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Inversion of satellite-derived free-air gravity-anomaly data has been used to map crustal thickness and continental lithosphere thinning in the South China Sea. Using this, we determine the ocean–continent transition zone structure, the distal extent of continental crust, and the distribution of oceanic lithosphere and continental fragments in the South China Sea. We construct a set of regional crustal cross-sections, with Moho depth from gravity inversion, spanning the South China Sea from offshore China and Vietnam to offshore Malaysia, Brunei and the Philippines to examine variations in ocean–continent transition structure and ocean-basin width. Our analysis shows a highly asymmetrical conjugate margin structure. The Palawan margin shows a narrow transition from continental to oceanic crust. In contrast, the conjugate northern margin of the South China Sea shows a wide region of thinned continental crust and an isolated block of continental crust (the Macclesfield Bank) separated from the Chinese margin by a failed oceanic rift. The Dangerous Grounds are predicted to be underlain by fragmented blocks of thinned continental crust. We use maps of crustal thickness and continental lithosphere thinning from gravity inversion together with free-air gravity- and magnetic-anomaly data to identify structural trends and to show that rifting and the early seafloor-spreading axis had an ENE–WSW trend while the later seafloor-spreading axis had a NE–SW trend.

This conference addressed the question of whether fluvial-deltaic and coastal reservoirs deposited in tropical environments exhibit different facies relationships than those of temperate systems. Although this conference concluded that there is nothing distinctly unique about clastic depositional systems in tropical environments, the magnitude of the processes that operate in these environments differs from nontropical environments. Because these processes influence the distribution of reservoir and nonreservoir facies, geologists need a fundamental understanding of the contrasts between tropical and nontropical environments when interpreting subsurface data and building depositional models. We highlight the following features that seem to be more favorably developed in tropical environments. (1) Many tropical systems exhibit complexly stacked channel and overbank deposits. Although some fluvial deposits have the classic meandering pattern, seismic attributes indicate that many tropical channel belts are anastomosing systems that intersect each other at large angles. Subsurface correlation without the benefit of three-dimensional seismic data is all but impossible. (2) In contrast to deltaic deposits in temperate climates, which exhibit high sand/shale ratios near the delta with sand content decreasing basinward and laterally away from the delta, tropical deltas tend to be muddier near the river mouth and have cleaner, well-sorted sand bodies better developed laterally in the shoreface and shallow-marine environments. (3) Modern tropical coasts are home to large estuarine environments dominated by mangroves. Estuarine deposits interpreted in the subsurface and observed in outcrop are sand rich, yet little sand is observed in modern mangrove estuaries. Mangrove estuarine deposits also contain abundant organic-rich layers, thin coals, and dispersed organic matter that may favor the development of a self-sourced petroleum system. The flushing of these systems during relative low stands may be an important mechanism not only for reservoir development, but also for dispersing organic matter into deep-water systems. This conference was dominated by discussions of depositional systems in Southeast Asia, which have a strong tectonic overprint. Further research on tropical systems from the South American and African conjugate margin could provide important tests for the ideas highlighted above.

The West Baram Line separates NW Borneo’s two petroleum systems. Oligocene sandstone and Lower Miocene carbonate reservoirs of the gas-prone Luconia system lie SW of that line. Northeast of the West Baram Line, the oil-rich Baram-Balabac Basin produces from Middle Miocene to Early Pliocene sandstones deposited in a foreland basin on the western side of the mountainous interior of Borneo. On the present-day shelf exploration efforts spanning nearly four decades have focused on the Champion and Baram deltas and associated extensional growth fault structures. Many of these structures have experienced youthful inversion owing to ongoing tectonic shortening. Recent discoveries prove this petroleum system extends into deep water beyond the modern shelf edge where an active fold-thrust belt has formed above autochthonous rifted continental crust of the Dangerous Grounds province in the South China Sea. New regional mapping (c. 100 000 km2) integrating seismic, borehole and gravity data shows that the Baram-Balabac Basin is segmented into four structural domains whose boundary zones trend NW-SE similar to the strike of the West Baram Line. Domain boundaries appear to control the position of the basin’s palaeoshelf edges, turbidite depositional systems, major unconformities and the position of the basin’s largest fields. These observations suggest that the domain boundaries are the expression of deep structures, probably within the underlying rifted continental crust. Two of the domain boundaries can be projected onshore to align with fault systems separating contrasting geological elements indicating they represent features of tectonic interest. The onshore geology of NW Borneo represents the early history of the Baram-Balabac Basin. The onshore geology is reviewed and new outcrop, biostratigraphic and palaeomagnetic data from Sabah are discussed in light of current models for the region’s tectonic evolution. A hybrid model is proposed in which the Baram-Balabac Basin post-dates the Sarawak Orogeny. The Sarawak Orogeny, which appears to be more regionally extensive than previously believed, is attributed to Eocene to Early Oligocene collision of the Dangerous Grounds and Reed Bank with Sabah and Palawan. In the proposed model Oligo-Miocene subduction of oceanic crust under NW Borneo is minimal. The Sabah Orogeny and younger inversion events are related to underthrusting of the Dangerous Grounds driven by both the opening of the South China Sea and by NW-directed subduction beneath SE Sabah in the Semporna-Dent Peninsula. In this context the structural complexity of the Baram-Balabac foreland basin reflects the adjustment of deep crustal blocks to far-field tectonic stress.

