2002 年 67 巻 1 号 p. 16-29
The Pattani Trough is an elongate N-S trending Tertiary rift in the Gulf of Thailand. In the Pattani Trough, large quantities of gas and condensate are charged in the central trough and lesser oil is accumulated in the frank areas and northern trough. The stratigraphic units of the trough which have a total of about 10,000m of the Tertiary sediments, were initially developed from the Oligocene to present time. The initial syn-rift sediments of Oligocene consist of lacustrine deposits, and thick early to middle Miocene sediments are composed of fluvial and partially deltaic/marginal/marine deposits. The gases in the Pattani Trough are derived from the thermal cracking of type III kerogens and, the cracking of pre-existing oils. The oils are waxy and heavy, and interpreted to be lacustrine algal origin. Two petroleum systems with two different source facies, the Oligocene lacustrine algal facies and the Miocene fluvial coaly facies, are operating in the trough. Two-dimensional basin modeling was conducted to examine generation, migration and accumulation of hydrocarbons across the trough. Oil generation from Oligocene source rocks initiated in the early Miocene, and gas generation mainly occurred in the middle to late Miocene in the central trough. CO2 contents generally increase with depth from a few percent to around 25% in the trough. Occasionally some wells encounter gas reservoirs containing CO2 above 60% and as high as 91%. Anomalously high CO2 in the Platong gas field was found, and its distribution was identified to be large plume-like shape related with a nose structure. Based on carbon isotopic analysis, anomalously high CO2 has inorganic origins which include magmatic activity and decomposition of carbonates. Basement lithologies and high thermal activities in this region suggested that anomalously high CO2 was derived from thermal decomposition or hydrothermal dissolution of carbonate basements.