The Stacked-Channel Reservoir Sands of SE Asia (New Depositional Concepts and Implications for Reservoir Prediction)

Lambiase, Joseph ¹
(1)Petroleum Geoscience Program, Chulalongkorn University, Bangkok, Thailand.

A number of Late Tertiary SE Asian reservoir sandstones have been recognized as stacked channel sands, with their depositional environment variously interpreted as braided stream, meandering river or distributary depending on their specific sedimentary character and well log signature. These channel sands have been viewed as representing standard progradational fluvial units because they exhibit the fining-upward trends defined in traditional depositional models. However, recent studies of SE Asian outcrops and subsurface successions indicate that many fining-upward, stacked channel sands were deposited by aggradational back-filling of lowstand channels during a subsequent transgression, rather than progradation. Sands deposited near the paleo-shoreline often display clear evidence of tidal influence, including double mud drapes, flaser bedding and marine trace fossils; tide-generated sedimentary structures and trace fossils become more abundant and diverse upward in the succession, indicating progressively more marine influence with time. The aggradational, back-filling process was facilitated by the extremely high sediment supply rates that prevailed during the Late Tertiary which allowed sedimentation to keep pace with accommodation, thereby generating a transgressive, fining-upward succession that closely resembles a progradational fluvial deposit.

It is difficult to predict the reservoir volume and geometry of stacked channel sands deposited by aggradational back-filling when the reservoirs are sub-seismic, as they are in much of SE Asia. Estimating reservoir volume is not straightforward because the thickness of the sand unit is controlled by the rate of sand supply versus that of transgression and is not related to channel width, so there is no proportional relationship between thickness and channel width that can be used for volume estimation. Predicting reservoir geometry is equally problematic; the vertical facies and lithologic trend within a back-filled succession, and thus its well log signature, also are a product of the rates of sediment supply and subsidence so that a meandering river channel can be back-filled with a succession that closely resembles the straight channel of a braided system.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.