposition of the basement floor of the MeervlakteValley is uncertain. It is also not clear, but supposed,that the Meervlakte Valley may be underlain by asedimentary basin.

The Northern Divide Ranges Sub-Terrane

The Meervlakte sub-terrane is separated from thePacific Ocean by a second parallel terrane composedof a series of mountain ranges which are known as theNorthern Divide Ranges with peaks of up to 3,000meters high. The Northern Divide Ranges consist of agroup of individually named mountain ranges whichcommence on the Northern Irian Jaya coast in the"Bird's Neck" area where they merges with theCentral New Guinea Mountain Ranges. From there,the ranges carry on eastwards along the northern coastof New Guinea island, separating the northern CoastalPlain from the Meervlakte Valley in Irian Jaya andfrom the Sepik River Basin in Papua New Guineabefore disappearing offshore near the city of Wewak.Its offshore continuation and expression is unclear.Onshore it is approximately 50 km wide and 570 kmlong. The Mamberamo River exits the Meervlakte at apass in the portion of the Northern Divide Rangesknown as the Van Rees Mountains.

Mapping within the Northern Divide Ranges indicateexposures of Paleogene and Neogene sedimentaryrocks which are presumed to be present in thesubsurface. Over 7,000 meters MamberamoFormation turbidites of Mio-Plio-Pleistocene agehave been measured in outcrop. Ophiolites areassociated with early Paleogene sequences. Novolcanic lavas are observed but ash-fall volcanics arenoted in some sedimentary units of the PaleogeneDarante and Auwewa Formations.

It is our opinion that the Northern Divide Rangesrepresent a paleo-subduction complex andaccretionary prism which now divide a forearc basinburied under the Meervlakte Valley, to the south,from a successor basin opening along the Yapen Faultwrench zone, to the north. Subduction has stoppedalong the Northern Divide Ranges complex and the

complex was uplifted to form the mountain rangesnow observed. It is not clear, but possible, that thisNorthern Divide Ranges was once contiguous withthe Niengo Basement Platform subduction complex tothe NW by displacement along the Yapen WrenchFault zone.

The Northern Coastal Plain Sub-Terrane

The northernmost and third sub-terrane borders andparallels the Northern Dividing Ranges to the southand the Caroline Sea to the north. The low-lying andswampy plain occupies a strip of the coast parallelingthe Northern Divide Ranges between the city ofJayapura, in the east, to Waropen Bay in the west, adistance of approximately 350 km. The plain is at itswidest along the course of the Mamberamo Riverdelta. The distance between the mouth of theMamberamo River (known as Cape D'urville) and thepoint the river exits the Northern Divide Ranges isapproximately 70 km. The plain is marked by an axialridge of active mud volcanoes which are presentalong the strike of a large, regionally significant,wrench-fault known as the "Yapen Fault Zone"(Williams and Amiruddin, 1983).

NORTHERN NEW GUINEA RELATEDFEATURES

Cendrawasih Bay

Cendrawasih Bay has a water-depth of over 4,500 feetat its central point. It has a more-or-less flat bottomand its northern and western slopes rise steeply asopposed to the gently rising southern and easternslopes. The presence of deep water, Early Tertiary,marine sediments on the SE coast of the Vogelkopimplies an open marine connection for CendrawasihBay with the Pacific Ocean during most of theTertiary. Cendrawasih Bay appears to be underlain byoceanic crust and possibly a Cendrawasih depressionwhich may be an actively subsiding basin (Dow andHartono, 1982). The relationship between the westernmargin of the North Irian Basin and Cendrawasih Bayocean basin is poorly understood. Seismic coverageacross the boundary area is available.

Yapen Island

Yapen Island is made up of an east-west trending,mountainous belt with peaks over 4,900 feet. Theoldest rocks exposed on Yapen Island are ophiolites

consisting of serpentinites, peridotites and schists.These are overlain by volcanic deposits includingporphyritic rocks, tuffs and breccias which areinterbedded with some greywackes and shales. Thesesediments have been dated and are correlative to theAuwewa Formation of Paleogene age (Atmawinata et.al., 1989). The total Paleogene sequence isapproximately 7,000 feet. Overlying the Paleogene isa sequence of 3,300 feet of pelagic limestones andmarls which become more reefal to the south asobserved in outcrop.

