The Banda Arc: Geology and Petroleum Potential
Tim Charlton
Consultant regional geologist
charlton@manson.demon.co.uk
The Banda Arc: Geology and Petroleum Potential | ||||||||||
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The Petroleum Potential of East Timor An oil industry consultancy study by Tim Charlton (see Reports available) Newly independent Timor-Leste (East Timor) is situated in the eastern half of Timor island, located between eastern Indonesia and northern Australia. East Timor has had little onshore hydrocarbon exploration during the last thirty years or so through the period of Indonesian annexation, although active exploration was underway during the final years of Portuguese colonial rule up to 1975. Hydrocarbon prospects for East Timor are widely considered to be only moderate due to perceived tectonic complexity, but in this study (see reports available) a much higher degree of prospectivity is inferred. 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 East Timor, as indicated by more than thirty documented oil and gas seeps (Figure 5). 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. Geochemical studies also suggest similarities between Jurassic source rock sequences in Timor and those on the adjacent (and hydrocarbon productive) Australian Northwest Shelf.
Reservoir and seal The primary reservoir target is an unnamed shallow marine siliciclastic succession of Upper Triassic-Middle Jurassic age encountered subsurface in the Banli-1 exploration well in West Timor (Figure 6) (see also The Petroleum Potential of West Timor, Figure 8). Equivalent lithologies on the Northwest Shelf (the Malita and Plover formations, and lateral equivalents) form reservoir rocks in the Jabiru, Challis and Skua fields amongst others. These potential reservoir sequences are likely to be sealed by shales of the Middle Jurassic Wai Luli Formation.
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 recognised north of Betano (Figure 7). 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 likely vertical closure of several hundred metres. A number of small oil seeps are located over the interpreted 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.
Further targets A number of additional inversion anticlines are identified in East Timor, e.g. in the Aliambata, Iliomar, Suai and Suete areas. A second, related type of potential structural trap is an extensional rollover anticline at Pualaca formed during Permo-Mesozoic extension, but not inverted during Neogene collision. The Pualaca structure is associated with voluminous surface oil seeps. A further potential structural trap type that at least warrants further investigation is large domal culminations above northward-directed late stage backthrusts, some of which are associated with oil seeps. Structural and stratigraphic traps (sedimentary pinchout, rollover anticlines) are potentially also present in Plio-Quaternary syn- to post-orogenic basins along the south coast of East Timor, but any hydrocarbons trapped within these young basins are likely to be sourced from the underlying Mesozoic succession rather than from the Plio-Quaternary basinal sequences. | |||||||||
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