Petroleum Prospectivity Of The West Timor Trough Will Jones, Anand Tripathi, Rajesh Rajagopal and Adrian Williams, PGS Reservoir, 24 Oct 2011

The Western Timor Trough lies immediately south of the Island of Timor, within Indonesian territorial waters (Figure 1). It is a northeast to the southwest elongated basin which covers an area in excess of 40,000 km2. Exploration so far has been very limited with only four wells offshore and these are located on the southeastern edge of the trough near the boundary with Australia. The wells are all dry holes and this has led to the area being considered non-prospective.

Timor Island appears to represent an extension of the Australian continental shelf, perhaps originally analogous to the Exmouth plateau, which now forms a series of back thrusts to the southeast as a result of the collision of the Australian and Eurasian plates (O'Brien et al. 1999). The collision also caused buckling of the Australian plate in front of the thrust complex to produce the Timor Trough (Keep et al. 2002).

This review is based on a new 2D seismic survey consisting of 4300 km of data which ties to the four wells in the area (Napoleon-1, Manta-1, Mina-1 and Belalang-1) and also to wells in the adjacent Ashmore Platform and Sahul Syncline areas in Australian waters (Figure 2). The data were recorded in 2010 with a dual sensor and have a typical record length of 11 s. The line spacing averages around 15 km and line length ranges from 9 to 25 km. Three lines were Beam depth migrated, resulting in improved imaging, particularly in the accretionary wedge. Interpretation of this seismic data has given rise to a more optimistic picture, in particular that there is a source kitchen under the axis of the trough which is capable of charging untested structures to the southeast.

Regional geology

The stratigraphy underlying the Timor Trough has not been tested directly by drilling but can be inferred from the wells on the Ashmore Platform, Vulcan Graben and Laminaria High in adjacent Australian waters and also from outcrop and well data on Timor Island, in particular well Banli-1 (Charlton 2006). The stratigraphy in the two areas is generally similar although the sandier Permian to Cretaceous section in the Bonaparte Basin gives way to a more distal carbonate and clay rich succession on Timor (Figure 3).

The oldest Australian continental margin sequence unit intersected in the Indonesian West Timor area is the Late Permian Hyland Bay Formation (Napoleon-1). Undifferentiated Sahul Group sediments overlie this unit over most of the West Timor region but interbedded sandstones and shales of the Osprey Formation (a turbidite sequence) onlap Mount Goodwin Formation shales and siltstones on the eastern margin of the Ashmore Platform.

Jurassic sediments are missing in wells on the Ashmore Platform and also in the West Timor wells Napoleon-1 and Manta-1. This is partly due to erosion during the late Triassic Fitzroy Movement, during the onset of sea floor spreading that produced the Callovian Unconformity, and during the Valanginian. However, the Ashmore Platform was a high area during the Jurassic so that the Jurassic may not have been deposited in this area (Cadman and Temple 2003).

The Late Cretaceous and Cenozoic succession deposited on the Ashmore Platform is typically a platform carbonate sequence interrupted by unconformities resulting from fluctuating sea levels.

The onshore West Timor stratigraphy summarised in Figure 3 is deduced from outcrop geology and the Banli-1 well. Banli-1 encountered Cretaceous and Palaeogene deepwater limestones and radiolarian shales. These were tectonically emplaced above a volcaniclastic unit (lowermost Cretaceous?), shallow marine siliciclastics (Upper Jurassic) and the upper sandy facies of the Wai Luli Formation (probably upper Middle Jurassic). The deepest formation penetrated in Banli-1 is the fluvio-marine Late Triassic Malita-equivalent sandstone but the oldest rocks exposed on Timor Island are red crinoidal limestones of the Permo- Triassic Maubisse Formation (Charlton 2006).

Structural interpretation

Seismic data quality differs greatly between the two sides of the Timor Trough. To the southeast it is very good with horizons down to the upper part of the Permian at around 8 s being well imaged. However, these deeper horizons rapidly disappear under the accretionary prism to the northwest on the Timor side of the Trough. Here little can be discerned on the seismic data below the top 2-3 s, although Beam depth migration has recently produced significant improvements.