Biak & Supiori Islands

Biak and Supiori islands are separated by a narrowstrait that at ebb tide is only five meters wide.Ophiolites are exposed on Supiori Island in amountain range with a highest elevation of 1,020meters above sea level. Biak Island is relatively flatwith modest hills. Surface geology consists mostly oflimestones and distinct levels of old reef terraces areprevalent. The Auwewa Formation is exposed onSupiori Island near Korido and consists of much thesame lithology as found elsewhere in northern IrianJaya i.e. conglomeratic basalt, tuffs and breccia.Sedimentary rocks of the "Supiori Group" aredescribed by Masria et.al. (1981) as unconformablyoverlying the Auwewa Fmn and considered to beEocene to Oligocene in age. The lithology isdominantly limestone and marl with rare and thinfossiliferous interbeds. This unit is overlain by atuffaceous marl of Miocene age. How these unitsrelate to better known stratigraphy on the Irian Jayamainland is unclear.

TECTONIC ELEMENTS OF THE NORTHERNNEW GUINEA TERRANE

The regional tectonic elements which comprise theNorthern New Guinea Terrane are described in moredetail below, starting at the northern edge of theterrane marked by the New Guinea Trench andworking southwards to the foothills of the CentralMountain Ranges which define the terranes southernmargin where it joins the northern edge of the MedialNew Guinea Terrane. The following regional tectonicelements are depicted in map view on Figure 1 andsome are portrayed in cross-section on Figure 5.

The New Guinea Trench

The "New Guinea Trench" once marked the edge ofthe subducting oceanic plate which formed the

magmatic arc now welded onto Medial New Guinea.Now it appears largely inactive and it constitutes theboundary between the Caroline-Pacific Plate and theIndo-Australian Plate. The New Guinea Trench islocated approximately 100 km offshore the northernIrian Jaya coast and its trace parallels the orogen nowrepresented by the Central New Guinea MountainRanges. The New Guinea Trench trends in a generallyWNW to SSE arc convex to the north. It can be tracedto run the length of the island of New Guinea and tothe SE it bifurcates once into two separate trenchesmidway down the northern Papua New Guinea (PNG)coast. The east-west trending bifurcation is called theManus or West Melanesian Trench and the southerncontinuation is sometimes referred to as the BismarkTrench. At its NW end the New Guinea Trenchappears to die out but may in fact be offset to thesouth just NW of Biak Island and may for a shortdistance form the so-called Biak Trench beforeending in the Waigeo transform fault. The NewGuinea Trench once may have been continuous withthe Philippine Trench before being offset by theWaigeo transform fault.

The Niengo Basement Platform

A prominent and broad, structurally high basementplatform which we call the Niengo BasementPlatform, has been defined offshore by drillingbetween the New Guinea Trench and the northerncoast of Irian Jaya . This platform makes up the floorof most of the northern portion of the North IrianBasin. Local depocenters are evident on the platformand probably represent small, incipient, trench-slopebasins.

The southern margin of the Niengo BasementPlatform extends onshore, as evidenced by basementpenetration in the Niengo-#1 well and theMamberamo-#1 well at depths of 6,460 and 4,025feet respectively. Offshore, the platform has beenencountered in the R-#1 well and the O-#1 well atdepths of 6,888 and 4,750 feet respectively. In the R-#1 and Mamberamo-#1 wells the basement rocks aremade up of mafic rocks described as andesites andbasalts. In the O-#1 and Niengo-#1 wells basementrocks consist of metamorphic limestones and maficvolcanics. Sedimentary rocks covering the platformare largely Plio-Pleistocene in age. Older sedimentaryrocks observed further to the south in the borderingNorth Irian Basin contain thick, Late Miocenesediments not yet observed on the Niengo Basement

Platform, implying absence through non-depositionon this paleo-high.

Seismic data indicate the upper surface of the NiengoBasement Platform to be highly irregular althoughcomparatively shallow in depth. A map of thebasement in the northeastern portion of the YapenPSC indicates that the platform rises generally to theNE to a two-way time of less than 1.0 second in theextreme NE corner of the Yapen PSC. Locally,numerous highs and lows are mapped on thebasement surface with relief between 3.5 seconds inthe deep lows and 1.8 seconds on the highs. Thehighest point of the platform seems to be at itssouthernmost extent near the Niengo-#1 andMamberamo-#1 wells. No seismic data exist over thenorthern extent of the platform where it meets theNew Guinea Trench and the character of it in thatdirection is not known. To the south, the NiengoBasement Platform is bordered by a deep sedimentarytrough or basin with a thick accumulation of Tertiarysediments. The border between the Niengo BasementPlatform and this sedimentary deep is marked by avery steep dip of the platform edge to the south. Thereis no known evidence for a faulted contact, but such isimplied. Seismic data clearly indicate the Tertiarysediments onlapping the steep southern edge of theplatform. Within only a few kilometers to the south ofthe platform edge the depth to basement has increasedfrom 1.6 to over 8.0 seconds two-way time.

