Abstract
Petroleum samples collected from shallow holes at two locations on Tongatapu, Tonga, have been analysed by computerized gas chromatographic—mass spectrometric and stable carbon isotopic techniques. The gross composition, absence of n-alkanes and isoprenoids in the aliphatic hydrocarbon fractions, presence of ring A/B demethylated hopanes, and narrow carbon isotopic composition of the hydrocarbon fractions indicate the samples are moderately to severely biodegraded crude oils. The nearly identical terpane distribution and carbon isotopic composition of samples from both locations indicate a common origin. The aliphatic hydrocarbon fractions contain a thermodynamically stable assemblage of steranes and triterpanes, indicating an advanced stage of thermal maturity for the oil samples. These results imply that, if the oils have been generated within sediments on the Tonga Platform, the source beds are probably in the more deeply buried and thermally mature western section of the platform.
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