Depth conversion of the interpreted time horizons was carried out using stacking velocities generated during the data processing. It is accepted that the depths calculated will be a little too large, particularly in the deeper areas, but a good tie was found with the top Permian at the Napoleon-1 well, giving confidence in the method used.

The area southeast of the axis of the Timor Trough may be divided into three provinces:

The northeastern area consists of an eastwest dome with the top Permian lying at 3000-7000 m depth. In this area, the Permian

and Triassic sections show a clear pattern of normal faulting which can be mapped at top Permian and top Triassic levels with moderate confidence, given the line spacing of the available data. The faults cut through the whole Permian and Triassic section producing a horst and graben pattern (Figures 4 and 5). The horsts plunge at either end, producing a series of elongated closures which individually trend southwest to northeast but are arranged en echelon so as to be aligned eastwest as a group. The fault blocks are often tilted and eroded at top Triassic level, equivalent here to the Callovian unconformity, producing a marked angular unconformity over much of the area. The faults are generally continuous between the top Permian and top Triassic so that structures mapped at these events are valid for horizons throughout the entire Triassic succession. Faults at top Triassic are more subdued than at top Permian because of erosion at the higher horizon. The horsts narrow upwards, so areas of closure within the Triassic are smaller than those mapped at top Permian. Some of the faults continue with a much reduced throw through

the overlying section which consists of a thin Jurassic and Cretaceous section capped by thick Cenozoic carbonates.

The southwestern end of the area is a Triassic depocentre where the Permian descends to a maximum depth of perhaps 13,000 m. Here the Triassic section is faulted into a succession of tilted fault blocks with a few horsts. These faults appear not to affect the Permian surface but rather sole out just above it (Figures 6 and 7). This might indicate that the lower Triassic here consists of incompetent shale, perhaps equivalent to the Locker Shale/Mount Goodwin Formation, and may have source potential.

The third province is a graben underlying the deepest part of the Timor Trough, called here for convenience the West Timor Graben, in which the top Permian sinks to a maximum of perhaps as

much as 18,000 m (Figure 8). This is aligned with the coast of West Timor along a more easterly trend than the Permo-Triassic faults. It is inferred that a Jurassic section is present in this graben which could contain both mature source rocks and reservoir rocks.

Northwest of a line approximately along the deepest part of the Western Timor Trough, the geology is very different from the southeastern side. The Permian and Triassic succession almost disappears abruptly, although there are hints that the bedding is beginning to rise towards the northwest. The most significant difference is the appearance of a fold and thrust belt, where

anticlines are constrained between southeast verging thrust faults and northwest verging antithetic thrusts faults (Figure 9). Northwest of this is a zone as much as 3 s thick which appears to consist of stacked imbricate thrusts verging to the southeast. This is followed towards Timor Island by a zone of listric normal faulting. The imbricate thrusts and listric faults appear to rest on a detachment surface and may represent gravity sliding off the uplifted Timor block. Recent Beam migration processing has revealed reflectors dipping to the northwest beneath the detachment. This can be interpreted as one of the anticipated major thrust slices of Australian continental crustal rocks which from the backbone of Timor (Figure 9).

Banli-1 appears to have penetrated the detachment. The upper part of the well was drilled through a highly thrusted succession of Cretaceous and Cenozoic carbonates with dips from zero to more than 50°. At about 900 m depth the well crossed a major fault below which there was a succession of mudstones and sandstones of Middle Jurassic to late Triassic age within which the dip angles decreased from 18° at the top to 4° at the bottom (Sani et al. 2006). Banli-1 well is close to the end of one of the dual-sensor seismic lines and the detachment surface can be correlated between the two (Figure 10).

Hydrocarbon potential Only five wells have been drilled on the Indonesian side of West Timor, but several have been drilled at the southeastern end of the Laminaria High where it abuts the Sahul Syncline and Nancar Trough (parts of the Bonaparte Basin) (Figure 1). There, key discoveries such as Corallina, Buffalo, Bluff and Buller occur in the Middle to Late Jurassic succession which is buried to depths of 3 to 4.5 km. Further to the west, on the Ashmore

Platform proper, a few unsuccessful wells were drilled to test the Plover Formation and/ or Triassic fault blocks, either sealed by Early Cretaceous mudstones and shales (where present), or by Late Cretaceous and Cenozoic carbonates. Of the wells in Indonesian waters, only Napoleon-1 tested the Permian, the others bottoming in the Triassic.