It is our opinion that the Niengo Basement Platformrepresents an accretionary prism or subductioncomplex of metamorphosed pelagic sediments andoceanic crust which has been scraped off the uppersurface of the descending oceanic plate and accretedonto the northern end of a forearc basin. Thesubduction complex has grown and been uplifted by"backthrust" thrust faulting in a direction oppositethat of the subducting slab in a process of"subcretion" or "underplating", (Dickinson, 1995).There may be a genetic relationship to the NorthernDividing Ranges which we also believe to be anaccretionary prism although we are unable todemonstrate a temporal relationship.

North Irian Basin

Coeval with the creation of the intra-oceanic islandarc that would ultimately end sutured to the northernIndo-Australian Plate, a forearc basin begandeveloping between the New Guinea Trench and the

magmatic arc, most likely in the Paleocene. Thisbasin we refer to as the North Irian Basin. Previousauthors including Pertamina (1999) have referred toas many as four separate basins in this area namedvariously the Waipoga, Waropen, Biak, Jayapura orNE Irian Basin. From our work it appears the entirearea can be more accurately described as a singlelarge basin with numerous interconnected andseparate depocenters or sub-basins sharing a similarand correlative basin fill.

The North Irian Basin lies within the Northern NewGuinea Terrane as described above. It is an elongateBasin with its long axis trending generally east-west,and basic dimensions are generally 200 to 250 Kmwide by 600 to 700 km long by 3 to 8 km deep.

The oldest sediments in the North Irian Basin areknown as the Auwewa Formation, which is believedto be Paleocene to Early Oligocene in age. Rocksmaking up the Auwewa Formation consist of andesiteand basalt conglomerates, pelagic limestones, hemi-pelagic shales and greywacke dominated turbidites.The Auwewa Formation is observed in outcrop tounconformably overlay mafic oceanic volcanics andophiolite sequences. Such deposits above an oceaniccrust floor within a tectonic setting between amagmatic arc and an ocean trench are diagnostic of aforearc basin( Dickinson, 1995).

Commencing from the onset of the collision of the arcwith the Indo-Australian Plate, the North Irian Basinwas compressed, resulting in rapid basin subsidencewhich was accelerated by rapid deposition andloading of sediments derived from erosion in thenearby, uplifting Central New Guinea MountainRanges. There is also evidence that imbricate back-thrusting of the down-going oceanic slab underplatedthe North Irian Basin and that the ophiolite exposedon the north flank of the Central New Guinea Rangesis itself a back-thrusted sheet derived from the down-going oceanic slab.

The northern limit of the North Irian Basin liesoffshore, somewhere south of the New Guinea Trenchon the Niengo Basement Platform. No seismic dataare available to mark its exact position. The sedimentthickness of the North Irian Basin progressively thinsin the northerly direction and onlaps the NiengoBasement Platform. At the Mamberamo-#1 well,Mamberamo C-Member lies directly on basement andat the R-#1 well the Mamberamo D-Member lies

directly on basement. To the west, the North IrianBasin extends beneath Waropen Bay and seems tomerge into Cendrawasih Bay, the latter of which iscurrently a subsiding oceanic basin. To the east theNorth Irian Basin rises and outcrops near Jayapuraand the Papua New Guinea border in the BorderRanges of mountains. The North Irian Basin lies on-trend with the Sepik River Basin which is located inneighbouring Papua New Guinea. There is apossibility that the two may have at one time been acontinuous basin.

