Tim Charlton, Banda Arc Geologist

reports

The petroleum potential of West Timor

The only petroleum exploration well drilled in Timor in the last 30 years is the Banli-1 well, drilled in Indonesian West Timor by Amoseas Indonesia Inc. in 1993/4. The well was drilled in the Kolbano structural block near to the south coast of the island (see Petroleum Potential of East Timor, Figure 4). The Kolbano block comprises a thin-skinned fold and thrust belt consisting of imbricated Australian continental margin strata, primarily Cretaceous and Tertiary in age, but with Upper Jurassic rocks exposed in a pseudoantiformal core (the Pasi Inlier). The well location was chosen primarily on the basis of gravity data, with the well sited on a large positive gravity anomaly (as mapped on gravity data available at that time).

The geology encountered by Banli-1 is summarised in Figure 8. The upper 900m of the well column consists of strongly deformed Cretaceous and Tertiary strata, with a complex imbricated thrust structure directly comparable to observed structures in the surface Kolbano foldbelt. Below this, in a structural transition zone consisting of rocks of earliest Cretaceous to Middle Jurassic age, structural disruption is much less intense than in the upper structural domain, and no duplications of stratigraphy have been identified, although bedding-parallel thrusts are interpreted, particularly in the Middle Jurassic shales of the Wai Luli Formation. Below the Wai Luli Formation the well intersected a remarkably simple succession of Middle Jurassic to Upper Triassic sandstones, equivalent to the Malita and Plover formations of the Australian Northwest Shelf. This lower structural domain dips uniformly to the SE, with dip values decreasing regularly downhole from 18°SE immediately below the Wai Luli shales, to 4°SE at T.D.

 

banli-1 section

Figure 8: Banli-1 (southern West Timor) well section

Banli-1 is interpreted to have drilled through the near-surface Kolbano thin-skinned fold and thrust belt, into a large subthrust inversion anticline (Figure 9). The downhole decrease in dip values in the lower structural domain is the pattern that would be expected from mild inversion of an original extensional half-graben basin where the graben-fill sediments had the characteristic downward-fanning of bedding dip. The syn-rift Malita and Plover-equivalents form potential reservoir sequences, whilst the overlying Wai Luli shales provide a potential seal. Stratigraphically deeper and/or laterally equivalent restricted marine shales similar to those exposed on the northern flank of the Kolbano block may have provided source rocks to charge the sub-Kolbano structure. New gravity data (Boz et al., 2014) suggests that the sub-Kolbano inversion anticline is a basement-cored structure, and the crest of the inversion anticline in the cover succession should be located just to the south of the Bouguer gravity maximum, several kilometres north of the Banli-1 well location (Figure 9).  

sub-kolbano x-sec 2

Figure 9: Kolbano (southern West Timor) cross-section

As approximated by the size of the Kolbano Bouguer gravity anomaly (Boz et al., 2014), the sub-Kolbano inversion anticline could be more than 20km long and 5-10km broad - comparable in dimensions to anticlines exposed at the surface in East Timor such as the Cribas and Aitutu anticlines. Vertical closure of the sub-Kolbano anticline above its intersection with Banli-1 may be as much as 300-500m. The sub-Kolbano inversion anticline is thus large enough to contain a giant hydrocarbon accumulation, but the Banli-1 well was located so far off the anticlinal crest that the structure is effectively untested.

Further prospectivity

In addition to the sub-Kolbano inversion anticline outlined above, I have identified three more potentially large inversion anticline exploration prospects/leads in onshore West Timor. One of these is a recently identified feature in the onshore part of the Eni-operated West Timor PSC, but the other two are in currently unlicensed areas of SW West Timor. All are located in areas of much less extreme topography than the sub-Kolbano target. Offshore to the south of West Timor I have identified three exploration leads: the South Bena structure (see the Banda Arc structure section), a further potential inversion anticline in the West Timor PSC, and another structure in an unlicensed area to the west. These exploration leads are described in detail in my report on the prospectivity of the Indonesian Banda Arc.

Further exploration potential is possible in SE West Timor. The Besikama Basin is a synorogenic basin similar to, and immediately across the border from the Suai Basin, the primary focus for exploration in Timor-Leste. This little studied area is known to have at least one oil seep and several active mud volcanoes.

 

 

© Tim Charlton
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