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ABSTRACT: The Phitsanulok, Lacustrine Basin, Onshore Thailand

RIGBY, S. M., A. A. BAL, H. M. BURGISSER, D. K. HARRIS, M. A. HERBER, S. THUMPRASERTWONG, and S. WINKLER, Thai Shell Exploration and Production, Bangkok, Thailand

The Phitsanulok basin is a north-south trending intracratonic rift that was formed during the Early Tertiary, by east-west extensional movements related to the collision of India with Asia. To the west the Phitsanulok basin is bounded by the Western Boundary fault, while the Eastern flank is densely faulted. Early rifting occurred during the Oligocene and Early Miocene and resulted in relative movement of basement faults producing local highs. A later compressional phase was accompanied by basic volcanism. Such composite tectonics resulted in complex trap geometries.

The Tertiary basin fill consists of a sedimentary wedge comprising dominantly of alluvial plain, alluvial fan and lacustrine deposits. Up to 8 km of Tertiary sediments were deposited in the Sukhothai Depression, the basin depocenter.

Lake Phitsanulok was a large (1000-4000 sq km) body of fresh water that provided the main environment for the accumulation of up to 400 m of organic-rich claystones. The lake margins accumulated coarser, clastic, deltaic deposits that constitute the main reservoirs of the Sirikit field. Soft lacustrine claystones provide both regional and intraformational seals.

The source rocks of the Phitsanulok basin were deposited in three separate environments: (1) lacustrine (oil prone source rock), which are the most important in terms of quantity and quality; (2) fluvial-lacustrine (marginal source rock for oil); (3) Marginal lacustrine swamp (gas prone source rock).

The Sukhothai Depression forms the main kitchen, and first generated oil at the beginning of the Middle Miocene. The resulting crudes are light (40 degrees API) and waxy with low sulfur and a high pour point.

 

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)