Most of the oilfields in Thailand are producing from offshore and onshore Tertiary basins. The Sirikit Field is the largest onshore oilfield in the Phitsanulok basin, Central Part of Thailand. Thai Shell Exploration and Production Co., Ltd., used to produce oil from this field from December 1982-2004. Currently this field is operated by PTT Exploration and Production Public Co., Ltd., under S1 Asset. More than 470 wells have been drilled and over 170 MMbbls of oil produced. Reservoir management studies into optimizing recovery and identifying unswept oil volumes are ongoing through water-flooding and infill drilling activities. Exploration effort is still continuing in the vicinity of the production blocks. The challenge for any exploration and production company in the onshore basins will be to accurately delineate the small accumulations and maximizing recovery. These can be achieved by acquiring 3D seismic and improve oil recovery such as the use of gas lift, pumps and water- flooding. Oil is being produced mainly from the lacustrine sediments of the Yom, Pratu Tao and Lan Krabu Formations (Supamittra and Suryanto, 2007).