The Gulf of Thailand contains several structurally complex trans-tensional basins. These are made up of asymmetrical grabens filled with non-marine to marginal marine Tertiary sediments as old as Eocene. Underlying the graben sediments are a variety of Paleozoic marine carbonates, granitic intrusive rocks, and metasediments. Many of the basins contain thick sequences of oil-prone source rocks, but the limited lateral extent of these deposits, combined with vatiations in heat flow and depth of burial of the source rocks, causes the distribution of hydrocarbons to be complex and difficult to predict. Numerous exploration opportunities remain, but the outlook is for a large number of smaller discoveries. The Thai Department of Mineral Fuels maintains a current concession map of the Gulf of Thailand.