The southern boundary of the North Irian Basin isunclear, principally due to lack of seismic control andthe presence of recent alluvium cover. Large outcropsof North Irian Basin sediments are exposed in theNorthern Divide Ranges of mountains which wouldindicate that uplift of this range occurred much laterthan subsidence in the North Irian Basin. It clearlyimplies that the North Irian Basin extends further tothe south, probably under the Meervlakte Valley. TheMeervlakte Valley is floored with recent alluvium,obscuring older rocks that may prove the basin existsthere. We consider it likely the North Irian Basinextends under the Meervlakte Valley, and that likelysouthern boundary of the basin is marked by theobserved ophiolite outcropping in the northern slopeof the Central New Guinea Ranges. This ophiolitemay represent a fragment of the same oceanic crustnow making up the floor of the North Irian Basin andmay be evidence that the basin extends that far.Detailed outcrop information in this area is largelyunavailable.

Numerous local deeps may occur along thelongitudinal axis of the North Irian Basin. Faulting oflate Tertiary age may have resulted in sufficientdisplacement along the North Irian Basin axis, in botha horizontal and a vertical sense, to regard some localdeeps or depocenters as associated sub-basins.Notable among these is the so-called Teer River Sub-basin which contains an important oil seep. Anotherdepocenter appears to occur at the front of theNorthern Divide Ranges beneath the present-dayMamberamo Delta. Local deeps also seem to occur onboth flanks of the Yapen Fault Zone. Another deepappears to occur at the southern margin of WaropenBay, associated with the Naufi Fault Zone. The so-called Waipoga Trough may also be a related sub-basin of the North Irian Basin.

We characterize the North Irian Basin as a hybrid

basin was initiated as a forearc basin and floored byoceanic crust, but once subduction ceased thearchitecture of the basin became dominated by acomplex, but dominantly compressional stress regimeresulting in thrusting and major wrench-faulting,presumably in response to the, now-oblique,convergence of the Caroline-Pacific Plate and theIndo-Australian Plate. This subsequent deformationhas dramatically altered the character of the NorthIrian Basin. The major structural manifestations ofthis intra-basinal tectonic deformation are discussedin the following section.

STRUCTURAL GEOLOGY

Due to the remote and difficult field conditions onlyreconnaissance-style field geology has beenperformed. Most of the known information regardingstructural style comes from remote sensinginvestigations including SLAR, air photos andsatellite imagery. Some gravity, magnetic and seismicsurveys have been conducted but these are largelyconfined to offshore and basinal areas. Ourknowledge of the structural geology relationshipswhich are probably evident in outcrop in the NorthernDivide Ranges and northern slopes of the CentralNew Guinea Ranges, is minimal at best. Thefollowing structural features are illustrated on Figure1.

Yapen Fault Zone

The Yapen Fault Zone appears to be a positive flowerstructure formed by a wrench fault on which asubstantial amount of sinistral strike-slip movementhas taken place. The Yapen Fault may be acontinuation of the Sorong Fault System evidentfurther west in the Vogelkop. Yapen Island andperhaps Biak-Soepiori Islands to the north, may beslivers which were transported westwards, along thisfault from their original position somewhere nearpresent-day Jayapura.

Relative motion of the Caroline-Pacific Plate and theIndo-Australian Plate is now dominated by strike-slipmotion which may or may not be related to transformfaulting. This strike slip motion may be expressed inthe Sorong-Yapen-Bewani fault systems (possibly allpart of a single genetically related zone) extendingfrom east to west over most of the length of NewGuinea. Motion along the fault is sinistral and someresearchers have suggested that as much as 600 km

horizontal displacement occurred (Waschsmuth andKunst, 1986), based on the theory that sedimentsexposed on Yapen Island were originally depositedtogether with those of the Cyclops Mountains locatednear present-day Jayapura. Subsequent andcontinuing strike-slip motion now evident along theYapen Fault Zone may be localized on transform ortranscurrent faults which were active fault in theoceanic crust prior to being partially subducted underthe arc-Indo-Australian massif.

The Waipoga Fault Zone

The Waipoga Fault Zone is the name given to a large,sinistral, strike-slip fault which forms the southeasternedge of Cendrawasih Bay. The fault strikes SW-NEand appears to be displaced or bent and truncated bythe Yapen Fault Zone which the Waipoga Fault Zoneintersects near the east margin of Waropen Bay. Thefault is parallel to, and en echelon with, the GauttierOffset, as described below.

The Waipoga Trough

The Waipoga Trough is a deep, narrow depressionformed between the Waipoga Fault Zone and the IrianJaya mainland. Its origin and relationship to the NorthIrian Basin are not clear but for petroleum explorationpurposes it can likely be treated as a depocenter in theNorth Irian Basin.

