Abstract

Geologically the Khorat Plateau consists of stacked Permo-Carboniferous, Triassic, Mesozoic and Tertiary basins. Hydrocarbon exploration of these basins began in earnest in 1971, when Union Oil drilled Kuchinarai-1, with minor gas shows.

From 1982 to 1984 Esso drilled 8 separate structures, of which the Nam Phong gas field is in production and the Phu Horm gas field is under appraisal (Amerada Hess). Mud losses and a gas kick in Chonnabot-1 were interpreted as indicating another significant gas bearing structure.

A further 12 structures were drilled between 1990 and 1994 by several operators, with just 1 significant flow of gas from Dong Mun-1.

A fourth round of exploration drilling began in 1997. In Laos, Pakse-1 (Hunt) and Naxay-1 (Monument), were both dry, with the Permian carbonate reservoir target reported to be absent in both wells. In Thailand Phu Wiang-1 (Amerada Hess), a redrill of Chonnabot-1, encountered significant overpressure and gas shows at numerous levels. However, the Permian reservoir target proved to be mixed marine clastics and carbonates rather than the prognosed fractured platform carbonate. The Permian and late Triassic sections failed to flow significant amounts of gas. Cairn’s Si That-2, drilled up dip of Esso’s 1983 well, was abandoned after testing a non-commercial volume of gas.

To date the platform carbonates of the Permian Pha Nok Khao Formation have proved to be the only productive reservoir and was the target in 18 of the 21 exploration wells. Only 10 of these actually encountered the carbonates, in 5 of which it proved to be of reservoir quality, with dolomitisation and fracture enhancement being the key elements in productivity. In 4 cases it flowed significant volumes of gas.

There is a proven commercially viable gas prone hydrocarbon system within the Khorat Plateau. This is an energy starved region, with a large population providing a ready market for any gas. Despite past disappointments this huge basin will continue to attract exploration interest.