In 1979 two concessions in the Central Plains of Thailand were granted to Shell. Following magnetic and seismic surveys two exploration wells both of which found hydrocarbon indications were drilled in the northern concession. While the southern concession was relinquished further drilling led to the confirmation to an economic discovery in the Sirikit Field which was opened for production in early 1983. At the same time further exploration over the whole basin has continued with 16 wells to date.

The geometry of the Cainozoic Phitsanulok Basin is controlled by two fault zones-Uttaradit in the north and Mae Ping in the south. These faults probably have had long complex, tectonic histories. The Western Boundary Fault was formed as a response to extensional movement of the Taphan Hin crustal block moving northeast away from the Khao Luang block. Maximum Cainozoic fill adjacent to this fault is at least 8 km. Implied east-west extension is of the order of 15 km.

The basin fill consists of Oligocene to Lower Miocene alluvial clastics with local development of lacustrine facies which was followed by wide spread fluvio lacustrine sedimentation. During Middle Miocene time, major tectonic and/or climatic change resulted in the disappearance of the paleo-lake. Alluvial sedimentation with increasing clastic supply proceeded up to the end of Middle Miocene time when the regional stress regime changed and local inversion took place in the east and southeast of the basin. The resulting unconformity is followed by deposits of fluvial systems which are precursors of the present day systems.

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