POTENTIAL HYDROCARBON BASED ON STRUCTURAL GEOLOGY AND STRATIGRAPHY IN SERAM BASIN HALMAHERA, EASTERN INDONESIA

Fajar Ramadhan Geological Engineering Department, Diponegoro University, Semarang

Jl.Prof.Sudharto,S.H Tembalang, Semarang – 50275Phone: 085718206266

e-mail : fajar_ramadhan@aol.com

ABSTRACT

Indonesia get decrease in oil and gas reserves of about 2.4% per year (Panuju, 2010). These discoveries are found dominantly in Tertiary producing basin which has been explored since early 19th century, only one that found in non producing basin. Jurassic play system was found in producing Seram Basin. Seram and the Banda Sea lie between the passive margin tectonics of Australia’s Northwest Shelf and the active margin tectonics of New Guinea, both of which have played an important role in the structure, facies distribution and hydrocarbon prospectivity of the area. A restored cross section across Seram and a 3D model reconstruction of the Miocene evolution of the Banda Arc reveal the history of the area. The Proto-Banda Sea is considered to have formed in the Permian, including a marginal basin with Permian oceanic crust. Extension was terminated by Triassic orogenesis in New Guinea supplying vast amounts of Triassic detritus (Kanikeh) to the stretched Banda margins. In the Late Triassic, the sediment supply was diminished in part due to the renewed onset of extension along the New Guinea margin. It is notable that the Triassic orogeny was very similar to the Miocene to Recent orogeny in New Guinea. As Triassic sediment supply was reduced, carbonate banks were locally built up (Manusela reservoir) surrounded by starved source rock facies. Stratigraphy of Eastern Indonesia from Palaeozoic – Mesozoic is mostly dominated by terrestrial to shallow marine deposits that resulted from tensional tectonic event, while the Tertiary to Recent sediments indicate a shallow marine to deeper marine deposition of carbonate with minor siliciclastic sedimentary rocks.Hydrocarbon potential of Eastern Indonesia mainly developed in the Mesozoic petroleum play for siliciclastic play and in the Neogene limestone for limestone play. Eastern Indonesia has the upside potential for hydrocarbon accumulation, mainly for Paleozoic siliciclastic play or late Tertiary deeper marine play.

Keywords: Structural, Stratigraphy, Hydrocarbon, Seram Basin, Eastern Indonesia

INTRODUCTION

Indonesia's energy demand is increasing but it is not accompanied by the production of oil and gas resources which declined approximately 2, 4% per year (panuju, 2010). The most significant discovery from the last two decade in Eastern Indonesia is the Jurassic-Permian Play System in Eastern Indonesia (Tangguh, Oseil and Abadi). These discoveries are found dominantly in Tertiary producing basin which has been explored since early 19th century, only one that found in non producing basin. Jurassic play system was found in producing

Hydrocarbons are organic chemical compounds of carbon and hydrogen, and may be in a gaseous, liquid or solid phase. Carbon always forms four chemical bonds, while hydrogen forms but one. The simplest hydrocarbon is methane, with a single atom of carbon bonded to four atoms of hydrogen. Ethane has two carbon atoms, each bonded to each other and three hydrogen atoms each. Hydrocarbons can get larger and more complex as the number of carbon atoms increases. Carbon atoms may double- or even triple-bond to one another, and a single carbon atom may be bonded to three or four other carbon atoms, producing branched chains. These hydrocarbons are used in myriad industrial applications in addition to their use in fuel. The hydrocarbons derived from crude oil are the basis of the enormous array of plastics that are essential to virtually all aspects of modern life.Seram Basin, producing Bintuni basin and non producing south eastern edge of Palung Aru basin. Bintuni Basin has been produced oil from 1956 (Mogoi and Wasian discovery in 1938) but the oil was produced from tertiary sediments (Miocene Kais Limestone, only around 7 MBBLS oil was produced from 1956-1961), meanwhile production in Seram basin started in 1913 (Bula Field discovery in 1897) from Pliocene Fufa Formation and until now still producing around 45 BOPD.The first discovery of Jurassic Play System at Eastern Indonesia is in 1988 when East-Nief-1 well in Seram basin tested Jurassic Manusela carbonate and recovered 1300 bbls of oil and 1.23 MMSCFD, since than the Jurassic Play System are being tested in others area. The effort are pay off by the discoveries of several fields, which is Tangguh Complex Giant Gas Field from 1990 to 1998 exploration campaign which tested Jurassic Sandstone Reservoir which known as Roabiba-Aalenian at Bintuni Area, Oseil Field was found by Oseil-1 well and tested 3800 BOPD oil from Manusela Carbonate at Seram basin, and Abadi Giant gas field which found by Abadi-1 well in 2000 and tested 25 MMSCFPD, 260BOPD (52.5 API) & 178 BWPD from Jurassic Plover sandstone in SE edge of Palung Aru near Timor Gap.Jurassic plays system has found more than 4 BBOE reserve with success ratio for finding the play system around 56% in Bintuni Area, 66 % in Palung Aru and 66 % in Seram Basin.

STATE GEOLOGICAL

Seram and the chain of islands continuing in E/ SE direction all share a very complex fold-thrust belt geology, with N-directed thrusting and with fragments of continental blocks, metamorphic rocks and ophiolite complexes. Deformation is less intense West of Seram, on Buru island. Large ophiolite bodies and metamorphics are present in SW Seram and Buru. (figure 1)Paleozoic metamorphics are overlain by M-L Triassic Kanikeh Fm flysch-type clastics, overlain by Late Triassic reefal and deepwater limestones (commonly interpreted as Jurassic, but paleontological evidence appears to exclusively suggest a Late Triassic age; e.g. Martini et al., 2004). This series has been interpreted as a Late Triassic intra-cratonic rift sequence.The Triassic is overlain by a highly condensed Early-Middle Jurassic limestone (e.g. Wanner & Knipscheer1951) or may locally be missing completely, and Late Jurassic Kola Shale and may represent continental breakup and onset of spreading. The Nief Fm pelagic limestone sequence of latest Jurassic (calpionellids), Cretaceous (Globotruncana) and Paleo-Eocene ages represents the oceanic drift or very distal passive margin stage of the Buru- Seram microplate.Similarities in stratigraphy and structure between Seram and Timor have been noticed by many authors. There are also similarities with the Triassic stratigraphy of nearby Misool, but the Jurassic- Paleogene of Seram- Buru is in more distal facies, and lack the rich macrofossil faunas of Misool. There is also evidence of consumed oceanic crust between Misool and Seram (see below), so the present-day proximity is not necessarily the same as the paleo-position.Widespread folding and thrusting of Eocene and older rocks, with the formation of the 'Salas Block Clay'

olistostome or melange, suggests a major collisional event, but the exact age of this remains uncertain. It is probably related to ophiolite obduction at the S/SW side of Seram, which have a Late Miocene onset of exhumation age (around 8 Ma; Linthout et al. 1996). (Figure 2).This young fold and thrust belt outcrops on Seram and continues N of Seram up to 100 km offshore (e.g. Teas et al. 2009), where it looks like a continuation of the Banda Arc accretionary complex. This foldbelt is commonly described as merely a zone of young thrusting within the 'Birds Head' part of the Australian continental margin between Misool and Seram Islands (e.g. Granath et al. 2011), but the width of this belt requires 100's of km of shortening. Also, a S-dipping subducted slab is clearly imaged below Seram by tomography and by earthquake distributions, and remnants of a Plio-Pleistocene volcanic arc are present S of Seram (Ambon). In my opinion it is difficult to not view Seram as a continuation of the (now mostly extinct) Banda Arc subduction complex.The Seram Island is part of the Outer Banda Arc forming a nonvolcanic arc, metamorphic belt, and acretionary terranes (Figure 3). This area, the continuation of Timor, is previously located in the collision zone between the Australian Continent and the Banda Subduction Zone, where the NW margin of Australia moved towards the Banda Subduction. Zone. The NW shelf of Australia itself was generated due to the break-up of the Gondwana Land during Jurassic (Powel, 1976; Veevers, 1982). The Banda Subduction Zone was resulted from the oceanic crust subduction beneath the Banda Volcanic Arc that was located to the north of Australia, during the Eocene time (Hartono, 1990). To fit the Seram stratigraphic section (Figure 4) into a regional context it was compared to that in the Browse Basin along Australia’s NW Shelf

METHODOLOGY

This paper is based on study both published literatures and unpublished data encompassing recently some joint study results with oil companies in Eastern Indonesia and also the field work carried out in the surrounding Tanimbar Islands. The main target of the field work is to investigate the characteristics of reservoir rock and source rock that developed in this area which then extrapolate to the interpretation of the sub-surface beneath the concession area of JOGMEC - INPEX Corporation and Outer Banda Arc region in general.To address the goal of the paper, literatures from various publications and unpublished data were collected, examined, analyzed and put in the interpretation and synthesis. By re- studying various stratigraphy data of eastern Indonesia which is supported by the primary data in the Tanimbar Islands, some exploration wells report as well as seismic lines in the Arafura Sea – Outer Banda Arc and with reference to the results of previous studies primarily related to tectonics, the concept of tectono-stratigraphy of Eastern Indonesia can be summarized in this paper.

RESULTS AND ANALYSIS

Source RockThe potency for hydrocarbon is mainly controlled by the presence of mature source rocks. Without charging the system from mature source rock to the reservoir in the traps, the petroleum system will not work properly, except there are other charging systems from other basins nearby. Pre-Tertiary source rocks are widespread in Eastern Indonesia but significant source rock accumulation is primarily restricted to three periods, namely, Permian, Late Triassic and Early-Middle Jurassic periods. Proven Tertiary source rocks are identified in restricted areas of Eastern Indonesia but have been shown to be prolific hydrocarbon producers. The source rocks of Paleozoic-Mesozoic are mostly of infra-rift origin, while the Tertiary source rocks are related to syn-orogeny sediments. The characteristic of petroleum system in some field in Eastern Indonesia (table 1).ReservoirThe Reservoir rocks belong to the Mesozoic and Tertiary. The Mesozoic reservoirs are sandstone and

carbonate rocks, while the Tertiary rock consists of Miocene limestone and sandstones and also Pliocene sandstones. The limestone geometries mainly are platform and buildup. The porosity is mainly secondary type due to dissolution and cracking. Trap and SealThe traps in Seram Basin are mainly structural traps which include thrust and normal faults, while minorities are stratigraphic traps of carbonate buildup. For the seal rocks, the petroleum system in Eastern Indonesia is mostly provided by the syn-orogeny and passive margin shales. This rocks type is usually deposited in a wide depositional environment and therefore can act as a regional seal rock.

DISCUSSION

The tectonic model and cross sections that have been presented indicate that the Seram margin has been subjected to over 100 km of shortening with overall shortening of the order of 50%. As the section is roughly perpendicular to the margin and the compression is inferred to have been highly oblique (sinistral transpression) the true shortening may be considerably greater. Much of the shortening is inferred to have been taken up by overthrusting of the accretionary prism, but there was also substantial shortening of the continental margin sediments, as indicated by the antiformal stack in the Manusela Beds and the thrusting of the Upper Nief beds to the northeast of Seram.In the Oseil and East Nief area, the Manusela carbonate reservoir porosity dominantly occurs in fractures. The structural-tectonic model indicates two different directions of thrusting, which will have a bearing on the orientation of fractures within the Manusela carbonates. The Early Pliocene thrusting was towards

070°+/-10° and open fractures at that time are likely to have been parallel to this direction. Late Pliocene

to Present thrusting was towards 040° including in the Manusela carbonates, and open fractures are likely to be parallel in the same orientation. However, a full evaluation of the stress field through time is needed to predict the full range, intensity and width of fractures.The Oseil and Bula oilfields, and the prolific seeps along Nief Gorge, which trends 040°, demonstrate the Late Triassic to Early Jurassic source system. The structural model suggests rapid Pleistocene burial and heating by overthrusting, so that the system is active today. In terms of hydrocarbons, it is possible that there are more Manusela carbonate plays, but there may also be reef plays in the Upper Nief duplexes and structural- stratigraphic traps in the inferred Middle Jurassic sandstone reservoir.Comparing Seram to Timor, whilst there are broad tectonic similarities due to their positions around the Banda Arc, there are structural and stratigraphic differences. As Seram lay along the northern margin of the proto-Banda Sea, it is likely that it received more Triassic sediment than the Timor margin due to Early to Mid Triassic uplift and erosion in New Guinea. In the model presented here, this would have led to a large accretionary prism to overthrust Seram, but less so for Timor. In addition, the collision in Seram was highly oblique, whilst that in Timor was more orthogonal. This may have caused more fracturing in Seram, but potential carbonate plays in Timor may have been less buried.A clear problem with all the models and hypotheses presented above is the general paucity of data in the Banda Arc region with which to test the ideas. Hopefully, ongoing exploration will help to rectify this.

CONCLUSIONS

The present tectonic setting of Eastern Indonesia Region is closely related to the long history of NW Shelf Australian tectonic evolution since the Paleozoic to Tertiary. Tensional tectonic events mostly occurred during Paleozoic to Mesozoic which may be contemporaneous with the slivering process of Gondwanaland to form Southeast AsiaMainland. In the Neogene period, the tectonic events in Eastern Indonesia were the compression due to subduction – collision of Australian Plates into Eurasian and a number of smaller micro continents

which are considered to be of Australian affinity.Stratigraphy of Eastern Indonesia from Palaeozoic – Mesozoic is mostly dominated by terrestrial to shallow marine deposits that resulted from tensional tectonic event, while the Tertiary to Recent sediments indicate a shallow marine to deeper marine deposition of carbonate with minor siliciclastic sedimentary rocks.Hydrocarbon potential of Seram Basin mainly developed in the Mesozoic petroleum play for siliciclastic play and in the Neogene limestone for limestone play. Eastern Indonesia has the upside potential for hydrocarbon accumulation, mainly for Paleozoic siliciclastic play or late Tertiary deeper marine play.

ACKNOWLEDGEMENTS

Acknowledgements are addressed to consortium of JOGMEC and INPEX Inc. for funding field work in the Tanimbar Islands during the joint study of Selaru area. We also would like to acknowledge our colleagues including Budianto Toha M.Sc, Dr. Salahuddin Husein, Sarju Winardi M.Eng, and Dr. Hendra Amijaya for having shared continuous and fruitful discussions.

REFERENCES

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Table 1. Petroleum System Characteristic of Some Fields in Eastern Indonesia

AREAFIELD RESERVOIR SOURCE ROCK SEAL

TRAPAGE FM. LITH. AGE FM LITH. AGE FM. LITH.

BINTUNI

WiriagarM

id.JurassicK

embelangan

Group

SandstonePermian

Jurassic

Ainim

Yefbie

Carbonaceousshale & coal shale & coal

Late Jurassic

UpperK

embelangan

Claystone and shale

anticlineVorwata pinchoutOfaweri Pop-up Roabiba anticline

SALAWATIKlamono Miocene

MioceneKais

KlasefetLimestoneLimestone Early

PlioceneKlasaman Shale

MioceneMiocene

KaisKlasefet

Intraformati

onalShale

Thrust anticline

WalioKasim

SERAM OseilEarly-Mid.

Jurassic Manusela LimestoneTriassicJurassic

KanikehManusela

Calc.shaleCarbonate

Late Jurassic

Kola ShaleThrust

anticline

BANGGAI Tiaka JurassicEarly

MioceneLate

Miocene

BobongTomori

Mtindok

SandstoneLimestoneSandstone

JurassicEarly.

Miocene

BuyaSalodik

Marine shaleShale &

carbonateLate

MioceneMatindok Marine

shaleThrust

TIMORAbadi Mid. Plover Sandstone Early. Plover eq. Marine shale Early Echuca shale Normal FaultBayu-Undan

Mid.Jurassic

ElangPlover

SandstoneSandstone

Mid.Jurassi

c

ElangPlover

SandstoneSandstone Early

CretaceousEchuca Shoals

shale

Figure 1. Early geologic map of Seram (Rutten 1929; from Rutten and Hotz, 1920)

Figure 2. N-S cross-sections through NW Seram, showing N-directed folding and thrusting of metamorphics-granite complex over folded Mesozoic sediments (mainly Triassic) (Rutten and Hotz,1920)

Figure 3. Schematic regional cross-section across Seram, with a vertical exaggeration of x ~2.5. See Figure 1 for location

Figure 4. Simplified stratigraphy for Seram courtesy of Kufpec (Indonesia) Limited.