IX.1 INTRODUCTIONThe first geological research after the second world-war on the Halmahera region has been carried out by Van Bemmelen, as it reported in his famous book: "The Geology of Indonesia (1949).Obi Island is separated from Bacan and Halmahera by a strand of the Sorong Fault strikeslip system. Its paleogeographic relations to the other two islands are unknown. Obi differs from Bacan and Halmahera by the absence of volcanic rocks of Neogene age. It appears not to have a direct relation to a presently active subduction system.

IX.2 REGIONAL GEOLOGY IX.2.1 Tectonic Setting The Halmahera Island has complex tectonic history, since the area is situated in a triple plates junction of Philippine Oceanic Plate, Australian Continental Plate and Eurasian Continental Plate (Figure 1). In the North of the region, the Philippine oceanic plate moves eastward and subducted beneath the northern part of Halmahera Island. This plate subduction continued to the southern part of Phillipine Arc. Whilst in the South, the Australian continental plate moves relatively along the NE and is bounded by the sinistral Sorong Fault. An oceanic plate of Molucca adjacent to the Halmahera Island subducted in the West beneath the SangiheArc and in the East beneath Halmahera (Figure 2).To the South, there are three plate movements along the Sorong Fault Zone, which drifted to the West continental fragments from the northern part of Papua New Guinea. In the southeast part of Halmahera Island, the platelet comprises of thin layer of Palaeogene coaly limestone. The limestone deposited overlies Pre-Tertiary ultrabasic igneousbasement rock.A NW-SE trending fault of Akamalo separated the Halmahera region from its northern part. The Akamalo Fault may be activated by the Sorong Fault System which trends East-West. The fault activation resulted in deep basin formation in the Weda Bay. The basin has been filled of 20.000 feet thick of Palaeozoicum to Pleistocene sedimentary rocks. The Weda Bay Basin itself has wide of 15.000 Km2.Widi Shelf is a relatively basement high adjacent to the southwest part of the Weda Bay Basin, which separated one each other by strike slip Widi fault. Miocene reefal carbonates growth up on the Widi Shelf and has been growing up recently on the northern and eastern parts of the shelf. Whereas, in the southern boundary of Widi Shelf there was possibility that grabens developed on the Miocene Carbonate Unit. The shelf itself has slowly been tilted to the northwest and passed up the Miocene carbonate level. The

uplifting, in turn, resulted in reformation of the Tertiary Southern Halmahera Basin, which its provenance was the southeast onshore of Halmahera. The basin has wide of 1.700 km2, and filled by a 15.000 feet thick of Tertiary sediment.A compressional zone of the uplifted Neogene bounded the Western part of Southern Halmahera Basin and the Weda Bay Basin by sediment. The western area to the compressional zone was deposited by Oligocene fore-arc sediment and OHA volcanic, as the results of Palaeogene arch subduction.

IX.2.2 Stratigraphy Halmahera Basin Sedimentary rocks with age of Mesozoic and Tertiary were deposited more than 20.000 feet in the Weda Bay Basin, whereas, in the southern Halmahera Basin more than 15.000 feet sediments were deposited since early Tertiary (Figure 3). The onshore Halmahera stratigraphy is quite confusing because of limitation of the outcrops, especially in the southern part of the island. In contras, the offshore stratigraphy is clear and even can be correlated to the wells in the outside basins, such as Salawati basin to the southeast.The basement rocks in the Weda Basin consists of turbidity sandstones, shale and coal derivated from Sibela-Tapas Formation, which was metamorphed and equals to Devonian Kemum Formation in the Bird Head.Pre-Mesozoic Carbonate Formation unconformably overlies the Permian fluvio-deltaic sandstone, shale and coal basement The upper part of the pre-Mesozoic Formation composes of oAdized clastics of red bed. Jurassic-Cretaceous (Mesozoic) rocks consist of intercalation of shale, sandstone and fluvio-deltaic shale which unconformably overlies the pre-Mesozoic sediments.The Tertiary rocks unconformably deposited overlie the older sediments. In general, the Tertiary rocks compose of shallow marine limestone, marl, shale, sandstone and coal.In the Miocene time thick limestone of reefal, banks and platform carbonates was conformably deposited overlies the older sedment in the Abda Bay Balm but it deposited unconformably overlies the older sediment in the SoAh Halmahera Balm The carbonate had been growth up to the Pliocene time, before it covered by marine claystone. In the Plio-Pleistocene time, tectonic activities increased in Halmahera and its suuounding region, coinaidentally with ophmurn of movement intensity along the Sorong Fault Zone, so that baln subsidence and rocks deposition in the Weda Bay Basin increased very fast. This Plio-Pleistocene sequence composed by volcano-clastic deposits at the western pat of Halmahera, adjacent to the recent volcanic arch and shallow-deep marine claystone deposited in the Weda Bay Bast.Obi Basin

