Transgression-Driven Avulsion of Fluvial Channels
Petroleum Geoscience Program, Chulalongkorn University, Bangkok, Thailand.
Recent studies of subsurface fluvial systems on the Sunda Shelf indicate that back-filling of fluvial channels during transgressive events generates sand-filled channels that have excellent reservoir potential and are an important component of the overall stratigraphic architecture. The back-filling mechanism appears to be progressively decreasing water surface slope caused by tidal intrusion that migrates landward during transgression, thereby continually decreasing sediment transport competence and capacity, which causes deposition in the channel near the upstream limit of tidal influence. The channel will avulse if transgression and back-filling continue to decrease the water surface slope until it falls below some threshold value, which is the same process that causes avulsion in progradational systems.
Cored successions of back-filled fluvial channel sands in the Gulf of Thailand gradually fine upward into tidal flat sediments and generate bell-shaped well log signatures, as is expected if transgression and back-filling continue until the channel is filled completely. However, other back-filled fluvial channel sands display a rapid transition into significantly finer tidal sediments; these have the blocky log signatures that are traditionally interpreted as braided stream deposits. The abrupt vertical lithofacies change may reflect channel avulsion when the fluvial sand supply is switched off suddenly and replaced by lower energy tidal sedimentation.
Water surface slope is decreased tens of km upstream by even a modest tidal range so that much of the effected channel reach is well upstream of any direct indicators of tidal influence and most, and at times all, of the back-filled sediment is strictly fluvial. Consequently, some completely fluvial successions with abrupt vertical lithofacies changes and blocky well log signatures may be the product of channel avulsion during transgression. Misidentification of transgressive, back-filled meandering channel sands as regressive, braided stream deposits has profound implications for sequence stratigraphic analysis and prediction of reservoir architecture.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012