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ABSTRACT: Source rock characteristics onshore Thailand: Implications for oil and gas prone sources in the offshore basins of Thailand

Jardine, Eric1, Rui Lin1, Paul Taylor2, and Joseph Curiale2
(1) Unocal Thailand, Ltd, Bangkok, Thailand 
(2) Unocal Corp, Sugarland, TX

More than twenty Tertiary-age rift basins exist in Thailand, extending 1,500 kilometers from the southern Gulf of Thailand to the country's northern border. The larger basins are proven prolific producers of significant gas and oil reserves. In most basins, lacustrine depositional environments occurred initially during Oligocene and Early Miocene rifting, changing to fluvial settings in the Miocene and Pliocene. Primary offshore basins are generally deeper and have higher heat flows. Mature source rocks offshore are mostly gas-prone fluvial-deltaic coals and shales, whereas onshore they are mainly oil-prone lacustrine shales. Lacustrine sources also occur offshore but are poorly documented due to deep burial.

Fifty-five Tertiary-age lacustrine and fluvial-deltaic source rock samples from four onshore basins were analyzed to better understand oil and gas generation from correlative sequences offshore. Lacustrine oil shales contain 5% - 20% organic carbon, have hydrogen indexes over 500, oxygen indexes below 70, pristane/phytane ratios below 3.0, high gammacerane, and low bicadinanes. Produced oils offshore have molecular characteristics compatible with similar lacustrine sources. Lacustrine oils offshore and onshore are alike, exhibiting high wax, high pour points, low gas/oil ratios, and little sulphur.

Thirteen immature samples representing kerogen end-members were pyrolyzed to define source rock kinetic parameters. Lacustrine Type I samples convert to hydrocarbon in a narrow temperature and activation energy range. Fluvial-deltaic coaly Type III samples have a much broader generation range. The kinetic parameters were used to refine a thermal modelling program improving predictions of timing, rate, and yield for oil and gas generation in Thailand basins.

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