(and implications for deltaic reservoirs in SE Asia)

by Joseph J Lambiase.

Traditional delta models maintain that: 1) shoreline and sand body geometry and facies distribution reflect dominant hydrodynamic processes, 2) retrogradational stratigraphic successions are rarely, if ever, preserved, 3) reservoir quality is best in progradational, topset sands, and 4) reservoir-degrading bioturbation is less on deltas than on open marine coastlines because reduced salinities inhibit burrowing organisms. However, on three modern Borneo deltas (Baram, Mahakam and Trusan), and in their outcropping and subsurface successions: 1) facies distribution reflects dominant hydrodynamic processes but shoreline and sand body geometry do not, 2) retrogradational successions dominate the preserved stratigraphy, 3) topset beds have significant tidal influence and do not form high-quality reservoirs, and 4) bioturbation is greater on the deltas relative to adjacent marine coastlines.

The disparity between the models and the observations is caused by the setting of the Borneo deltas in rapidly subsiding basins with very high sedimentation rates, low energy marine environments and a tropical climate, which is markedly different than the temperate climate, stable open ocean coastline with moderate sediment supply setting that is implicit in standard models. Retrogradational successions are generated because high sediment supply rates allow deposition, rather than drowning, during transgression. Low energy marine environments cannot rework delta morphology so that shoreline and sand body geometry reflect relic hydrodynamic conditions and differ significantly from facies distributions that maintain equilibrium with hydrodynamics. Low wave energy allows significant tidal influence despite low tidal ranges, which generates relatively heterogeneous and laterally discontinuous topset sands. Also, abundant organic matter in muddy river waters promotes bioturbation that mixes mud into shoreline sands, thereby reducing reservoir quality.

The application of standard delta models to deltas with a setting similar to the Borneo examples leads to inaccurate prediction of sand body geometry and reservoir quality. Many of the Tertiary deltas of SE Asia are in this category; accurate assessment of their reservoirs can be achieved only by applying a model that integrates the appropriate depositional, hydrodynamic and climatic environments.