Tim Charlton, Banda Arc Geologist

reports

 

The petroleum potential of Timor-Leste

Timor-Leste (East Timor) is situated in the eastern half of Timor island, located between eastern Indonesia and northern Australia. Timor-Leste has had little onshore hydrocarbon exploration since 1975 through the period of Indonesian annexation and the first years of independence, although active exploration was underway during the final years of Portuguese colonial rule up to 1975. Hydrocarbon prospects for Timor-Leste are widely considered to be only moderate due to perceived tectonic complexity, but in the present writer's opinion the territory has a much higher degree of prospectivity, both in the onshore area and immediately offshore. In particular, several large and structurally simple inversion anticlines are interpreted, with the potential to host giant hydrocarbon accumulations.

Hydrocarbon seeps and source rocks

Hydrocarbons have clearly been generated in Timor-Leste, as indicated by more than 60 oil and gas seeps now known (Figure 4). Upper Triassic to Lower Jurassic restricted marine source rocks, described as ‘world class’ sources for oil, are comparable to proven source rocks in the geologically similar and hydrocarbon productive island of Seram to the north of Timor. Upper Triassic bituminous shales in Timor locally contain up to 23% TOC.  

timor hc highlights

Figure 4: Petroleum highlights of Timor

Reservoir and seal

The primary reservoir targets are shallow marine siliciclastic successions of Upper Triassic-Middle Jurassic age (the 'Malita-equivalent' and 'Plover-equivalent' encountered subsurface in the Banli-1 exploration well in West Timor: Figure 5) (see also The Petroleum Potential of West Timor, Figure 8). A possible correlative of the 'Malita-equivalent' seen at outcrop in Timor-Leste is the Foura Sandstone (Figure 5; Charlton & Gandara, 2014). These potential reservoir sequences are likely to be sealed by thick shales of the Middle Jurassic Wai Luli Formation, and a newly identified Late Jurassic shale succession, the Tchinver Shale (Figure 5). The Tchinver Shale could also have source rock potential although this has not yet been tested geochemically.

 timor strat

  Figure 5: Timor stratigraphy and comparison to the Australian Northwest Shelf

Maturation

Prior to Neogene development of the Timor collision complex, most of the Mesozoic potential source sequences remained thermally immature due to a relatively thin Cretaceous-Tertiary sedimentary cover. Present-day thermal maturity is interpreted to result from structural burial within and beneath the Timor fold and thrust belt. Consequently, active migration from mature source sequences is likely to be taking place at the present day, after the development of the main target trap structures.

Structures

The most attractive structures for hydrocarbon exploration are inversion anticlines developed from Permo-Mesozoic grabens or half-grabens beneath complex Late Jurassic-Tertiary imbricate thrust stacks. One such inversion structure was intersected by the Banli-1 exploration well drilled by Amoseas Indonesia Inc. in West Timor (Petroleum Potential of West Timor, Figure 9).

In East Timor a comparable inversion structure is interpeted north of Betano (Figure 6). As with Banli-1, previous exploration wells in the Betano area (Betano-1 and -2) were located on a gravity high, interpreted here as marking the footwall block of the inversion structure. Two distinct anticlinal culminations are recognised on the inversion trend, situated respectively to the northwest and northeast of Betano village. The better-defined Northwest Betano structure is approximately 12km long, up to about 5km broad, with a potential vertical closure of several hundred metres. A number of small oil seeps are located around the periphery of the structure, potentially from the spillpoint of the anticlinal trap. Source rocks for the Northwest Betano structure are likely to be oil-prone, restricted marine sediments of Late Triassic to Early Jurassic age, deposited within a pre-inversion graben which developed during Permian-Triassic continental rifting. The reservoir, by analogy with Banli-1, potentially comprises Late Triassic-Middle Jurassic Malita/Plover equivalent shallow marine siliciclastics. The seal is shales of the Jurassic Wai Luli Formation, which also forms the basal décollement for complex near-surface geology. Significantly, pre-Middle Jurassic rocks have not been encountered in the Betano area, indicating that the deeper sequence has not been breached by either tectonism or erosion.

 nw betano x-sec 2

Figure 6: Cross-section through the Northwest Betano inversion structure 

Prospectivity by sub-area

Our fieldwork in Timor-Leste has identified six sub-areas with significant prospectivity (Figure 7). We have identified some 23 prospects and leads onshore, several of which are potentially very large, and most of which have associated oil and/or gas seeps.

study areas

Figure 7: Timor-Leste prospective areas

1. Suai

Several natural oil seeps are known from this area, including an abundant, high quality natural oil seep at Matai. This seep oil was used unrefined to run Timor Oil trucks.
The Suai Basin was the focus for Timor Oil exploration 1960-1972. 16 wells were drilled, 9 encountering hydrocarbons, 2 of which were subcommercial discoveries (Matai-1, Cota Taçi-1).
Timor Oil targeted the Neogene syn/postorogenic Suai Basin, but the best oil shows were found in the succession below the basin, including in the Eocene Dartollu Limestone and underlying metamorphic basement.

We have identified 6 exploration prospects/leads in the Suai area, all inversion anticlines in the sub-Suai Basin succession. Two structures are potentially large (potentially multi-hundred million barrel prospects).

2. Betano

Several small natural oil seeps occur in the Betano area.
Two wells were drilled by Timor Oil (Betano-1 & -2), the latter with minor oil shows.

We have identified 3 exploration prospects/leads in this area, 2 of which are potentially very large (see Figure 6 above).

3. Aliambata

This area was the primary focus for exploration in East Timor prior to World War 2. 'The beach ... is literally drenched with oil' (1925 field report).
5 wells have been drilled to significant depth (>100 feet) – all encountered substantial oil.

We have identified 3 substantial exploration prospects/leads in this area.

4. Pualaca-Sahem Valley

Pualaca has 2 prolific and high-quality oil seeps (up to 5 barrels per day artisan production), and a large gas seep. A further large oil seep is located further south in the Sahem valley at Fatu Lulik.
Pualaca and the Sahem valley form a non-inverted, lightly deformed Permo-Mesozoic mini-graben.

We recognise a potential subthrust inversion anticline and an updip pinchout trap within the axis of the Pualaca-Sahem graben.

5. Aituto-Bazol

A 50km-long inversion anticlinal trend in the interior of the island.
Numerous gas seeps (+1 oil seep) are associated with the Bazol-Aituto anticlinal trend.

We have remapped the Bazol Anticline and defined a significant prospect/lead with flanking gas seeps. The Aituto anticlinal trend has moderate to good but as-yet insufficiently defined exploration potential.

6. Lautem

Minor oil seeps have been reported but not yet confirmed from this area (possibly these are no longer active).

2 very large surface dome structures suggest the possibility of large subsurface inversion anticlines (high risk, potentially high reward structures – these will require seismic data to validate).

 7. Offshore Suai Basin

The one well drilled offshore Timor-Leste to the north of the Timor Trough thrust front (Mola-1) targeted updip pinchout in the synorogenic Suai Basin and an underlying thrust anticline on the southern flank of the basin. The well tested non-commercial gas in the sub-basin section (no reservoir).

From limited access to old seismic data, we have identified 2 large exploration leads in this area. One is a separate culmination on the anticlinal structure tested by the Mola well. The other is a nearshore inversion anticline located at the western end of a mini-graben (downdip connection to potential source rocks). We are confident that fuller access to modern seismic data in this area will permit the identification of further prospective structures.

 

© Tim Charlton
Wesite design in Guildford by: Sitec IT Ltd