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