The central Myanmar (Burma) oil family—composition and implications for source

Abstract

Geochemical characteristics of 31 Miocene through Eocene oils/seeps, an Eocene coal and an Eocene resin from the central Myanmar (Burma) basin system are examined. The oils/seeps vary widely in API gravity (8°–52°), sulfur concentration (26–3550 ppm), δ 13C (−27.6% to −23.4%) and molecular composition. This variability is caused by varying amounts of biodegradation superimposed upon a single genetic oil family, and the difference between aliphatic and aromatic hydrocarbon carbon isotope ratios (i.e. δ 13C arom − δ 13C aliph) provides a useful gauge of the extent of biodegradation. The projected geochemical characteristics of unaltered central Myanmar oils are as follows: (a) API gravity = 35°–37°; (b) [S] = 0.05–0.10%; (c) δ 13C aliph = −27.8% to −27.6%; (d) 24-ethylcholestanes are the dominant steranes; and (e) 20S-24-ethyldiacholestane, various bicadinanes and oleanane are present. The ratio of 18α(H)-22,29,30-trisnorneohopane to 17α(H)-22,29,30-trisnorhopane ( T s/ T m) is postulated as a maturity parameter and, on this basis, relative bicadinane concentration increases (and the trans-trans-trans/cis-cis-trans bicadinane ratio decreases) with increasing thermal maturity in this oil set. Geologic arguments suggest a deep Paleogene source for these oils. Two geochemical arguments that support this inference are (a) the occurrence of saturated and unsaturated C 15 and C 30 cadinane monomers and dimers in pyrolyzates of an Eocene resin and the kerogen from an Eocene coal, and (b) identical compound-specific carbon isotope ratios for selected isoprenoids and n-alkanes in a typical central Myanmar oil and the hydrous pyrolyzate expelled from an Eocene coal. We propose an Eocene resinous shale/coal source for these oils, with the oldest (Eocene) reservoirs filling first and the youngest (Miocene) reservoirs filling last, consistent with the observation that the least mature oils are present in the oldest reservoirs. According to this model, surface seepage and near-surface oil could result from subsurface traps that are filled to spillpoint.