East Timor is well-endowed with oil and gas seeps and has 26 wells, including two small oil fields, whereas West Timor has only one well, Banli-1, with minor oil shows and several gas bearing mud volcanoes (Figure 11). There are also areas of gas hydrate, identified by a bottom simulating reflector, along the deeper parts and northwestern side of the Timor Trough.

Geochemical studies of the oil seeps and potential source rocks on Timor Island indicate that there are Upper Triassic and Jurassic source rocks with both gas and oil potential, comparable with the Jurassic source rocks of the North West Shelf. Late Triassic and Jurassic source rocks with TOC up to 23% and hydrogen index up to 396 have been reported (Charlton 2006).

The proven Late Jurassic source rocks of the Bonaparte Basin are generally either absent or thin and shallowly buried (immature) across the Ashmore Platform. The Jurassic is also absent in West Timor wells Napoleon-1 and Manta-1. However, the new seismic data indicates that thicker Jurassic strata appear to be present in the northeast of the offshore West Timor area, particularly in the Timor Graben (Figure 8).

Temperature measurements in Napoleon 1 indicate a temperature gradient of 36 ºC per km for this well. Applying this gradient to the depth-from-seabed map for the top of the Triassic gives a present day temperature map for the base of the Jurassic interval (Figure 11). Assuming onset of maturity for gas at 120 ºC and overmaturity at 200 ºC, the Jurassic would be mature along the edge of the West Timor graben. From here, gas would be able to migrate into the more westerly of the Permo- Triassic horst blocks. On the other hand it would encounter a more tortuous migration pathway as it moved perpendicular to the major structural axis into the more easterly horst blocks. The projected temperatures at top Permian indicate maturity for gas generation in the lower part of the Triassic over much of the crest of the dome, except for the highest parts. Source rocks higher in the Triassic would be mature along the margins of the graben in the northwest and around the depocentre in the southwest.

Vitrinite reflectance data is available in the Napoleon-1 and Manta-1 wells and is in broad agreement with the predictions from the temperature gradient. In Napoleon-1 the top of the Triassic is immature (Ro = 0.4) whereas the lower Triassic and Permian are early to mid/late mature for oil (Ro = 0.72-0.81). At Manta-1 the lower Triassic is up to early mature for oil (Ro = 0.34-0.48). Extrapolation of vitrinite reflectance profiles to surface for both Napoleon-1 and Manta-1 wells confirms that the section is at its maximum depth of burial

at the present day. Spore colour index analysis also suggests similar maturity outcomes (Robertson 2000).

The primary reservoir sequence in the Western Timor Trough is expected to be Late Triassic- Early to Middle Jurassic 'Malita-equivalent' and `Plover-equivalent' sandstones. (Charlton 2006). The deltaic and marginal marine Plover Formation contains excellent reservoir quality sandstones in the nearby Laminaria and Corallina Fields. Another potential Jurassic reservoir unit likely to be present in this area is the Elang Formation which hosts a number of commercial discoveries such as Bayu-Undan, Corallina, Elang, Kakatua and Laminaria fields.

Potential secondary reservoir targets also include widespread shelf carbonates and localised sandstones in the Permian and Triassic. Tertiary marine carbonate build-ups and patch reefs are also developed in the shallow section. However, the reservoir characteristics of the carbonates, especially those of the Permian, are likely to be highly variable and may be tight and recrystallised such as encountered in Napoleon-1 and Sahul Shoals-1. Nevertheless, in Kelp Deep-1, the Permian Hyland Bay Formation flowed gas at a rate of 339,800 m3/day (Cadman and Temple 2003). On the other hand Triassic sandstones frequently average up to 20% porosity (e.g. Ashmore Reef-1, North Hibernia-1, Sahul Shoals-1 and Napoleon-1).

Permian reservoirs will most likely be sealed by a thick, regionally distributed, basal Triassic claystone. The top Triassic and Jurassic

reservoirs are likely to be sealed by Jurassic- Early Cretaceous claystones (where present) or by overlying carbonates. On Timor Island the Jurassic Wai Luli Formation shale is overpressured in the Banli-1 well (Sani et al. 1995), demonstrating its sealing capacity.

