Geological Synopsis,

Offshore Timor-Leste

Geir K. Ytreland, August 2005

 

The geology offshore Timor-Leste is a continuation of the Australian Northwest Shelf (Australian Plate) geology, however, the island of Timor constitutes the collision front with the Banda Arc. As such the Permian-Tertiary succession continues relatively undisturbed from the NW Shelf, through the JPDA, under the deepwater Timor Trench until some point close to the southern shores of the island of Timor; where the stratigraphy becomes severely folded, upthrusted, faulted and tectonised. Another distinguishing feature is the presence of a thick (up to 3 km) succession of Pliocene/Pleistocene sedimentary sequence overlying the previously described Permian-Tertiary between the deepest part of the Timor Trench and the island. This Pliocene/Pleistocene sedimentary wedge or prism represents the erosional material derived from the Timorese hinterland/source during the uplift the last 6 MM years. This wedge consists mainly of shale and mudstones (as seen in Mola-1 well), and the presence of this presumably impermeable cap rock may have significant consequences for the exploration on the north side of Timor Trench, see discussion below.

 

In general; at least four play models can be envisaged for the offshore Timor-Leste areas now being opened for exploration:

 

  1. Middle Jurassic clastic reservoirs, sourced by Mesozoic gas- and oil prone marine and marginal marine source rocks (as seen in Bayu-Undan, Sunrise and other fields on the Timor-Leste part of the NW Shelf). Trap styles are mainly structural and numerous: tilted fault blocks, anticlines, horsts etc. However, possible sand pinch-outs and other stratigraphic trapping mechanisms are also observed on seismic
  2. Upper Jurassic/Cretaceous erosional fans (reservoirs) derived from nearby highs, indications of such can be seen on the northern shoulder of the NW Shelf /southern slope of the Timor Trench, e.g. north of Sunrise
  3. Intra-shale reservoirs (fans, turbidites, conturites, reefs) within the Pliocene on the northern side of Timor Trench. Gas was encountered in Mola-1 around 2600 m depth, however, no significant reservoirs were found associated with this gas in the well. The source of this thermogenic gas is speculative, but nevertheless constitutes a potential play model for this particular area
  4. Permian carbonate reservoirs; only one well has penetrated the pre-Triassic sequence in the general area: Kelp Deep, which indeed encountered gas in tight Permian limestones. This Permian limestone horizon is widely recognized on seismic in the area, on both sides of Timor Trench, and it is possible that carbonate reservoirs with sufficient porosity may be found in the areas now being opened for exploration

 

All oil/condensate and gas fields in the JPDA (NW Shelf) have lost the bulk of its liquid (oil) leg due to preferential leakage of oil, mainly through faults, as the gas cap has expanded and displaced the liquids below it. This is today clearly visible by the thick residual oil legs seen in the main reservoirs in the fields (Bayu-Undan etc), and also by the well defined leakage features seen along current faults and large pockmarks seen on the seabed today (easily observed on the current 2D seismic offered in this round). It is very likely that this leakage has taken place over the last 0-6 MM years, with the activation of faults during the Pliocene, and this process is still active.

There is reason to speculate however, that in the area between the axis of the Timor Trench and the island of Timor; i.e. the area overlain by the above mentioned Pliocene/Pleistocene sedimentary wedge (roughly half the area being offered in this round), this substantial leakage may not have taken place. This is because the presence of a thick shale/carbonate sequence may have acted as a very efficient seal, preserving the liquid hydrocarbons trapped in the Mesozoic (and ?Permian) section below. Features can be seen on seismic in this area, including direct hydrocarbon indicators (DHI’s), shallow trapped gas, and most noticeably; the well-known oil seeps onshore Timor-Leste which contain light oils, and which most likely have been displaced from deep reservoirs located to the south. Consequently, there is a significant and realistic potential to make large oil discoveries, in Mesozoic reservoirs, in traps (structural or otherwise) located in the area between the deepest part of the Timor Trench and all the way up to the southern shores of  Timor-Leste. The current 2D seismic suffers reduced quality in this area due to the presence of the thick shale succession overlain by carbonates (at seabed). However, indications of potential traps can clearly be seen on the current seismic, and 3 potential plays/prospects are noted on the present data. It is expected that future exploration efforts by the industry, and properly designed (3D) seismic surveys, may identify a number of drillable structures in this area, where a combination of compressive and extensional tectonic forces has the potential of generating a number of structural plays, in addition to possible stratigraphic ones.

 

Most of the oil from the active onshore seeps is light, 25-38 degree API, very low sulphur and sourced from the Mesozoic.

 

In summary, the acreage opening up for exploration can offer very low geological risk, is surrounded by oil and gas discoveries on all four sides, can demonstrate known commercially proven petroleum systems and world-class gas/condensate fields in the JPDA and the potential for significant oil accumulations having been preserved to the north of the Timor Trench. A number of play models can be envisaged, and more than twenty plays/prospects have already been identified on the current seismic, in a wide variety of water depths.