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A Depositional Model for Transgressive Fluvial Channel Sandstones in SE Asia

Abstract

Recent studies of a number of SE Asian Tertiary outcrops and subsurface reservoir successions indicate that many fining-upward, stacked channel sands interpreted as braided stream, meandering river or distributary successions were deposited by aggradational back-filling of lowstand channels during a subsequent transgression. The depositional mechanism is a progressive decrease in sediment transport capacity and competence as ongoing relative sea level rise continually reduces water surface slope, coupled with high sediment supply rates, that quickly fills accommodation with progressively finer, fluvially-derived sediment until the channel becomes inefficient and avulses. The sedimentary character of the channel fill successions varies considerably; some feature increasingly marine conditions upward from near their base while others have only minor marine influence near the top of the channel sand, although all become finer-grained and exhibit increasingly lower-energy sedimentary structures upward. Near the paleo-shoreline, distributaries were back-filled with a strongly marine succession that is similar to progradational tidal point bar successions except that marine influence increases upward. Further upstream, there was less marine influence, marine indicators first appear much higher in the succession and the stratigraphic column closely resembles fining-upward fluvial point bar successions deposited by meandering rivers. Proximal back-filled channel successions exhibit much less fining-upward and resemble braided stream deposits. They include relatively fine sandstones that look like sandy braided stream deposits except they have minor marine influence. They grade laterally upstream into coarse, often pebbly sandstones with minimal marine influence. Back-filled channel successions have been recognized from all across SE Asia; they range in age from early Miocene to early Pliocene and were deposited in syn-rift, transitional and post-rift tectonic settings. Their stratigraphic columns are similar to their progradational fluvial counterparts, but sand body geometries, connectivity and reservoir volumetrics are markedly different. Recognition can be difficult but is necessary for accurate reservoir characterization and sequence stratigraphic interpretation.