Ophiolitic Basement ComplexAll members of a complete ophiolites sequence (oceanic crust) have been identified in the Obi geology, hom ultramafic rocks to gabbros and volcanics. The ultramafic complex must have a tectonic contact with the metamorphosed sedimentary unit in the basement of Obi and with the Jurassic Leleosasso Formation, which is assumed to overlay that unit. The nature of the contactis not dear but may be an unconformity. Based on similarities with the ophiolites in the basement of Halmahera, a Cretaceous age of the ultramafic complex can be expected. The complex is widely present along the western and northwestern coast of Obi as well as in the central parts of the island.Metamorphic Basement Complex .The rocks are assumed to represent a corlineMal fragment Contacts of the metamorphc rocks on Tapas Island are only with unconformably overlying Pleistocene reef limestones. The rocks are of unknown age. The metamorphosed sediments occur rather isolated from the surrounding geological units. Leleosasso FormationLeleosasso Formation consists of calcilutites associated with deformed shales and quartz sandstone turbidities, where several beds are internally laminated. It is deposited in a continental slope environment. No contacts have ever been reported. The Jurassic age for these sediments is based on the similarities of the formation with Jurassic sediments elsewhere in Eastern Indonesia. The largest occurrences of the Leleosasso Formation are shown on the southern and eastern coast of Obi Latu, to the NW of Obi. Paleogene Volcanics and Volcaniclastics Widely distributed volcanic and volcaniclastics rocks in Obi are pre-Miocene age and are assumed to be Paleogene to Early Miocene in age (Sudana and Yasin, 1983). They have an unconformably position underneath Miocene Limestones of the Fluk Formation. The unit occupies the central part of Obi.Fluk FormationMassive calcilutites with abundant larger foraminifera and shell fragments outcrop at the northern coast of Bisa Island, to the North of Obi. In southeastern Obi, Middle to Late Miocene calcarenites are unconformably overlying volcanic rocks. These limestones belong to the Fluk Formation of Early-Middle Miocene age (Sudana and Yasin, 1983). Composition and age indicate that limestones are the lateral equivalent of the TingTeng Formation of Halmahera and the Ruta Formation of Bacan. `Woi, Obit andAnggaiFormationThe Early to Middle Miocene limestones of the Fluk Formation are unconformably covered by a large variety of Late Miocene to Pleistocene clastic sediments and coral reef limestones. The sediments are subdivided into three formations; the Woi consists of sandstone, conglomerate and marl, the Obit composed by volcanic breccia and andesitic

lava with tuffaceous sandstone and clay intercalation, and the Anggai consists of limestone.

IX.3 PETROLEUM SYSTEM IX.3.1 Source Rock Halmahera Basin Analoging the Weda Bay Basin to the Bird Head region, therefore, the hydrocarbon source rocks potential are shale, claystone and coal from swamp deposit with age of late Permian. Transition deposit of early-middle Jurassic coaly shale and Palaeocene marine deposits of shale and fine grained limestone are also source rocks for the basin. In addition to these, Eocene transition deposits of coal and shale, Miocene deep marine deposits of shale and fine grained limestone and Late Miocene marine deposit of shale are also source rocks forthe Weda Bay and South Halmahera Basins.Burial history and maturity analysis on the Weda Bay Basin indicates that early hydrocarbon generation was in Early Jurassic and continued to Recent time for the other source rocks. Almost of the structures created during Middle Jurassic, Cretaceous, Early Tertiary, Oligocene, Middle Miocene and Pliocene time are possible for facilitating for hydrocarbon accumulation. The Mesozoic deposits, which experienced basin subsidence during Late Miocene-Pliocene, are being maturity reference for any potential source rocks in the basin.Burial history and maturity analysis on the South Halmahera Basin indicated that thermal maturation for the source rocks was happened during early Tertiary Late Miocene and covering the area of 500 km2. If the hydrocarbon generation and migration really existed, than they should be accumulated during Late Miocene-Pliocene time and even continued to the Recent. This condition applied to the Salawati Basin.

Obi BasinThe few isolated outcrops of Jurassic sediments reported from Obi Island have been describe as black shales (Hall et al, 1988) but there are no source rock data from these sediments. One oil seep has been reported from Gommu Island, to the south of Obi, which has not been confirmed a second time or could be proven natural.

IX.3.2 Reservoir Rock Weda Bay Basin

The potential reservoir rocks in the Weda Bay Basin are Late Permian fluvio-deltaic sandstones. The sandstone has good reservoir quality equal to Permian Aifam sandstones, which produced 40 MMCFGPD from well Mogoi Deep-1, to the southeast Bintuni Basin. The other Early-Middle Jurassic fluvio-deltaic sandstones equal to the reservoir rocks in the giant oil field Wiriagar /Tangguh, which has gas reserve of 17 TCF. Carbonaceous fluvio-deltaic sandstone and Palaeocene-Eocene sandstone and carbonate also have moderate-good reservoir quality. Miocene carbonate equals to the hydrocarbon producing Kais limestone in Salawati Basin. Tough seismic analysis predicted that carbonate build-up in the Weda Basin was not well developed but high energy shelf/platform carbonate would have wide distribution in the basin.South Halmahera BasinThe main reservoir rocks in the South Halmahera Basin is Miocene carbonate which equals to Kais Carbonate. The reservoir carbonate composed by reef carbonate and shallow shelf carbonate deposit. In the Salawati Basin, the same reservoir has moderate - excellent reservoir quality. Reef carbonate of Kais in the oil fields: Walio, Kasim and Cendrawasih have porosity of 25-30%. In several oilfields in the Salawati Basin, such as Kasim, Cendrawasih, Jaya, Sele, Linda, Matoa, Baratlaut Obi and Arar, the main reservoir rocks are the Miocene Kais limestones. This carbonate possibly well developed in the south Halmahera Basin.Obi BasinThe entire Tertiary stratigraphy of Obi is very poor in quartz. Sandstones are all lithic in composition and poorly sorted or very fine grained. Coral reefs offer the best reservoirs in the area.