The Ramu basin lies along a plate boundary where the Finisterre terrane is colliding with the Indo-Australian plate. Estimates for the age of initial collision range from early Miocene to middle Pliocene. Two unsuccessful wells (Keram 1 and Tsumba 1) drilled to basement and two-dimensional seismic data show that folded and faulted early to middle Miocene carbonates and clastics (the Wogamush sequence) are overlain by relatively undeformed Pliocene marine clastics (the Wewak sequence) along a regional unconformity. The pre-Pliocene section, which is at the crux of resolving the age of initial collision, has been correlated previously to the Finisterre terrane. Clastics within that section, derived from older terranes south of the basin, imply an early Miocene age for colli ion. I propose that Miocene sedimentary and volcanic rocks in the two wells are correlative with the Wogamush beds of the Maramuni arc. The Ramu basin can then be viewed as having a two-stage evolution. During the Miocene, the basin was part of the Maramuni arc, the polarity of which is unresolved. A collisional successor basin developed in the late Miocene as the Finisterre terrane (Adelbert block) collided with the arc. Thrust faults on the northeastern side of the basin, truncated by a regional unconformity, are interpreted to mark the suture of the Adelbert block. A northern earliest Pliocene sediment source for the basal Wewak sequence was probably the Finisterre terrane, but multiple source areas are inferred for the rest of that sequence. Middle Pliocene inversion of the basin's n rtheastern flank, characterized by reverse faulting and forced folding, is attributed to plate boundary reorganization caused by rifting in the Bismarck Sea. The Ramu basin has numerous untested structures related to both collision and basin inversion. Folded Miocene carbonates (Wogamush sequence) pose a considerable risk regarding the presence of reservoir, and the Wewak sequence is dominated by volcanolithic arkoses with poor reservoir quality on the northeastern side of the basin, where they are in structural closure. Gas-prone source rocks are present, but are largely immature. Reservoir and charge considerations place the Ramu basin in the very high risk sector for exploration.

The present-day North New Guinea basin is a Plio-Pleistocene successor basin that formed subsequent to accretion of the Finisterre volcanic arc to the Australian Plate. The Ramu, Sepik, and Piore infrabasins formed in a forearc setting relative to the continental Maramuni magmatic arc. The evolution of these infrabasins was strongly influenced by accretion of the composite Torricelli-Prince Alexander terrane to the Australian Plate. Regional reflection seismic data and tectonic subsidence-subsidence rate calculations for seven wells drilled in the North New Guinea basin reveal a complex history. The timing and magnitude of subsidence and changes in subsidence rates differ between each of the Miocene infrabasins. A diachronous middle to late Miocene unconformity generally truncates infrabasin sequences. The Nopan No. 1 in the Sepik basin, however, has a complete middle Miocene to Pleistocene sedimentary record. This well records late Miocene negative subsidence rates documenting that the Nopan anticline grew as erosion occurred elsewhere in the region. This circumstance suggests that the major, sequence-bounding unconformity results from regional uplift and deformation, rather than changes in global sea level. The Plio-Pleistocene evolution of the North New Guinea basin has two profound implications regarding hydrocarbon exploration. First, the late Pliocene structural inversion of parts ofmore » the basin hinders stratigraphic and facies correlation inferred from the present setting. The recognition of basin inversion is particularly important in the Piore basin for predicting the distribution of potential reservoir facies in the Miocene carbonates. Second, the subsidence data suggest that although potential source rocks may be thermally within the oil window, these rocks may not have had sufficient time to mature owing to their recent burial.« less