Searching for the Missing Link: The Regressive System Tract - Seismic Stratigraphic Evidence from the Southern Gulf of Thailand

摘要

High resolution seismic stratigraphic and geomorphic analysis of some 600 km2 of offshore 3D seismic data from the southern Gulf of Thailand reveal the preservation of a complete eustatic cycle on a Quaternary tropical coastal plain that is periodically affected by oscillating sea levels. While the dominantly aggradational interfluvial coastal plain records only the classic deltaic Parasequence Set signatures (PSS) of the Highstand Systems Tract (HST), Lowstand Systems Tract (LST), and Transgressive Systems Tract (TST) stages capped by regionally pervasive ravinement surfaces, the erosionally incised intrafluvial rivers on the plain record an additional PSS, which is defined as the Regressive Systems Tract (RST). The RST is represented by downward stepping fluvial terraces in response to downward stepping base levels. Placing coastal river deposits within a sequence stratigraphic framework and the recognition of the RST afford significant insight into both the dynamic response of rivers to eustatic changes and to their petroleum system potential. The four complete PSS observed within the intrafluvial alluvial plain and their major characteristics are: 1. HST: Highest base level where channels are widest, have the lowest gradients, and carry the finest sediment load. Such reservoirs would have poorest sand quality and seal potential. 2. RST: Falling base level where channels tend to have lower width/thickness ratios and lower sinuoisities. Stages of erosional downcutting into the preceding HST deposits develop with the formation of terraces of lateral accretion units which can be seismically imaged as alternating oblique bands of amplitude polarity reversals in cross section and zebraic in plane section. Good reservoir sand quality and vertical but laterally disconnected geometries occur. 3. LST: Lowest base level with the occurrence of maximum downcutting of principal channels and the development of serrated subordinate tributaries. Fluvial channels exhibit low sinuosities with potentially good reservoir continuity, sand quality, and charging potential. 4. TST: Rising base level in which channels progressively widen and the subordinate tributary boundaries flood, yielding less favorable reservoirs but with potentially good source and seal potential. KRegressive Systems Tract;;3D Seismic;;Fluvial Geomorphology;;Tropical Coastal Meandering River;;Pleistocene;;Gulf Of Thailand;;Sunda Shelf