Subsurface geology of some upper cenozoic sediments in the upper gulf of Thailand with special reference to off-shore Hua Hin basin

Abstract

The Hua Hin basin is elcngated in the N-S direction and parallel with the coast of Phetchaburi and Phrachuap Khirikhan provinces in the Upper Gulf of Thailand. This basin is approximately 9 kilometres wide and approximately 100 kilometres long. The Upper Gulf of Thailand is surrounded by Permo-Carboniferous clastic rocks, Permian limestone and Mesozoic clastic rocks. The Hua Hin basin was formed by extensional force closely related with the opening of the Gulf of Thailand. The normal faults controlled the development of basin and bounded the western edge of basin and parallel to the Hua Hin –Pranburi coast. These normal faults are the result of the extension in this region. The extension force in the Gulf of Thailand is believed to have been caused by plate motions in the S-E Asia, especially the collision of India with Asia, and the interactions of the Three Pagodas, Ranong-Khlong Marui and pranburi–Hua Hin faults. In the Upper Gulf of Thailand with special reference to off-shore Hua Hin basin, the Lithostratigraphic units had been proposed as informal name and composed of the Hua Hin Group and Choa Phraya Group in ascending order. The Hua Hin Group comprises of 2 formations, namely, Nong Kae Formation and Phetchaburi Formation in ascending order. The Nong Kae Formation unconformably overlies the pre-Tertiary basement rocks and was deposited during Late Eocene/Oligocene to Early Miocene. This formation is lacustrine facies and is characterized by the interbedding of shale with limestone of approximately 300 to 2,800 metres thick. The Phetchaburi Formation unconformably overlies the Nong Kae Formation and was deposited during Early Miocene to Late Miocene. The lithological characteristics of Phetchaburi Formation deposited within the Hua Hin basin are interbedding of shale and limestone in the lower part and a series of fining-upward sequence of sandstone, siltstone and shale intervening with the sequence of shale/claystone, limestone/dolomite and sandstone in the upper part. In the basement high areas, the lithological characteristics of this formation is shale interbedded with sandstone. The formation is fluvio-lacustrine facies with the thickness ranges from 300 to 1,500 metres. The Choa Phraya Group is represented by 2 formations, namely, Krabang Formation and Sattakut Formation in ascending order. The Krabang Formation unconformably overlies the Phetchaburi Formation and was deposited during Late Miocene to Late Pliocent. This formation is characterized by the sandstone interbedded with shale/claystone/clay of fluviatile facies with the thickness of 300 to 500 metres. The Sattakut Formation unconformably overlies the Krabang Formation and was deposited during Late Pliocene to Holocene. The formation is characterized by the association of sand and clay of shallow marine facies with the thickness is approximately 300-400 metres. Despite the fact that so far there is no commercial hydrocarbons discovered in the Hua Hin basin, the potential source rocks are the lacustrine facies of Nong Kae and Phetchaburi Formation. The determination of time-temperature maturity of the basin reveals that the oil window is between 2,900 to 3,700 metres beneath the sea bed. The potential traps are both stratigraphic and/or structure types of Nong Kae, Phetchaburi and Krabang Formations.