PETROLEUM GEOCHEMISTRY OF OILS FROM MYANMAR – A PRELIMINARY ASSESSMENT

Jørgen A. Bojesen-Koefoed⁺¹, Michael B. W. Fyhn¹, H. Peter Nytoft^1*, Lars H. Nielsen¹, Niels H. Schovsbo¹ and Ioannis Abatzis¹

¹ Geological Survey of Denmark and Greenland (GEUS), 10 Øster Voldgade, DK-1350K Copenhagen.

^* Retired.

⁺ Corresponding author: jbk@geus.dk

Key words: Myanmar, Central Myanmar Depression, Salin Basin, Chindwin Basin, Rakhine Coastal Belt, petroleum, oil composition, biomarkers, source rocks, petroleum systems, Eocene.

Myanmar has a long history of petroleum production, but little information has so far been published pertaining to the detailed composition of the country’s oils. The present paper reports the results of the analysis of a total of 28 Cenozoic oil samples collected from producing fields/wells or natural seepages in the onshore Salin and Chindwin Basins of the Central Myanmar Depression and from Ramree Island in the Rakhine Coastal Belt. In addition, a set of 68 mudstones and coals from four Oligocene – Paleocene formations were collected along the western margin of the Central Myanmar Depression and were analysed for petroleum generation potential. Data on five oil samples from the Assam Province of India, situated to the north of the Central Myanmar Depression, were also included in the study for comparison purposes.

Previous studies have suggested that the Eocene succession in the Central Myanmar Depression includes organic-rich source rocks. However, none of the analysed rock samples show any potential for the generation of appreciable amounts of liquid petroleum components. The samples were collected at the basin margin and only cover a small fraction of the succession, and they are probably not therefore representative of the actual source rocks present in deeply buried kitchen areas in the Central Myanmar Depression.

The oils from the Central Myanmar Depression appear to have been generated from the same overall source type, which is dominated by terrigenous, higher land-plant –derived kerogen. Several different parameters consistently show clear north–south trends in the Salin Basin with respect to both thermal maturity and source facies. Oils from the Chindwin Basin in the north of the depression can be distinguished from those of the Salin Basin further south using subtle variations in biomarkers. The oils collected from Ramree Island represent a different basin and are easily distinguished in that they are generated from a predominantly marine source rock, albeit one with a significant terrigenous input. However, one sample from Ramree Island was generated from a predominantly terrigenous source, which may suggest the existence of two viable petroleum systems in the Rakhine Coastal Belt or of marked facies variations within the source succession.

Oils from the Central Myanmar Depression and the Assam Province of India are remarkably similar, thus suggesting generation from highly similar source rock types, presumably of Eocene age. This could be a result of these two areas being physically connected during the Eocene as suggested by modern plate reconstructions.

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