IX.3.3 Seals Halmahera Basin Seals in the Weda Bay Basin composed of intraformational shale and claystone, which covered the pre- Miocene carbonate reservoir.Seals at the lowest level of Miocene reef consisted of Miocene calcareous shale, and deep marine sandstone. The off-reef equals to the Miocene carbonate. Whereas the top seal should be the same with the lowest carbonate build-up level, when the very thick mudstone of late Miocene Weda Group produced excellent seals for the highest reef level. Ifthe potential Weda reservoir really existed, it should exist within the claystone.Plio-Pleistocene sequence of volcano-clastic presents in the west Halmahera, close to the Recent volcanic arc. Whereas, in the Weda Bay Basin deposited a shallow-deep marine claystone.Obi Basin

The Obi stratigraphy is poor in clays. Real clay units are apparently missing and this is thought to be the immediate effect of the composition of the eroding areas that provided the clastic sediments.

IX.3.4 Traps Halmahera Basin There are two main hydrocarbon traps in the Halmahera block, they are: structure and stratigraphy traps.Structural TrapsThe structural traps are anticlines and faults, which dominated in the northern part of the Weda Bay Basin. The anticlines associated with faults. The anticlines have North-South trend and perpendicular to the main compressional stress, which trends East-West, same as the plate collision.The first period of regional fault system is trending East-West, as shown by East-West trending Akemalo fault. This crossed the northern part of Weda Bay basin. Some prospects and leads are identified along the fault.Stratigraphic TrapsThe stratigraphic trap in the South Halmahera Basin comprises of carbonate build-up, which well developed in the basin. Equals to those in the Salawati Basin, the Miocene Carbonate Formation was well developed and has a shallow shelf facies with strong energy and reef facies. The other traps are sandstone lenses and slope and floor fans deposits.Obi BasinHydrocarbon traps in the basin compose of stratigraphy and structure traps. The main stratigraphy trap is coral reef limestones of Late Miocene-Pleistocene Anggai Formation. The structural trap is anticlines related fault.

IX.3.5 Migration PathwayThe source rocks are present within the deeper part of the basin and oil migration into the reefs through the Widi Fault System. During Late Miocene, migration took place from a kitchen area to the south. Some of oil trapped in reef reservoir before tilting of the Widi Shelf and subsequently retained. More promising migration path may occur from the East of the Widi Shelf where the bounding fault is oriented North-South.

IX.4 Hydrocarbon Play Weda Bay BasinThe potential play in the Weda Bay Basin is Late Permian fluvio-deltaic sandstones, which has good reservoir quality equal to Permian Aifam sandstones in the Bintuni Basin.

Hydrocarbon potential in this play sourced from shale, claystone and coal, which deposited in the swamp with age of Late Permian.The other potential play in the Weda Bay Basin is Early-Middle Jurassic fluvio-deltaic sandstones, carbonaceous fluvio-deltaic sandstone and Palaeocene-Eocene sandstone.Miocene carbonate equals to the hydrocarbon producing Kais limestone in Salawati Basin is the other objective play which sourced by argillaceous shale and Early Miocene Mudstone and Coals. The hydrocarbon trapping in the Weda Bay Basin is mainly structural trap, which dominated in the north part of Weda Bay Basin (Figure 3).South Halmahera BasinThe main potential hydrocarbon play in the South Halmahera Basin is Miocene carbonate which equals to Kais Carbonate. The reservoir carbonate composed by reef carbonate and shallow shelf carbonate deposit.Eocene transition deposits of coal and shale, Miocene deep marine deposits of shale and fine grained limestone and Late Miocene marine deposit of shale are the source rocks for the Miocene Carbonate Play. The Hydrocarbon trapping in the South Halmahera Basin is mainly stratigraphic trap comprises carbonate build-up (Figure 4).Obi BasinThe main hydrocarbon play for the basin is coral reef carbonate play of the Late Miocene-Pleistocene Anggai Formation.

ReferencesDarman H., and Hasan Sidi F., 2000, An Outline of The Geology of Indonesia, Published by

IAGI-2000, pp 131-140.Mobil-GRDC, 1993, Geology and Hydrocarbon Prospects of Halmahera, Bacan and Obi, East

Indonesia.