The majority of petroleum produced in Thailand comes from offshore Tertiary basins in the Gulf of Thailand. The Ko Kra Ridge splits the Gulf into two parts. The western portion contains ten main basins of various sizes. Discoveries have been made in Chumporn and Songkhla Basins. The eastern part is composed of Pattani, Khmer and Malay Basin. Pattani and Malay are hydrocarbon prolific basins. Sediments are non-marine in origin, mainly fluvio-lacustrine deposits. However, the eastern portion was influenced by marine incursion during the early Miocene age. Hydrocarbons are found in Oligocene and Miocene sandstones. Source rocks are of Oligocene and Miocene sediments. The Oligocene sources are oil-prone source rocks which contains Type I kerogen. The Miocene sources are fluvial flood plain and delta plain sediments which mainly consists of Type III kerogen and tend to yield gas. A variety of trap types present in the Gulf such as anticlinal faults, tilted faults, rollover and buried hills. A number of oil and gas fields have been found in Pattani and North Malay Basin. Most of the gas fields are distributed all over the central part of the Pattani basin while oil fields are gathered in the shallower part of the basin margin. Blocks are available for bid throughout the Gulf particularly in the northern and southern portions.