The definition of ultra-high temperature (uHT) conditions varies depending on regional average well temperatures. In the Gulf of Thailand (GOT), uHT is defined as a well with a bottom-hole static temperature (BHST) exceeding 450°F (230°C). Recently, a national oil company began requiring a drilling fluid stable up to 520°F (270°C) for exploration wells, especially when the actual BHST is uncertain. This paper describes the development and stability of a drilling fluid approved by the operator for these extreme well conditions. Typical exploration wells in the Gulf of Thailand require 6 1/8 -in. hole for their primary target interval. Beside the extreme BHST, the slim and deviated well geometry (up to 60°inclination), and the potential for highly acidic gas (CO 2 and H 2 S) contamination, presented extreme conditions for prolonged successful open-hole wireline logging operations (up to 4 days), which was the most critical objective for the exploration wells. Successful open-hole wireline logging is also imperative for perforating and further production from the typical thin layers of sandstone reservoir.New additives provide a thermally stable non-aqueous fluid (NAF) enabled logging and evaluation under the severe down-hole conditions. The novel emulsifier and fluid-loss-control agents were introduced after undertaking an extensive laboratory-testing program. Apart from the newly introduced products, fluid engineering and improved field practices also played an important role in the success of these exploration wells.The successful achievement with this novel NAF facilitated the planning of additional uHT exploration wells for the near future. In addition to the successful logging program, the novel NAF minimized non-productive time and mets the objectives set for the exploration wells comparing to the conventyiaonl NAF.