Reservoir geology of the Sirikit oilfield, Thailand: Lacustrine deltaic sedimentation in a Tertiary intermontane basin

The Sirikit oilfield, situated some 400 km N of Bangkok, Thailand, is a fault-bounded structure within a major half-graben basin, comprising a relatively undeformed eastern sector and a western area characterized by a complex pattern of isolated fault blocks. The 8 km thick basin-fill succession includes alluvial fan-fluvial deposits overlain by the fluvio-deltaic Lan Krabu Formation, which contains two main oil reservoirs. A reservoir geological model has been produced based on cores and wireline log suites from some 45 wells. The geology is characterized by cycles of regressive coarsening-upward sequences, up to 15 m thick, which reflect cyclic progradation of fluvial-dominated deltas into a large relatively shallow lake in a humid tropical climatic regime. The cycles are commonly topped by subaerial flood-plain claystones. Distributary channel sandstones show evidence of stage fluctuation and intermittent abandonment. Individual mouth-bar reservoir sandstones range in thickness from 1 to 5 m and form complex sheet-like bodies of lateral extent up to 20 km 2. Thus the Sirikit deltas have little inferred geometrical similarity with contemporary bar-finger models postulated for river-dominated lacustrine deltas. The sheet-delta geometry may be due to a combination of high frictional forces related to similar river and lake water densities, shallow lake depth and a large but variable fluvial sediment load. This combination of factors resulted in rapid dumping of sediment, the plugging of channel mouths and subsequent channel bifurcation. © 1989 The Geological Society.