Subsurface geology of the Southern part of tertiary Mergui Basin, Andaman Sea

Abstract

The study of subsurface geology of the southern part of the Tertiary Mergui basin, Andaman Sea, is based on well information and 2D seismic data set to explain subsurface geology, geologic structure, depositional environment and basin evolution with preliminary assessment on the petroleum potential within the study area. The Mergui basin is located in the southern part of Andaman Sea. The development of basin is controlled by N-S normal faults that bound western edge of the basin. These normal faults are the result of the extension in this region that probably related to the collision of India with Eurasia plate and interaction of strike-slip faults in this region. The Tertiary sedimentary sequence of the southern part of Mergui basin can be subdivided into 4 units, namely units A, B, C, and D. The unit A comprises outer neritic to middle bathyal shale in the lower part, upper bathyal sandstone in the middle part, and the upper part is outer neritic to middle bathyal siltstone claystone and shale that located in restricted sub-basin. The unit A that is located at the basement high is composed of limestone. This unit deposited in an outer neritic to middle bathyal basinal plain, mid-fan turbidite, and reef shallow-marine environment. The age is Late Oligocene to Early Miocene. Unit B consists of siltstone and claystone which occasionally interbeded with limestone, sandstone and shale. The age of this unit is Early Miocene to Middle Miocene. The environment of deposition is upper to middle bathyal basinal plain. Unit C consists of siltstone and claystone. The unit that located at the basement high is composed of limestone or carbonate builds up. The age of this unit is Middle Miocene to Pliocene. This unit deposited in upper to middle bathyal basinal plain, and reef of shallow-marine environment. Unit D consists of clayey siltstone. The age of this unit is Pliocene to Holocene. This unit deposited in upper to middle bathyal basinal plain with the shelf onthe eastern part of study area. The potential source rocks of the basin are marine shale and reef limestone in Units A and C., which, geochemicals analysis indicates oil and gas and gas prone types. The sandstone in unit A and reefal limestone in Units A and C are of fair to good quality reservoir. The marine claystone, siltstone and shale in all units are effective hydrocarbon seal. Most hydrocarbon accumulation is in structural and stratigraphic traps. The Lopatin{174}s method and geothermal data suggest that petroleum generation might have been the oil generation at a depth below the oldest sedimentary unit, 1000 meters in Kantang-1a well and 1200 to 1300 meters in Kraburi-1 well.