Play Types

The main plays in the area comprise fluviodeltaic reservoir sands in Triassic and Jurassic horst blocks charged with gas from Permian, Triassic or Jurassic source rocks. Secondary plays

include carbonates and sands in Permian horst blocks, and Tertiary carbonate build-ups. The best source kitchen is believed to be located in the northwest of the study area, although there is potential for mature source rocks to be distributed along almost the entire length of the Timor Trough. Bottom simulating reflectors indicate a potential gas hydrate play in the northeast of the area (Figure 10).

The Upper Permian provides widespread structural targets in horsts and tilted fault blocks beneath a Lower Triassic sealing shale section. Although larger displacement faults at this level may appear to cut well up into the Triassic section or even to surface, there is a high probability of shale against shale juxtaposition through the basal Triassic section. Most likely charge would be gas from the Permian or Triassic section where it is favourably located downdip. Note that there is also potential for deposition of distal turbidite sandstone reservoirs within the Permian at some locations. The main risk with the Permian play is reservoir quality, as Permian limestones are known to be relatively tight at a number of locations where they have been penetrated elsewhere in the Bonaparte Basin.

Triassic fault blocks are ubiquitous throughout the area and have the potential to trap large volumes of gas in multiple stacked "Malita Equivalent" sandstone reservoirs. This is a commercially well-established, high potential play on the Australian side of the Timor Trough. Seal will depend on shales within the Lower Jurassic section or within the lower part of the Cretaceous section. The most likely charge would be gas from the underlying or downdip

Triassic section. The main risks with this play are cross-fault leakage of individual sands, and leakage through reactivation of fault movement post-entrapment.

Where fluvio-deltaic to shallow marine deposits of the Lower to Middle Jurassic "Plover Equivalent" section overlie the Triassic deposits, there is potential for additional reserves, not only within fault blocks but also within stratigraphic or combined structural/ stratigraphic traps on the flanks of the main high trends.

Seals would then be provided by Upper Jurassic marine claystones or Lower Cretaceous deepwater radiolarian shales, and charge is expected to come from laterally equivalent marine shales deposited contemporaneously with the Plover Formation further out in the basin.

During the Tertiary there was widespread carbonate deposition throughout the region, primarily in inner and outer slope environments and, as a result, there is potential for development of carbonate build-ups on highs in shallow water, and also for carbonate banks and other related features in more distal settings. The main risks with these plays, however, are their lack of overburden (seal) and access to adequate charge.

Within the accretionary prism on the Timor side of the Trough, there are two possible trapping mechanisms: The detachment surface at the base of the imbricate thrust and listric fault zone could provide a regional seal to underlying reservoirs, particularly if the Lower

  • Middle Jurassic shales form an overpressured detachment zone as appears to be the case at Banli-1 (Charlton 2006).

The main play along the accretionary prism is therefore likely to be Jurassic sandstone reservoirs incorporated in large relatively undeformed thrust blocks sealed by Jurassic shales of the detachment zone. Several closures on the detachment surface have been identified but in the absence of clear seismic imaging below this level, it is difficult to map individual leads. A quite separate play is represented by the anticlines within the fold and thrust belt itself (Figure 9). The lithology involved in the anticlines appears to be Cenozoic carbonates which might have significant fracture porosity as a result of its strong folding and faulting.

Conclusion

Discoveries and shows onshore Timor Island confirm a working petroleum system with one or more sources providing a mixed oil and gas potential in the Permian and Triassic. Analogy

with the nearby Vulcan Sub-basin also suggests a high probability of source potential in the Jurassic in the offshore West Timor Trough. In particular the West Timor Graben underlying the axis of the trough may contain a Triassic and Jurassic kitchen capable of charging traps on the dip slope to the southeast.

The main risks for finding economic volumes of hydrocarbons in the Trough are likely to be charge issues and reservoir quality (particularly for Permian carbonate reservoirs) plus, to a lesser extent, trap configuration where accurate fault-plane mapping, timing of movement and cross-fault juxtaposition will prove to be significant factors.

