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ABSTRACT: Characterization of lacustrine source rocks, and the occurrence of lacustrine oil in the Pattani Basin, Gulf of Thailand

Taylor, Paul1, Eric Jardine2, Rui Lin2, and Marek Kacewicz1
(1) Unocal Corporation, Sugar Land, TX 
(2) Unocal Thailand, Ltd, Bangkok, Thailand

The petroliferous Pattani Basin in the Gulf of Thailand is one of at least twenty Tertiary-age rift basins that occur offshore and onshore Thailand. Previous geological and geochemical studies have concluded that two petroleum systems are operative in the Pattani Basin; one sourced from abundant Miocene coals and carbonaceous shales deposited under fluvio-deltaic environments, and a second system sourced from Oligocene/Lower Miocene lacustrine shales.

Organic rich lacustrine shales deposited during the late syn-rift and early post-rift stages of basin evolution are well documented in the onshore basins, but penetrations of equivalent hydrocarbon source facies in the offshore basins are extremely limited due to deep burial, thus their presence can only be inferred from oil geochemistry and seismic facies analysis.

Samples of lacustrine shale from two onshore Thailand basins, with total organic carbon values ranging from 6% to 20% and Hydrogen Index values from 500 to 800, were characterized by gas chromatography and gas chromatography-mass spectrometry to determine their detailed molecular composition and allow improved oil-source correlations in the Pattani Basin. The lacustrine organic matter was characterized by low pristane/phytane ratios, high gammacerane, and low levels of higher plant biomarkers such as bicadinanes and oleananes. Geochemical cross-plots derived from these molecular characteristics, in addition to gasoline-range hydrocarbon parameters, facilitated easy classification of source facies from Pattani Basin reservoir fluids.

The geochemical data coupled with basin modelling results and actual hydrocarbon occurrences show that oils and condensates with significant contributions from lacustrine source rocks are concentrated in flank and basement high locations within the Pattani Basin.

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