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Fluvial 3-D Seismic Stratigraphy of Southern Gulf of Thailand: New Insight into Geomorphology and Associated Reservoir Response to Changes in Sea Level

Zhai, Rui 2; Pigott, John D.1; Pigott, Kulwadee L.1; Uttarathiyang, Tattiya 3
1 Schoolf of Geology and Geophysics, University of Oklahoam, Norman, OK.
2 China University of Petroleum, Beijing, China.
3 PTTEP, Bangkok, Thailand.

Analysis of reflection geometries and wavelet properties of Quaternary fluvial facies extracted from some 600km2 of offshore 3D seismic data from southern Gulf of Thailand reveal systematically evolving stratigraphic differences in the coastal plain fluvial geomorphologic elements and associated changes in reservoir-source-seal quality. We interpret the seismic facies of these paleo-meandering rivers to represent depositional-erosional responses to changes in accommodation space and coastal plain gradient forced by allocyclic eustatic changes in sea level but modified by autocyclic changes in fluvial discharge.Five regional Type I unconformities of the Late Quaternary can be recognized. They punctuate the intervening episodes of deposition which can be subdivided into accretion-erosion “fluvial parasequences” caused by changes in autocyclicity of the river system and staged falling or rising base levels. Reflection character and reveal these sequential parasequence set attributes for these paleo-rivers LST-TST-HST-RST-LST, etc: (1)Lowstand System Tract (LST): Downcutting of principal channel, with development of serrated subordinate tributary boundaries. Fluvial channels exhibit low sinuosity (1.1), low channel width (250-500m), and low width/thickness ribbon meanders (<1). (2)Transgressive System Tract (TST): Channels progressively widen (1.2-2.2km) and the serrated subordinate tributary boundaries become flooded. Good potential source and seal potential occurs for horizontal charging of adjacent accretion units. (3)Highstand System Tract (HST): Channels are widest and become increasingly filled with low velocity mud. If this portion of the coastal plain is inundated by the encroaching marine waters, the channels will become totally submerged. Poorest reservoir but greatest seal and source potential characterizes the plugged and abandoned channels.(4)Regressive System Tract (RST): Highest channel sinuosity (1.8) and largest width/thickness ratios. Stages of erosional downcutting into the preceding HST deposits develop with the formation of terraces of lateral accretion units which are seismically imageable as alternating oblique bands of amplitude polarity reversals in cross section and zebraic in plane section which generally exhibit high velocity acoustic amplitude contrasts. Best reservoir quality occurs.Placing coastal river deposits in a sequence stratigraphic framework affords significant insight into the petroleum system potential.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009