The Gauttier Offset

The Gauttier Offset is named for an offset in the trendof the Gauttier Mountains, probably caused by asinistral strike-slip fault striking SW-NE. Horizontaldisplacement along the strike slip is severalkilometers and the fault causing it likely offset theNorth Irian Basin axis in a similar manner. This faultmay form the western boundary of the Teer Riversub-basin along the main North Irian Basin axis. Itseems to be related to the same stress regime causingthe Waipoga Fault Zone because both the Gauttierand Waipoga faults are parallel and en echelon.

The Apauwar Fault Zone

The Apauwar Fault Zone is clearly evident on radarimagery of the northern coast of Irian Jaya. TheApauwar Fault Zone strikes NNW-SSE and may beoffset by the Gauttier Offset described above. Itscontinuance east of the Gauttier Offset is sometimes

named the Nawa Fault. It is anomalous in that thestrike of the Apauwar/Nawa fault zone is unique andcounter to every other structural element recognizedin the North Irian Basin. Motion along theApauwar/Nawa faults seems to be more of a thrustthan a strike-slip. Its role in North Irian Basinstructure is not clear. The Apauwar, Nawa and Yapenfaults bound the Teer River sub-basin.

STRATIGRAPHY AND SEDIMENTATION

Sediment fill, as observed and measured in outcropwithin the North Irian Basin, has been continuousthrough the Tertiary with the exception of a regionaldepositional hiatus in the Late Miocene. This hiatusresulted in a major unconformity that can be tracedover large areas of the basin on seismic data. Thisunconformity marks the base of the MamberamoFormation and the top of the Makats Formation. Priorto the hiatus, almost 19,000 feet of dominantly clasticsediments had accumulated in the North Irian Basin.Lateral facies changes resulted in locally significantcarbonate buildups. These sediments are assigned tothe Auwewa, Darante and Makats Formations. Post-hiatus, deposition of the Manberamo Formationcontinued uninterrupted until the present except forlocal, minor unconformities. The MamberamoFormation consists of 4 divisions or memberstotalling 23,000 feet in thickness as measured inoutcrop. These thicknesses, would imply a total NorthIrian Basin fill of nearly 42,000 feet. Seismicevidence is available which indicates that the NorthIrian Basin is at least 25,000 to 30,000 feet deepalong its axis. Local, downwarped areas may existwhich are even deeper. Regional stratigraphy of theNorth Irian region is not well defined and lateralfacies relationships are poorly known from fewoutcrop observations. A generalized stratigraphicsection of the North Irian Basin is shown on Figure 6.

The principle source of sediment during North IrianBasin deposition must have been the island arc massifitself. Few other obvious sediment sources can beproposed. It is likely that almost all of the sedimentsof the North Irian Basin came from the south, firstfrom the forming island arc massif and later from theuplifted collisional orogen. The North Irian Basin asevidenced by the wells, contains good qualityreservoir rocks consisting of clean, quartzosesandstones ranging from coarse to fine grained,angular to rounded. Volcanoclastics may be present inthe deeper Auwewa and Makats formations but it is

evident that erosion of the continental portion of theorogen developing in the south gave rise to a largeamount of the sediments encountered in theMamberamo Formation.

PETROLEUM GEOLOGY

Since the 1950's, 12 petroleum exploration wells havebeen drilled in the North Irian Basin, including onestratigraphic test, and have resulted in four dry holes,two gas discoveries and one gas and oil discovery.Four wells encountering were abandoned short oftarget depth due to severe overpressure.

Reservoir Rocks

Mamberamo Formation sands in the R-#1 andNiengo-#1 wells exhibited good porosities on theorder of 23%. Good permeabilities were alsoindicated. The sands were described as quartzose andmedium to fine grained, poorly sorted and angular torounded. Notably, no volcanic or metamorphic rockfragments were described. No porosity occlusion dueto diagenesis was reported. Sands in these wells arelocated at a considerable distance from the arcprovenance as well as being displaced a considerabletemporal distance from the time the sedimentprovenance rocks were young and mineralogicallyimmature. Similarly, Mamberamo Formation sandsencountered in the E-#1 and H-#1 wells in thesouthwestern portion of the Yapen PSC encounteredgood quality reservoir sands with minimal volcanicrock fragments. Sands from this location weredescribed by the mud-loggers as sub-angular whichwould imply short distance transport or be consistentwith turbidite deposition.