Interpretation of the new dual sensor seismic data has identified a cluster of fault-dependent closures in horsts and tilted fault blocks on the crest of a dome occupying about half of the area of the Western Timor Trough. These could give rise to stacked traps throughout the Upper Permian and Triassic section. Further traps are possible in draping and onlapping Jurassic strata.

A quite distinct play is the trapping of hydrocarbons within the accretionary prism, particularly beneath the detachment. Closures have been mapped on the southeast side of the trough at Top Permian, Top Challis Formation, Top Triassic and Top Jurassic. In the accretionary prism they have been mapped on the detachment surface and on a Miocene marker.

References

Cadman, S.J. and Temple, P.R., 2003. Bonaparte Basin. Geoscience Australia, Australian Petroleum Accumulations Report 5, 2nd edition.

Charlton, T. R., 2006. The petroleum potential of West Timor. In: Proceedings, Indonesian Petroleum Association, Twenty Eighth Annual Convention & Exhibition 2001, 1, 301-317.

Edwards, D.S., Preston, J.C., Kennard, J.M., Boreham, C.J., van Aarssen, B.G.K., Summons, R.E. and Zumberge, J.E., 2004. Geochemical characteristics of hydrocarbons from the Vulcan Sub-basin, western Bonaparte Basin, Australia. In: Ellis, G.K., Baillie, P.W. and Munson, T.J., (editors), Timor Sea Petroleum Geoscience, Proceedings of the Timor Sea Symposium, Darwin, 19-20 June 2003. Northern Territory Geological Survey, Special Publication 1, 169-201.

Keep, M., Clough, M. and Langhi, L., 2002. Neogene tectonic and structural evolution of the Timor Sea region, NW Australia. In: Keep, M. and Moss, S. J. (ed) The Sedimentary Basins of Western Australia 3, 341-353.

O'Brien, G.W., Morse, M., Wilson, D., Quaife, P., Colwell, J., Higgins, R. and Foster, C.B., 1999. Margin-scale, basement-involved compartmentalisation of Australia's North West Shelf: a primary control on basin-scale rift, depositional and reactivation histories. In: The APPEA Journal, 39 Part 1, 40-63.

Robertson P.T. Utama Indonesia, 2000. Manta-1 Well, Timor Sea - A Petroleum Geochemical Evaluation of the Interval 2180 m to 3445 m and X-Ray Diffraction Analysis of two Selected Samples. Report No. 1504.

Sani, K., Jacobson, M.I. and Sigit, R., 2006. The thin-skinned thrust structures of Timor. In: Proceedings, Indonesian Petroleum Association, Twenty Fourth Annual Convention 1995, 277-293. 

Comments

William Jones of PGS Reservoir - Perth posted at 9:03 PM Monday, November 14th, 2011
We are grateful to Andrew Murray for his comments on our West Timor article. However there are a few points that need addressing. The comment about Manta-1 is fair enough but here we are quoting directly from Robertson (2000). The top Permian marker is at more than 10 km from the seafloor in the Triassic depocentre in the southwest and also along the West Timor Graben. This leaves plenty of room for the early Triassic to be at an appropriate depth for maturity within the offshore West Timor area. It is true that a linear temperature gradient based on one bottom hole temperature can only be an approximation to the truth. However the top Triassic is at more than 6000m depth along much of the West Timor Graben, which should be enough depth for maturity at any plausible temperature gradient. Offshore West Timor is indeed a frontier basin. However the hydrocarbon seepages on Timor Island indicate that there is a working petroleum system in the region. quote image
Andrew Murray of Perth posted at 12:06 PM Thursday, November 3rd, 2011
An interesting article but in the section on "Hydrocarbon Potential" we read: "At Manta-1 the lower Triassic is up to early mature for oil (Ro = 0.34-0.48)" . Well..no not really. Even by the most generous intepretation the earliest onset of oil generation is accepted to be 0.6% Ro. This is what the textbooks say but in reality values about 0.7 % are needed for commercial volumes, except for some exceptional sulphur rich source rocks. There are other issues to do with the linear extrapolation of temperature gradient from seabed to source rock depth to get a temperature map (temperature gradients are never linear - they shouldn't be) but those can be forgiven in view of the general uncertainties applying in a frontier basin. Over-optimistic identification of "oil kitchens" just causes confusion in the long-run.quote image
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