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The Pre-Tertiary petroleum system of onshore Northeast Thailand has been proven to produce hydrocarbons, in contrast to the offshore Gulf of Thailand, which is underexplored. Although hydrocarbons were found at the Pre-Tertiary interval in the Western Basins, a significant discovery has not been made since the 1990s. Therefore, as an E&P block operator in the Gulf of Thailand since 2019, Medco has an opportunity to explore the Pre-Tertiary prospects within the block.

In 2023, Medco undertook a field visit to onshore Southern Thailand to examine the Paleozoic and Mesozoic rocks, which are geologically similar to those in the Pre-Tertiary sediments of the Western Basins. The Ratburi Group consisting of Paleozoic limestone was deposited on the Sibumasu Terrane during the Middle Permian. The Thung Yai Group comprises Mesozoic rocks, including Jurassic shallow marine sediments of the Khlong Min Formation. This is subsequently overlain by the Cretaceous continental sediments of the Lam Thap Formation in a piggyback basin setting.

The Ratburi outcrops in the Phi Phi Islands provide a better understanding of the karstified Permian limestone. Its uppermost zone, called the "Epikarst," is the most porous and permeable zone, as evidently observed in several wells within the Western Basins. Meanwhile, the findings of oil stains in the Thung Yai sediments offer strong evidence of the working Pre-Tertiary petroleum system in the areas. It aligns with our depositional model and geochemical study, confirming that the Mesozoic sediments in the Western Basins have generated oil. Thus, the oil generated from Mesozoic source rocks is modeled to have migrated laterally through the epikarst along the top of the Ratburi limestone, reaching the focal areas.

The new insights into the Ratburi Epikarst and Mesozoic oil stains from the field serve as valuable references to build a new play concept for the Pre-Tertiary petroleum system in Medco's block.