Field work on the uplifted and exposed margins of the Khorat Plateau Basin during the 1950s and 1960s identified oil prone lacustrine shales in late Triassic half-grabens, although the vitrinite reflectance indicated that these had passed through the dry gas generating window. Other formations were seen to contain reasonable quality reservoir rocks, including platform and reefal carbonates (some dolomitised) of the Permian Pha Nok Khao Formation and widespread stacked, braided river channels in the Jurassic-Cretaceous Khorat Group. In addition, the basin had clearly been affected by compression related to the Himalayan Orogeny, with very large, relatively low relief anticlines, expressed at the surface as ranges of hills within the basin and along its western margin.

To the field geologists it was clear that the basin, which cover some 200,000 km2 had all the geological elements necessary for an extensive hydrocarbon. However, the region was fairly remote at that time and on the edges of the Vietnam conflict, therefore, initial exploration effort moved slowly.

Hydrocarbon exploration began in earnest in 1971, when Union Oil drilled Kuchinarai-1, with minor gas shows. There was then a lull in activity, until Esso drilled 8 separate structures in 1982 to 1984, of which the Nam Phong gas field (Esso) is now in production and the Phu Horm gas field (Amerada Hess) in under appraisal.

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