Organic geochemistry of the Neogene sediments from the Pattani Basin, Gulf of Thailand, palaeodepositional environments and hydrocarbon source potential
The Neogene sediment from the Pattani Basin, Gulf of Thailand, were subjected to detailed organic geochemical study. The results suggest that the Neogene source rocks have potential for hydrocarbon generation (as shown by high TOC content). Sequences 2, 3 and part of Sequence 4 are mature and capable of producing hydrocarbons, while those from Sequence 5 are immature. The top of the oil window estimated from biomarker maturity parameters (i.e. CPI and homohopane ratios) is at approximately 6000 feet. SCI data also support this proposal. The biomarker composition and distributions of the bitumen extracts reflect the abundance of higher plant contributions with varying amount of bacterial input, in oxic to sub-oxic conditions during deposition of the sediments. A strong contribution of terrestrially derived organic matter (higher plants) is indicated by the bimodal distribution of n-alkanes, OEP distribution of the n-alkanes in the range C23-C33, high Pr/Ph ratios, low S%, the occurrence and abundance of several biological markers, including oleanenes, oleanane, bicadinanes, tricyclic terpanes, abnormally high C31 homohopane R epimer, and high C29 steranes. Abundant coals in Sequences 5 and 4 contain higher plant material, interpreted as deposited in deltaic swamp environments. Kerogen typing shows that vitrinite (Type III kerogen) is the dominant organic matter in the samples from the Pattani Basin, suggesting potential to generate gas and condensate. The other organic macerals found include cutinite and spore/pollen (Type II kerogen), amorphous (Type I/II/III kerogen) and inertinite (Type IV). In general, molecular and microscopic examination of the Neogene succession of the Pattani Basin suggests a fluvio-deltaic environment, with peat swamps in Sequence 5 and the upper part of Sequence 4. This interpretation is confirmed by pollen analysis data. The age diagnostic pollen found in a few samples from Sequence 5 suggest the time of deposition as Middle Miocene to Pliocene.