assignment basin analysis

Foreland Basin And Its Hydrocarbon Potential

FORELAND BASIN AND ITS HYDROCARBON POTENTIAL

1. INTRODUCTION

Foreland basins are generally accepted to express downflexing of the lithosphere in front of tectonic loads (Price, 1973; Beaumont, 1981; Jordan, 1981; Karner and Watts, 1983; Lyon-Caen and Molnar, 1985; Stockmal et al., 1986; Flemings and Jordan, 1989; Sinclair et al., 1991; Watts, 1992). Foreland basins are elongate or arcuate, highly asymmetrical basins closely associate with continental collision zones (Allen & Allen, 2005). Foreland basins can be divided into two categories (DeCelles & Giles (1996)) (see Fig. 1):

- Peripheral (Pro) foreland basins, which occur on the plate that is subducted or underthrust during plate collision (i.e. the outer arc of the orogen). In the simply, it being understood that lie on the continental crust of the subducting plate. Ex: Indo-Gangetic Plain, north Alpine foreland foreland basin.

- Retroarc (Retro) foreland basins, which occur on the plate that overrides during plate convergence or collision (i.e. situated behind the magmatic arc that is linked with the subduction of oceanic lithosphere). In the simply, it being understood that lie on the continental crust of the overriding plate. Ex: Late Mesozoic-Cenozoic Rocky Mountain Basin, North America.

Both classes of foreland basin overlie cratonic lithosphere and are associated with crustal shorting in tectonically active zones. Some highly arcuate thrust belts and spatially restricted foreland basin are related to subduction zone roll-back and are commonly associated with important backarc extension (Fig. 1).

Foreland basin strata potentially contains a decipherable record of the rheology of the lithosphere and the tectonic history of the bounding mountain belt. Foreland basin area is important because it not only contains large reserves of hydrocarbons, coal but also the habitat, cultivation of human.

Characterization of petroleum foreland Basin:o Distinguishing features: multicycle basin on craton edge with adjacent uplift.

o Depositional History: 1st cycle mature platform sediments; unconformity;

2nd cycle orogenic clastics.o Reservoir: mostly sandstone, lesser carbonate; in both cycles.

o Source: overlying or interfingering shale; locally coal.

o Cap: shale or evaporite.

o Trap: mostly anticlines; some stratigraphic and combination.

o Geothermal Gradient: low to above average.

o Hydrocarbons: mixed crude, similar to interior basins in 1t cycle; above

average deep thermal gas. o Risks: trap efficiency; reservoir, source and seal development.

o Typical Reserves: <0.5- 5 billion bbl hydrocarbon/basin.

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Fig. 1 – Schematic illustration of peripheral foreland basins, retro-foreland basins, and basin related to subduction zone roll-back (Allen & Allen (2005))

Fig.2 – A typical foreland basin: The Permian basin of west Texas

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Fig. 3 – Foreland Basins in the Would

2. FORELAND BASIN SYSTEM

DeCelles & Giles (1996) provide a thorough definition of the foreland basin system. Foreland basin systems comprise three characteristic properties:

a) Orogenic Wedge: An elongate region of potential sediment accommodation that forms on continental crust between a contraction orogenic belt and the adjacent craton, mainly in response to geodynamic processes related to subduction and the resulting peripheral or retroarc fold-thrust belt;

b) Depozones: It consists of four discrete depozones, referred to as the wedge-top, foredeep, forebulge and back-bulge depozones (depositional zones) – which of these depozones a sediment particle occupies depends on its location at the time of deposition, rather than its ultimate geometric relationship with the thrust belt;

The wedge-top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge, including ‘piggyback’ and ‘thrust top’ basins. Wedge-top sediment tapers toward the hinterland and is characterized by extreme coarseness, numerous tectonic unconformities and progressive deformation.

The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the proximal flank of the forebulge. This sediment typically thickens rapidly toward the front of the thrust belt, where it joins the distal end of the wedge-top depozone.

The forebulge depozone is the broad region of potential flexural uplift between the foredeep and the back-bulge depozones.

The back-bulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential flexural subsidence cratonward of the forebulge.

This more inclusive definition of a foreland basin system is more realistic than the popular conception of a foreland basin, which generally ignores large masses of sediment derived from the thrust belt that accumulate on top of the orogenic wedge and cratonward of the forebulge.

c) The longitudinal dimension of the foreland basin system is roughly equal to the length of the fold-thrust belt, and does not include sediment that spills into remnant ocean basins or continental rifts (impactogens).

Fig. 4 – Foreland Basin system (DeCelles & Giles (1996))

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3. BASIN EVOLUTION3.1 Tectonic evolution

Foreland basins form because as the mountain belt grows, it exerts a significant mass on the Earth’s crust, which causes it to bend, or flex, downwards. The primary of foreland basin formation is moving tectonic load. Subsidence is differential and increases the closer to the Orogen, subsidence is not uniform along strike of fold and thrust belt. Variability of orogenic load is also control basin formation, it along strike tilts variability basin. Excepting for dynamic loading, all other types of supra- and sublithospheric subsidence mechanisms relate to the gravitational pull of ‘‘static’’ loads represented by the subducting slab, the orogen, or the sediment–water mixture that fills the foreland accommodation created by lithospheric flexural deflection (Fig. 5).

Fig. 5 – Proarc and retroarc foreland systems––tectonic setting and controls on accommodation (modified from Catuneanu et al., 1997a). Foreland systems form by the

flexural deflection of the lithosphere under a combination of supra- and sublithospheric loads.

The static tectonic load of the orogen and the sublithospheric dynamic loading are most often invoked as the primary subsidence mechanisms that control accommodation and sedimentation patterns in retroarc foreland settings. The static load of the sediment–water mixture is only of secondary importance in the formation of foreland basins, because accommodation by flexural deflection must be created first, before sediments can start to accumulate.

By Allen & Allen (2005), the plate tectonic scenario providing the context for the general evolution of foreland basin involves three stages (Fig. 6):

- Passive margin: the passive margin stage with orogenic loading of previously stretched continental margin during the early stages of convergence

- Convergent stage: inheritance of a passive margin, followed by an early convergent stage characterized by deep water conditions;

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- Convergent stage: convergent stage during which a subbaerial wedge is flanked with terrestrial or shallow marine foreland basin. Flexural foebulge unconformities are thought to be best developed during the early convergent stage, but may be buried beneath terrestrial sediments during the late convergent stage.

Fig.6 – Model involving orogenic loading of a previously stretched continental margin during the early stages of convergence (Stockmal et al. 1986; Watts 1992), modified by Allen et al.

(1991). The first orogenic loads are emplaced on a weaker lithosphere at considerable water depths.

The temperature underneath the orogen is much higher and weakens the lithosphere. Thus, the thrust belt is mobile and the foreland basin system becomes deformed over time. Syntectonic unconformities demonstrate simultaneous subsidence and tectonic activity.

3.2 Stratigraphy

In Catuneanu (2004), evolution phase of foreland basin divided into 3 phase, underfilled phase, filled phase, and overfilled phase. These phase determined fluctuation of relative sea level and flexural subsidence.

3.2.1 Underfilled Phase

Underfilled phase is initial phase of foreland basin infilling. Underfilled phase characterized by facies that has characterize of its rate subsidence. In basin which has rate of

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decline slowly, developing deep marine facies with flysch and agradation fluvial sequence and shallow marine. In basin which has rate of decline rapidly will develop submarine fan facies and agradation fluvial-delta sequence.

Fig. 7 – Ilustration Underfilled Foreland

Fig. 7A is initial phase development of foreland basin that form initial subsidence (dynamic subsidence<flexural uplift) on foredeep that form simultaneously with forebulge uplift. Then, Fig. 7B is final stage underfilled phase that occur when dynamic subsidence > flexural uplift, resulted basal unconformity from forebulge erosion (Catuneanu, 2004).

3.2.2 Filled Phase

This phase occur when the basin initially underfilled become to shallowing due to incoming sediment supply to the basin continuously, result of erosion from thrust belt lifted. This would causes the formation of stable shallow marine environmental in foreland basin, so that the stratigraphic record of this phase become shallow marine deposits and carbonate (forebulge, backbulge) and deep marine (foredeep).

Fig. 8 – Filled phase

3.2.3 Overfilled Phase

Overfilled phase is dominated by the formation of non-marine environment and reflect the stages of foreland basin evolution, when the sediment supply far exceeds the available accommodation space. This phase is the final phase in the development phase of the foreland basin. This phase is characterized by non-marine environment during the deposition zone and foreland basin molasses deposition.

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Fig. 9 – Overfilled phase. Depositional environmental on foreland basin is non-marine fluvial (Catuneanu, 2004).

3.3 Lithospheric behavior

Although the degree to which the lithosphere relaxes over time is still controversial, most geologists accept an elastic or visco-elastic rheology to describe the lithospheric deformation of the foreland basin. The composite lithospheric profile in Fig. 1 (average flexural profile in Fig. 2) changes through time in response to fluctuations in the amount of orogenic loading. Such orogenic cycles of thrusting (loading) and quiescence (erosional or extensional unloading) may operate over a wide range of time scales, both > and <1 My (Cloetingh, 1988; Cloetingh et al., 1985, 1989; Peper et al., 1992; Catuneanu et al., 1997b; Catuneanu and Sweet, 1999).

Fig. 10 – Flexural response to orogenic loading and unloading (modified from Catuneanu et al., 1997a). Renewed thrusting in the orogenic belt (loading) results in foredeep subsidence and forebulge uplift. The reverse occurs during stages of orogenic quiescence (erosional or

extensional unloading): the foredeep undergoes uplift as a result of isostatic rebound, compensated by subsidence of the forebulge.

For the loading model, the lithosphere is initially stiff, with the basin broad and shallow. Relaxation of the lithosphere allows subsidence near the thrust, narrowing of basin, forebulge toward thrust. During times of thrusting, the lithosphere is stiff and the forebulge broadens. The timing of the thrust deformation is opposite that of the relaxing of the

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lithosphere. The bending of the lithosphere under the orogenic load controls the drainage pattern of the foreland basin. The flexural tilting of the basin and the sediment supply from the orogen.

4. CHARACTERISTIC OF PETROLEUM SYSTEM OF FORELAND

BASIN

Petroleum system is the system that result and keep hydrocarbon. It’s divided in to 5 elements, they are source rock, reservoir, seal, trap and proper timing of migration.

Establishment and infilling of basin that occure during evolution controlled by tectonic and sedimentation. Tectonic and sedimentation are also affected the characteristic of petroleum system.

Source rock of petroleum system in foreland basin can be formed in early underfilled phase on wedge-top or foredeep is pelagic sedimentary rock with very fine material (flysh) and, the rock at back-bulge is deposit on fluvial and lacustrine environment. Whereas, the source rock can be formed at the end of underfilled phase of forebuldge deposited in deltaic-fluvial environment (Catuneuanu, 2004).

Reservoir of petroleum system in foreland basin can be formed in early underfilled phase on forebulge is aeolian deposit with unconformity, and also at late underfilled phase of wedge-top – foredeep is submarine fan deposit, on forebulge – backbulge is fluvial-deltaic deposit. Besides that, it can be formed on filled phase of forebuldge – backbuldge that formed carbonate platforms and ramps (Catuneuanu, 2004).

Seal of petroleum system in foreland basin can be formed on overfilled phase in non-marine environment like fluvial environment (Catuneuanu, 2004).

Hydrocarbon migration of foreland basin occure after kerogen in source rock is mature. The maturation of source rock in retroarc foreland basin is faster than peripheral foreland basin. It is caused by depleted of basement on retroarc-foreland basin is bigger than peripheral foreland basin.

The most common trap that can be found in foreland basin is thrust-fault. Thrust-fault can be formed by compression in collision zone. Besides that, the other trap which found is pinch-out. Pinch-out is mostly found on reservoir layers that wedge to slope from basement rock (Koesoemadinata, 1980).

5. CASE STUDY IN INDONESIA

In this case, there are two examples of petroleum system in foreland basin that resulted by collision in East Indonesia and have been proven oil and gas. They are Barito Basin and Banggai Basin.

5.1 Barito Basin

Barito basin is retroarc-foreland basin that resulted by collision of microcontinent southwest Borneo or Schwaner with Paternoster on late Jurassic until late Cretaceous. The result of this process also form Meratus Mountain as orogenic belt that reveal the outcrop of ophiolite. And then, the collision also form Barito Basin as retroarc foreland basin and Pasir-Asem Asem Basin as peripheral foreland basin (Satyana et.al, 2008).

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- Source rock Lower Tanjung Formation of Barito basin

deposited on early Eocene. It consist of carbonaceous shale, coarse sandstone, conglomerate sandstone and conglomerate. Lower Tanjung formation deposited on fluvial environment of underfilled phase of back-bulge part (Heryanto, 2010). Layer of carbonaceous shale is act as source rock of barito basin. Besides that, Lower Warukin formation deposited on Early Miocene, it consist of mudstone (dominated) the color is black greyish with intercalation of fine sandstone and coarse sandstone, the sedimentary structure are laminated and flaser. Lower Warukin Formation deposited in delta plain environment of late

underfilled phase of forebulge-backbulge part (Heryanto, 2010). Black mudstone of its formation is act as source rock.

- ReservoirIn barito basin, formation that act as reservoir are Middle Tanjung formation and Middle

Warukin Formation. Middle Tanjung formation consist of inserts blackish mudstone with coarse sandstone and coal. Sandstone of this formation is act as reservoir.

Middle Warukin Formation consist of black greyish mudstone inserts with fine-coarse sandstone, and the sedimentary structure are lamination of carbon and flaser. It’s deposited in deltaplain environment and rock that act as reservoir is fine-coarse sandstone of channel deposit. Both Middle Warukin and Middle Barito formation is form in underfilled phase of forebulge-back-bulge (Heryanto, 2010).

- SealSeal of barito basin are upper Tanjung formation and Lower Warukin Formation.

Upper Tanjung formation is deposited in deltaic environment on late Eocene, it consist of siltstone, fined-coarse sandstone wit sedimentary structure are wavy and flaser. Inserts of siltstone and fine sandstone is act as seal.

Lower Warukin formation is deposited on early Miocene on delta plain environment, it consist of black greyish mudstone inserts with fine-coarse sandstone, laminated carbon and its sedimentary structure is flaser. Mudstone of its formation is act as seal. Both upper Tanjung formation and Warukin formation that act as seal are formed in late underfilled phase of forebulge-backbulge (Heryanto, 2010).

- Proper timing of migration

Migration process in Barito basin happened after source rock (lower Tanjung formation) have been mature on middle Miocene and start to migration on late Miocene. Destination of its migration is coarse sandstone of middle Tanjung Formation that act as reservoir (Heryanto, 2010).

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Fig. 11 – Barito Basin (Red Circle)

Fig. 13 – Banggai Basin, eastern Sulawesi (purple color)


- Trap

Thrust fault in barito basin is act as trap (structural trap).

Fig. 12 – Migration and structural trap in Barito Basin.

5.2 Banggai Basin

Banggai basin is peripheral foreland basin from micro-continent collision of Banggai-Tukang Besi and Banggai-Sula which separated from the head of a bird (Papua) and move to the west by a Sorong-Fault with eastern Sulawesi ophiolite in the early Neogene. These collisions also formed basins in western Sulawesi such as Lariang Basin as retroarc foreland basin (Satyana et al., 2008).

- Source RockFormation Matindok on Banggai Basin sedimented on

Middle Miocene consisting of mudstone with sandstone inserts. Matindok Formation sedimented in shallow marine environments are also precipitate Tomori Formation below and

above Minahaki Formation on filled phase. The mudstone layer becomes the parent rock of Formation Matindok (PT PATRA NUSA DATA, 2006).- Reservoir

Anggota Mentawa of Minahaki Formation is carbonate reef act as reservoir. Minahaki formation deposited on shallow marine by filled phase above Matindok formation (source rock).

- SealShale of Matindok Formation becomes seal in Banggai basin. This formation deposited

on non-marine by overfilled phase.- Trap

In Banggai basin, the trap is carbonate reef and carbonate build-up (stratigraphic trap).

Fig. 14 – Migration and stratigraphic trap in Banggai Basin

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CONCLUSION

Foreland basin is one basin in a convergent plate boundary formed by collisions or subduction produce peripheral (collision) and retroarc (subduction) foreland basin. Deposition zone in foreland basin can divided to 4 zone, wedge top, foredeep, forebulge and backbulge. Infilling phase of foreland basin divided into 3 phase, underfilled, filled, and overfilled phase. From underfilled towards overfilled sea level rise, so that the environment of deposition of deep marine become increasingly shallow (fluvial).

Petroleum system that developed in Barito Basin (retroarc foreland) is in fluvial sediments, lacustrine until deltaic at Tanjung and Warukin Formation, and in Banggai Basin (peripheral foreland) is in shallow marine sediments at Matindok Formation and carbonate reef as the Mentawa member of Minahaki Formation.

REFERENCES

Allen, P. A., Allen, J. R, 2006, Basin Analysis – Principles and Applications, Blackwell Publishing, United Kingdom, p. 117-120, 256-257.

Catuneanu, O., 2004, Retroarc Foreland Systems – Evolution Through Time; Journal of African Earth Sciences, volume 38, issue 3, Elsevier Ltd., Oxford, United Kingdom, p. 225, 230-237.

Einsele, G., 1992, Sedimentary Basins – Evolution, Facies, and Sediment Budget, Springer-Verlag, p. 488-491.

DeCelles, P.G., and Giles, K.A., 1996, Foreland Basin Systems; Basin Research, volume 8, issue 2, Blackwell Science Ltd., Oxford, United Kingdom, p.105, 106, 108-114

Heryanto, R., 2010, Geologi Cekungan Barito, Badan Geologi Kementerian ESDM, Bandung, Indonesia, p. 51, 58-61, 79-81, 111-123

Koesoemadinata, R.P., 1980, Geologi Minyak- dan Gasbumi (2rd Ed.), Penerbit ITB, Bandung, Indonesia, p. 79, 80, 109-138, 173, 192-202

PATRA NUSA DATA, PT., 2006, Indonesia Basin Summaries (IBS), PT. PATRA NUSA DATA, Jakarta, Indonesia, p. 95-103, 126-136

Satyana, A.H., Armandita, C., Tarigan, R.L., 2008, Collision and Post-Collision Tectonics in Indonesia: Roles for Basin Formation and Petroleum System; Proceedings Indonesian Petroleum Association 32nd Annual Convention & Exhibition, Indonesian Petroleum Association, Jakarta, Indonesia, p. 1-12

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