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Hydrocarbons in the Timor Sea

By Myra Keep, structural geologist, University of Western Australia

Presented at the International Energy and Mineral Resources Conference
5 7 March 2003, Dili, Timor Leste.

Click here for a PDF file with additional maps and diagrams of Timor Sea geology that accompanies this paper (Slide references in the text refer to this file).

Introduction

The Timor Sea is an informal name given to a section of the northern Bonaparte Basin that covers the Zone of Cooperation and adjoining areas (Figure 1, Slides 2-3).  The area is divided according to the various physiographic features, including sub-basins, shelf areas and troughs.  Of importance to this talk are three areas, referred to as the Vulcan Sub-basin, the Sahul/Flamingo/Nancar area and the Greater Kelp-Sunrise High and Malita Graben ((Figure 1) (Longley et al., 2002).  These three areas contain the majority of hydrocarbon discoveries in the region (Figure 2). Bordering the Timor Sea to the north is the Timor Trough, a deep topographic low, which marks the edge of the Banda Orogen (Figure 1), a deformational belt which lies exclusively within East Timorese and Indonesian waters.

Figure 1: Satellite bathymetry map of the Timor Sea region, showing the location of major subdivisions and main wells
(modified from Longley et al., 2002). Click the map to see it larger in a separate window.

Figure 2: Satellite bathymetry with superimposed map of the main oil and gas fields
(modified from Longley et al., 2002). Click the map to see it larger in a separate window.

Important discoveries in the Timor Sea include: Scott Reef (1971), Sunrise-Troubadour (1974), Brecknock (1979), Jabiru (1983), Evans Shoal (1988) and Laminaria (1994).  All except Jabiru and Laminaria are gas/condensate discoveries. (Slide 6) Between the Brecknock (1979) and Laminaria (1994) discoveries 160 new wells were drilled, for a return of only 2.5 million barrels of oil equivalent (Kopsen, 2002).

Within the Timor Sea, the main petroleum traps are simple drape anticlines over underlying horsts and tilted fault blocks at the level of the regional base seal.  Ninety-seven (97)% of all hydrocarbons discovered to date have been found at the base regional seal levels (Longley et al., 2002). Only three fields (Maitland, Swan and Puffin) have been found above the regional seal.  Most of the oil is reservoired in Late Jurassic and Early Cretaceous rocks, and most of the gas is reservoired within Middle Jurassic to Triassic sandstones (Kopsen, 2002; Longley et al., 2002).

Petroleum charge varies throughout the region but in general the entire North West Shelf lacks a world class source rock, and in many areas the lack of an effective charge is due to the absence of an effective source rock (Longley et al., 2002).  However there is also a view that hydrocarbon exploration has been directed towards Triassic and younger sediments, and that Permo-Carboniferous sediments may constitute a hydrocarbon objective, especially in inshore areas (Kopsen, 2002).

For the Timor Sea, there is oil potential in the Jurassic sections of the Vulcan and Sahul-Flamingo-Nancar areas and the Vulcan Sub-basin.  There is also significant potential within the Cretaceous shales of the Vulcan and Sahul-Flamingo-Nancar areas.

Petroleum Geology

Vulcan Sub-basin (Slide 7)

At the end of 2001 this area contained 18 field discoveries and 67 exploration wells.  The historical technical success rate for the area is 27% for all discoveries, going down to 13% for fields larger than 20 million barrels of oil equivalent.  The sub-basin is thought to contain 1.3 trillion cubic feet of gas, 31 million barrels of condensate and 357 million barrels of oil (Figure 3).  Presently 170 million barrels of oil (48%), 31 million barrels of condensate (100%) and 1.2 trillion cubic feet of gas (92%) remain undeveloped.  The main fields include Jabiru, Challis, Skua, Oliver, Montara, Bilyara, Maret and Puffin, which have multiple sources (Longley et al., 2002).

Figure 3: Summary of estimated  oil, condensate and gas reserves for the Timor Sea
(modified from Longley et al., 2002).   Click the map to see it larger in a separate window.

Sahul, Flamingo and Nancar area (Slide 8)

Currently the area contains 19 field discoveries and 74 exploration wells.  The historical technical success rate is 26% for all discoveries, and 7% for fields larger than 20 million barrels of oil equivalent.  The area is though to contain 3.4 trillion cubic feet of gas, 233 million barrels of condensate and 337 million barrels of oil.  Presently 227 million barrels of oil (69%), 4 million barrels of condensate (less than 2%) and 50 billion cubic feet of gas (approximately 1%) remain undeveloped.  The area contains numerous significant oil pools on structural highs, with several levels of source rocks.  The main fields include Laminaria, Corallina, Bayu Undan, Buffalo and Jahal (Longley et al, 2002).

