A review and technical summary of the AAPG Hedberg Research Conference “Variations in fluvial-deltaic and coastal reservoirs deposited in tropical environments”

Abstract

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.