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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 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.

Tectonic subsidence analysis of borestratigraphic package is immature, consisholes north of the Bewani-Torricelli-Prince tent with observed low present-day geother Alexander Mountain (BTPA) range of Northern ma1 gradients. Present day oi l seeps in Papua New Guinea reveals rapid post-Eocene Northern New Guinea are anomalous when tectonic subsidence punctuated by two discompared to these computations. We suggest t inc t episodes of tec tonic upl i f t . These two hypotheses: Either the oil was generatpulses resulted from near-field changes in ed as a result of local (igneous) thermal strain produced by segmented evolution of events, or as our seismic data suggests, the the sinistral Northern New Guinea wrench oil migrated along conduits associated with fault system. Periods of slow tectonic subm a j o r t h r u s t f a u l t s y s t e m s f r o m d e e p e r sidence are marked by carbonate-dominated buried source rocks to the south. sediment deposition whereas episodes of greatest tectonic subsidence are predomina t e ly s i l i c i c l a s t i c . Our outcrop-to-borehole reconnaissance demonstrates that these sequences are primarily fine-grained, protected shelf-to-slope facies with uncommon biolithites, grainstones, and coarse-grained s i l i c i c l a s t i c s .

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

Basalts containing as much as 50% plagioclase phenocrysts that attain lengths of several centimeters are common on the northern islands of the Galapagos Archipelago. Mineralogic and geochemical evidence requires that the large anorthitic phenocrysts nucleated and grew from a less evolved magma than that in which they were erupted. We suggest that plagioclase-ultraphyric basalts (PUBs) form when a buoyant phenocryst-laden magma enters a chamber containing a denser, more evolved magma. As the new magma mixes with the old, anorthitic plagioclase phenocrysts may accumulate near the roof in a hybrid liquid. Thinner crust under the northern islands increases the probability that viscous plagioclase-rich magmas can ascend to shallow magma chambers and may account for the fact that PUBs are more common on the northern islands, than in the rest of the Archipelago.