Greater Kelp-Sunrise High and Malita Graben (Slide 9)

Currently the area contains 7 field discoveries and 16 exploration wells.  The historical technical success rate is 43%, dropping to 31% for fields greater than 20 million barrels of oil equivalent.  The area is thought to contain 20.2 trillion cubic feet of gas, 379 million barrels of condensate and 0 million barrels of oil.  Presently 54 million barrels of condensate (14%) and 10.7 trillion cubic feet of gas (53%) remain undeveloped.  The main fields include Sunrise-Troubadour, Evans Shoal, Lynedoch and Chuditch (Longley et al., 2002).

The total for the region (Slides 10 & 11) therefore has

bullet44 field discoveries
bullet157 explorations wells
bullet24.9 trillion cubic feet of gas (11.9 undeveloped)
bullet643 million barrels of condensate (89 undeveloped)
bullet694 million barrels of oil (397 undeveloped).

Opportunities

Deep water (Slide 12)

There are still large unexplored areas of the Timor Sea that have potential for undiscovered gas, with associated condensate.  Most of the large fields to date have been discovered using 2D seismic data.  Given the poor source rock, the probability for large oil fields is low (Figure 4).

Figure 4: Main petroleum systems of the Timor Sea
(modified from Longley et al., 2002).  Click the map to see it larger in a separate window.

Shallow water (Slide 13)

Potential hydrocarbon occurrences exist in shallower water closer to Timor Leste.  Geologically this inshore region has a similar geological setting to other known petroleum systems in Buru, Seram and Buton.  The East Nies and Bula fields of Seram may represent the types of potential accumulations on Timor Leste.  These small breached fields produce modest amounts of oil.  Given the complex geological setting of these islands, seismic imaging through the fold and thrust belts at the surface may prove problematic, as it has in Seram.  Offshore areas of Timor Leste, within the Viqueque and Suai basins appear to have similar geology, including large-scale shale diapers which breach the surface (Crostella, 1975).

Onshore (Slide 14)

Other opportunities for Timor Leste occur onshore.  Exposed rocks in Timor Leste are the only place the North West Shelf can be seen at the surface, and so it would be a popular destination for geological field trips for oil company personnel from Australia and around the world. 

Education (Slide 15)

Opportunities also exist in strengthening ties with Universities in strategic areas.  The geology of Timor Leste is relatively unknown in the modern era, and much remains to be learned and understood from studying onshore and offshore data.  Research into the rocks and especially the structures on Timor Leste will contribute not only to exploration for oil and gas but also to the general understanding of these types of geological environments.  Many opportunities exist for research and training for students from Timor Leste to study abroad, and for foreign researchers to visit Timor Leste.

References

Crostella, A., 1975, Geology and hydrocarbons of Timor, IPA.

Kopsen, E., Historical perspectives of hydrocarbon volumes in the Westralian Superbasin where are the next billion barrels? In Keep, M. & Moss, S.J., (Eds), The Sedimentary Basins of Western Australia 3, Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, 2002, 3-14.

Longley, I. Et al., 2002, The North West Shelf of Australia: A Woodside perspective, in In Keep, M. & Moss, S.J., (Eds), The Sedimentary Basins of Western Australia 3, Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, 2002, 27-88.

horizontal rule

Myra Keep is a structural geologist, currently based at the University of Western Australia.  She completed a BSc (Hons) at the University of London (1987), an MSc at the University of British Columbia in Canada (1989), and a PhD at Southern Methodist University in Dallas, Texas (1994).  Since 1994 she has worked for Mobil Exploration in Dallas as a seismic interpreter, worked as a researcher for the Fault Dynamics Group at the University of London, and was appointed the Mobil Lecturer in Structural Geology at the University of Aberdeen, before emigrating to Australia in 1996.  She is currently a Senior Lecturer in Structural Geology and Basin Tectonics at UWA, where she teaches structural geology and engineering geology. 

Her research focus is the structural and tectonic evolution of the North West Shelf of Australia, with particular focus on the Timor Sea.  All of the research is sponsored by oil and gas companies, and involves predominantly seismic interpretation of active structural systems.  She heads a small research group at UWA, examining petroleum systems around the North West Shelf.  She has held positions with the Petroleum Exploration Society of Australia WA Branch, and most recently was Co-Editor of the book The Sedimentary Basins of West Australia 3.  She is a Chartered Geologist, Fellow of the Geological Society and a member of the AAPG.

The Timor-Leste Institute for Development Monitoring and Analysis (Lao Hamutuk)
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