Geology and Petroleum Development in the Phitsanulok Basin, Central Plains of Thailand

The onshore Phitsanulok Basin situated at the junction between Uttaradit fault and Mae Ping fault is a Tertiary oil and gas province composed of a complex of half-grabens that were filled by lacustrine shales and continental clastics. The basin is formed by extensional rifting related to the collision of India with Asia. It is the largest and deepest basin in Central Plains of Thailand.

The presence of rich source rocks for oil, Early and Middle Miocene lacustrine shales, has made the Tertiary basins the main target of exploration during the past two decades, attracting a high level of operational activity. These sediments were interfingered and overlain by mouth bars, alluvial, and fluvial sediments. Oil and gas from the Early and Middle Miocene shales were trapped primarily in anticlines formed during the late Miocene.  The oldest sequence of the Phitsanulok Group consists of predominately low energy, alluvial plain and floodplain claystones. The coarse lithofacies comprises poorly sorted conglomeratic sandstones. This formation has been found hydrocarbon bearing in stratigraphic onlap traps surrounding paleo highs in the S1 concession. However, only one medium sized accumulation, the Sirikit Field, and several smaller deposits are on stream with approximately 18,000-19,000 bbl/d cumulative production. Over half of the exploration wells however did discover some hydrocarbons, mostly oil, and only  minority are dry holes without any shows.  This puts into question the hydrocarbon retention in the Tertiary basins or points to possibly invalid tests in the structurally complex traps.  Production from the Sirikit field commenced in 1982. The field has a STOIIP of 800 MMbbl. To date more than 250 wells have been drilled and over 170 MMbbls produced. Reservoir management studies into optimizing recovery and identifying unswept oil volumes such as waterflooding, primary infill and an exploration campaign are ongoing.  The challenge for any exploration and production company in this onshore Thailand’s basins will be to accurately delineate the small accumulations with 3D seismic and to maximize productivity through the use of gas lift, pumps and waterflooding. 3D static and dynamic modeling are also implemented to maximize the venture’s profitability. This has enabled the Sirikit field to show over 20 years a continuous growth in reserves and production rates. These are currently 19,000 bbl/d and have the potential to grow further. The application of similar techniques will increase the scope of the smaller, remaining prospects in the concession and in the other basins of the Central Plain of Thailand.   

Reference:

• Supamittra, C and Suryanto, D; Geology and Petroleum Development in the Phitsanulok Basin, Central Plains of Thailand; AOGS 2007 – 4^th Annual Meeting; Bangkok-Thailand