ABSTRACT: Source, migration and CO₂ occurrences in the Pailin Field, Gulf of Thailand

Martens, David¹, Rui Lin², and Robert Hickman¹
(1) Unocal Thailand, Ltd, Bangkok, Thailand 
(2) Unocal Thailand Ltd, Bangkok, Thailand

The Pattani Trough is an extensive Tertiary rift-sag basin in the Gulf of Thailand where several large gas fields have been discovered and developed since 1979. The structure in the main producing interval (Miocene) of the Pattani Trough is dominated by a series of highly-faulted half-grabens or "graben complexes". During the Oligocene, lacustrine deposition dominated in the basin (secondary source) followed by fluvial deposition in the Miocene and Pliocene (primary reservoir, source and seal).

Pailin Field, representing one of the half-graben systems, is located in the southern Pattani Trough. The field's petroleum system involves two distinctive hydrocarbon sources: a fluvial coaly facies and a lacustrine shale facies. Gas condensates from the lacustrine facies are characterized by low pristane/phytane ratios and high gammacerane, whereas those from the fluvial sources are characterized by high pristane/phytane ratios and high angiosperm resin markers (bicadinane "W" and bicadinane "T"). The lacustrine sources are deeply buried and overlain by high sand/shale ratio alluvial deposits, permitting vertical migration along fault planes and/or stacked sands. In contrast, the majority of the hydrocarbons in the field were sourced locally from Middle Miocene fluvial coals and carbonaceous shales. 

Hydrocarbon migration from the coaly facies was predominantly lateral; the migration model in conjunction with the thermal maturity analysis allows for accurate pay window predictions.

The Pailin gas contains appreciable amounts of CO₂ (5 to over 65%) derived from both organic and inorganic sources, which can be predicted pre-drill through a relationship between CO₂ partial pressure and temperature or depth. This understanding of source, migration and CO₂ distribution facilitates the development and production of Pailin Field.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia