Petroleum provinces are geographically separated into Northern and Central Plains, Northeastern, Andaman Sea, Southern and Gulf of Thailand. Mostly Tertiary in age, main producing reservoirs are distributed in various parts of the country, both onshore and offshore. Located in the Northeast, Pre-Tertiary basins are made up of sedimentary rocks dating from Carboniferous to Tertiary. Commercial hydrocarbon prospects have been found here, one of which is producing from Permian strata. New plays are awaiting discovery by new conceptual explorers.
Northeastern Thailand, or the so-called ‘E-San’ or ‘Khorat Plateau’, covers about 200,000 sq. km., or one third of the whole country. The topography consists of large rolling plains with mountain ranges to the west, south and the middle of the region. Thick sedimentary sequences consist of Khorat redbeds overlying Pre-Khorat Triassic, Permian strata and older rocks. Exploration data indicate that Permian platform carbonates are significant primary reservoirs, followed by Pre-Khorat Triassic fluvio-lacustrine clastics. Both Triassic and Permian shales are potential source rocks.
Since 1962, seismic surveys have been conducted and gas wells have been drilled. Nam Phong, Phu Horm and Dong Mun prospects are significant gas discoveries in Permian Carbonates. Underbalanced drilling, an advanced technique, was used to minimize formation damage and increase the productivity of one of the significant prospects in this region. The gas market is opening up for new discoveries mainly for power generation and other purposes.
The Khorat Plateau still contains significant Permian (reef, carbonate build-up, buried hill)/Triassic plays and prospects not yet fully explored and are awaiting further investigation. High-potential prospects lie in the vicinity of the Nam Phong and Phu Horm gas fields, including Phu Khieo Prospect. The economies of Southeast Asian countries are growing, characterized by constantly rising energy consumption. These high-potential blocks are still available during this 19th bid round.
Most Tertiary basins in Northern Thailand are intermontane basins that are generally small and narrow, but relatively deep and high in heat flow. Believed to form in a series of pull-apart basins, these basins are located in N-S trending half-grabens. Oil seeps were found over a century ago in Fang Basin in the North. In the midst of interest in petroleum utilization in late 1970s, Fang Oil Field became the first in Thailand. Significant amounts of oil currently come from it.
In the Central Plains, most basins are also generally characterized as N-S trending half-grabens formed by conjugate transtensional shear during early Tertiary age. Many oil fields have been discovered in Phitsanulok, Suphan Buri, Kamphaeng Saen, and Phetchabun Basins. On stream since 1981, Sirikit Oil Field, the first major commercial oil field, lies in Phitsanulok Basin. This field marked a golden era for hydrocarbon exploration in Thailand, which continues until today.
The main reservoirs in these areas are oligocene fluvio-lacustine sandstones, sealed and sourced by shales of similar age. Rollover and tilted fault blocks created traps found in these areas. However, Pre-Tertiary reservoirs (or buried hills) cannot be ruled out. While several blocks have been awarded to oil companies, many interesting blocks remain available during this bid round, including the westernmost block (Mae Sot Basin).
Gulf of Thailand
The bulk of petroleum produced in Thailand today comes from offshore. The Ko Kra Ridge splits the Gulf into two portions. The western portion contains sedimentary basins of various sizes. Some traces of oil, found over a decade ago, await further commercialization. The eastern portion includes Pattani and North-Malay Basins, both prolific for petroleum as seen in major gas, condensate, and oil fields developed to date.
Sediments in these Tertiary basins are mainly non-marine, fluviatile-lacustrine deposits. The eastern portion was influenced by marine incursion during early Miocene. Most hydrocarbons are found in Oligocene and Miocene sandstones. Source rocks are from Type I kerogen Oligocene sediments and Type II/III kerogen Miocene fluvial flood plains and delta plains, which yield gas and possibly oil. A variety of trap types present are both of stratigraphic and structural traps such as anticlinal faults, tilted faults, rollover anticlines, and buried hills.
Most gas fields are distributed all over the central to southern part of Pattani and North Malay Basins, while oil fields are found in shallower parts of basin margins and the upper Pattani Basin.
Blocks are available for bid throughout the Gulf, particularly in the northern marginal field and southern portions.
Covering 150,000 sq. km. to the west of the peninsula, the region is divided into two parts by the shelf margin: shallow water (<200 m.) and deep water. The main geological components from west to east includes East Andaman Basin, Mergui Ridge, Western Mergui Sub-Basin, Central High, Eastern Mergui Sub-Basin, Ranong Ridge, Ranong Trough, and Sunda Shelf. The largest basin of the area, Mergui is the northern extension of North Sumatra Basin and represents the transtensional back arc basin, lying mainly in deep water. Since 1975, 19 wells have been drilled and over 20,000 line-km of 2D-seismic acquired.
Oligocene/Miocene sandstones and carbonate buildups formed on the host and shelf margins are excellent in quality and serve as potential reservoirs. Oligocene and early Miocene shales with land plant-derived organic matters are potential source rocks. A variety of play types can be found both structurally and stratigraphically. A number of structural traps present are related to extensional and transitional tectonics. The untested stratigraphic plays of turbiditic Miocene sandstones, pinch-out of Oligocene sandstones against basement, and also Miocene carbonate buildups are the prime targets for future exploration.