Locally, throughout history of the North Irian Basin,carbonate deposition was widespread although likelyconfined to highs and shelf breaks during periods ofsmall clastic influx. Each of the Auwewa, Darante,Makats and early Mamberamo formations areobserved in outcrop to contain facies equivalents ofreefal limestones.

Source Rocks

Hydrocarbon source rocks of fair to good quality existin the North Irian Basin and the vicinity of the YapenPSC over a wide lateral and stratigraphic interval.Indications, based on scant data, are that source rocksassociated with the turbidite-dominated, clastic

formations from the Auwewa Formation through theMamberamo B and C-Members are predominantlygas-prone because of a high terrestrial by-derivedkerogen content. Oil-prone source rocks appear in theMamberamo D and E-Member sediments. Based onthe limited geochemical data available, the oil seepsin the Teer River and the 47o API oil tested in the R-#1 well, there is strong evidence that oil is beinggenerated, which lends credence to a postulatedthermal rather than biogenic hydrocarbon origin.

Maturity

The highest geothermal gradient we calculated forany point within the North Irian Basin, from verylimited available well data, is 1.670 F per 100 feet inthe R-#1 well. Crystalline basement was encounteredin the R-#1 well at a depth of only 6,888 feet and themaximum temperature recorded was 180o F, however,the well tested substantial gas and oil at a depth ofonly 5,809 feet. The reservoir interval is not hotenough to generate hydrocarbons nor are the rocksimmediately underlying it or those lying on top ofbasement.

Known Hydrocarbon Occurrences

The first documented report of a hydrocarbonoccurrence on the northern Irian Jaya coast dates from1916 when an exploration survey discovered a largelive oil seep along the Teer River. The Teer RiverSeep occurs in alluvium as a shallow pool 10 feetwide and consisting of a perpetually fed flow of light,orange-brown oil of 38 API gravity. Gas is observedto bubble through the oil pool. Gas analysis indicatesit contains mostly methane with some heavierhydrocarbons. Nearby outcrops of strongly folded andfaulted Makats Formation clastics and AuwewaFormation carbonates are prime candidates for thesource of the oil. A lesser seep appears ephemerallyin the rainy season at a site within the Apiawete Riverchannel, some distance away, and appears to beassociated with Mamberamo Formation sandstones.

The Niengo-#1 well tested gas at an estimated rate ofat least 4.5 MMCFD. The R-#1 well tested dry gas ata high rate of 21.6 MMCFD. A ten liter oil samplewas collected during the same test from the samezone. It is worth noting that the gas consisted of99.26% methane and no appreciable CO2, H2S or N2.The oil sample consisted of black oil, not condensate,of 47o API gravity and was part of an estimated 5

barrels of oil and water produced during the 30minute flow period.

CONCLUSIONS

The North Irian Basin is an elongate basin with itslong axis trending generally east-west. Basicdimensions of the North Irian Basin are generally onthe order of 200 to 250 km wide by 600 to 700 kmlong by 3 to 8 km deep. We characterize the NorthIrian Basin as a hybrid basin which began life as anforearc basin floored by oceanic crust. Subductionstopped in the Late Miocene and the remnant forearcbasin, now a sub-basin of the greater North IrianBasin, is expected to lie beneath the MeervlakteValley between the Northern Divide Ranges and theCentral Mountain Ranges of New Guinea. Subductionstopped due to translational motion between theCaroline-Pacific Plate and the Indo-Australian Plate.Convergence rotated to oblique relative motion,causing sinistral, strike-slip displacement between thetrench and the subduction complex. A portion of thesubduction complex was uplifted during this time toform and be preserved as the Northern DivideRanges. Another portion of what may be the samesubduction complex, or a localized equivalent we callthe Niengo Basement Platform, appears to bedisplaced from the Northern Divide Ranges by theYapen Fault and remains buried in the subsurface atthe northern edge of a successor basin which openedbehind the original subduction complex and formspresent-day Waropen Bay and holds the MamberamoRiver delta complex.

ACKNOWLEDGEMENTS

This paper was made possible due to the authors workon evaluating the geology and petroleum potential ofthe Yapen Block, offshore northern Irian Jaya. Theauthors would like to express their gratitude toPertamina and Apex (Yapen) Ltd. for access toseveral thousand kilometers of seismic, well reportsand regional geologic reports. Thanks to Mr. RichardB. Wells for editorial comments on the manuscript.

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