INTRODUCTION Neogene delta systems around Borneo contain numerous world-class hydrocarbon accumulations, with original in-place resources greater than 10 billion barrels of oil and 55 trillion cubic feet of gas. Exploration over the past ten years has been extended into the deep-water domain with significant discoveries, particularly in distal parts of the Mahakam and Baram Deltas where several active petroleum systems have been recognised within geological settings where there is a complex interplay between tectonics and sedimentation. In this paper we compare and contrast the structural development of the Mahakam Delta and West Sulawesi Fold Belt in the North Makassar Basin and the Baram delta system in the Northwest Borneo Basin. GEOLOGICAL DEVELOPMENT The study area lies within a complex tectonic region where three major plates -the Indo-Australian, Philippine- Pacific and Eurasia -interact and collide. Various aspects of the tectonic evolution have been discussed in numerous publications. Broadly speaking, there are three critical events in the Cenozoic history. The first is a Paleogene event, extensional in the Makassar Strait and largely compressional in Northwest Borneo, the second is a Neogene compressional event which resulted in the uplift and probable rotation of Borneo and subsequent significant increase in sediment production, while the third is the series of collisions which resulted in development of the West Sulawesi Fold Belt during the Pliocene. These events are shown schematically as Figure 2. BARAM DELTA - The Northwest Borneo Basin lies between the South China Sea in the northwest, the Sundaland Shield in the south and the Sulu Sea and Celebes Sea regions in the east (Darman and Darnit, 2003). The basin developed in response to rifting, sea-floor spreading, and subsequent strike-slip faulting during the Ter.tiary (Hall, 1996). The Neogene Baram Delta is a mud-rich wave-dominated system which prograded over the active convergent margin of Northwest Borneo. The delta province has evolved over a broad active deformation zone resulting from the complex interaction between deltaic growth faulting and basement tectonics (Sandal, 1996). In gross geometry, the delta is characterised by (i) an inner zone characterised by down-to-basin (growth) faults, and (ii) an outer zone of folding and associated thrust faults (Fig. 2). Within the inner, growth fault-dominated zone, two syn- depositional fault types are recognised: down-to-basin growth faults and landward-dipping counter-regional faults. The disposition of counter-regional faults strongly influences sedimentation patterns (Darman and Damit, 2003). Large faulted anticlines are possibly related to the inversion of older faults while other structural highs are cored by shale diapers whose location is related to basement structures (Sandal, 1996). The outer, deepwater fold and toe-thrust zone is largely generated by gravity -a deep decollement probably exists near basement but has not been imaged on the seismic available to us. Folding continues to the present day with the sea-floor topography reflecting underlying structure. Turbidites have accumulated within swales formed behind hanging-wall anticline ridges (Fig. 3). MAHAKAM DELTA In gross geometry, the Mahakam Delta is similar to the Baram Delta, characterised by an inner dominated by growth faults and an outer zone of folding and associated thrust faults. A schematic geoseismic section across the middle and outer parts of the delta system is shown as Figure 4. Three structural provinces are recognised in the delta system, the boundaries of which are postulated basement transfer zones developed during Late Eocene to Oligocene sag phase (Guritno et al., 2003). Higher uplift in the northern part of the basin generated a gravity-driven extension-toe-thrust system. The fault movement was probably initiated in the Middle Miocene and continues to the present day (Guritno et al., 2003; see also Fraser et al., 2003, Fig. 8). WEST SULAWESI FOLD BELT The West Sulawesi Fold Belt of western Sulawesi and its adjacent offshore area (Fig. I; Fraser et ai., 2003) occurs immediately west of the Palu-Koro Fault, a major crustal lineament, initially set up during the Eocene by spreading within the Celebes Sea. During the Plio-Pleistocene, prior extensional settings in the Makassar Strait became compressional as successive Australoid fragments collided with the southeastern comer of Sundaland. This recent collision not only assembled the island of Sulawesi into its current (ephemeral) K-shape but also formed the West Sulawesi Foldbelt. The foldbelt is progressively consuming the Makassar Straits, and eventual closure will cause major changes to the oceanic circulation of cool water flowing from the Northern Pacific into the warmer Indian Ocean. Near the western edge of the fold belt, compressional folds occur over a thin-skinned detachment within probably over-pressured Late Eocene or Miocene mudrocks (Fig. 5). DISCUSSION AND CONCLUSIONS In essence, structuring is similar in the Baram and Mahakam delta systems, albeit with a much stronger influence of older basement structures in the Baram system. Structuring in the Baram system is primarily controlled gravitational and sedimentary loading with strong influence of older basement-related structures. Sand/shale ratios and the degree of undercompaction playa decisive role in the deformation style, together with the degree of decoupling from the underlying basement (Sandal, 1996). In neither system is there any appreciable structuring directly related to tectonic events external to the delta system. West Sulawesi Fold Belt, however, is the direct result of transpression and collision, resulting from movements on the strike-slip fault system, which have (temporarily) created the island of Sulawesi. REFERENCES DARMAN, H. AND DAMlT, A.R., 2003. Structural control on sediment distribution in offshore Brunei Darussalam, South China Sea. Proceedings Indonesian Petroleum Association 29th Annual Convention, Jakarta, October 14-16, 2003, 347-353. FRASER, T.H., JACKSON, B.A., BARBER, P.M., BAILLIE, P. AND MYERs, K.,2003. The West Sulawesi Fold Belt and Other New Plays within the North Makassar Straits -a Prospectivity Review. Proceedings Indonesian Petroleum Association 29th Annual Convention, Jakarta, October 14-16,2003,431-450. GRAVES, J .E. AND SWAUGER, 1997. Petroleum Systems of the Sandakan Basin, Phillipines. In: Howes, J. V.C. and Noble, R.A. (Eds.), Petroleum Systems of SE Asia and Australiasia. Indonesian Petroleum Association, 799-813. GURITNO, E., SALVADORI, L., SYAIFUL, M., BUSONO, I., MORTIMER, A., HAKIM, F.B., DUNHAM, J., DECKER, J. AND ALGAR, S., 2003. Deep-Water Kutei Basin: A New Petroleum Province. Proceedings Indonesian Petroleum Association 29th Annual Convention, Jakarta, October 14-16,2003, 519-540. HALL, R. 1996. Reconstructing Cenozoic SE Asia. In: Hall, R. and Blundell, D. (Eds.), Tectonic evolution of SE Asia. Geological Society of London Special Publication 106. HALL, R., 2002. Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations. Journal of Asian Earth Sciences, 20, 353-431. Moss, S.J., CLARK, W., BAllLIE, P. W., HERMANTORO, A.E. AND OEMAR, S., 2000. Tectono-stratigraphic evolution of the North Makassar Basin, Indonesia. AAPG Conference, Bali (Abstract). SANDAL, T. (ED.), 1996. The Geology and Hydrocarbon Resources of Negara Brunei Darussalam. Brunei Shell Petroleum Co. Sdn. Bhd., and Brunei Museum.