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Controls on Fluvial Architecture During the Syn- to Post-Rift Transition in the Gulf of Thailand


Many of the important Gulf of Thailand reservoirs are fluvial sandstones within the early to late Miocene succession that includes the transition from syn-rift, fault-controlled sedimentation to post-rift, subsidence and sea level driven deposition. That transition was gradual and complex in the Gulf of Thailand; short-lived marine incursions began during the last stages of rifting while extensional faulting continued until well after rifting had ceased and in many areas post-rift subsidence included both a sag basin and a regional component. Consequently, the fluvial sandstones vary considerably with respect to channel size, orientation and sinuosity, making accurate reservoir characterization difficult as many of them are below seismic resolution. The stratigraphic architecture of the Miocene succession in the Songkhla Basin, a Tertiary rift basin in the southwestern part of the Gulf of Thailand, was investigated by integrating seismic geomorphology, well log data and biostratigraphic data. Horizon slices and computed stratal slices between the interpreted horizons were analyzed using RMS velocities, seismic inversion and spectral decomposition. The early Miocene fluvial succession has sinuous, 0.2 - 2.0 km wide NW-SE channel belts in the center of the basin. Channel belts became straighter and narrower (0.65 km) and changed orientation to NE-SW in the middle Miocene when the main depocenter shifted eastward after the main phase of rifting ceased, despite waning extensional faulting that persisted well into the late Miocene. Tidal creeks observed on seismic images supports biostratigraphic data from 4?? wells that indicates at least one period of marine influence, with the incursion coming from the northeast and suggesting extensive marine environments. Channel belt orientation changes to NW-SE again in the top middle Miocene where wide (1.9 km), sinuous channels dominate the post-rift succession. The general temporal variations indicate that tectonics was the main control on channel distribution, morphology and orientation and that relative sea level played a secondary role until at least the very latest Miocene. However, short-lived marine incursions and episodic faulting were important locally.