Abstract: The Central Myanmar (Burma) Oil Family: Composition and Implications for Source

Joseph A. Curiale, U Pe Kyi, Ian D. Collins, U Aung Din, U Kyaw Nyein, U Maung Nyunt, Charles J. Stuart

Geochemical characteristics of 31 Miocene through Eocene oils/seeps and two Eocene source materials (coal and bulk 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), ^dgr¹³C (-27.6^pmil to -23.4^pmil) and molecular composition. This variability is caused by differing amounts of biodegradation superimposed upon a genetically-constant oil family, and the difference between aliphatic and aromatic hydrocarbon carbon isotope ratios (i.e., ^dgr¹³C_arom - ^dgr¹³C_aliph) provides a useful gauge of the extent of biodegradation. The pre-biodegraded central Myanmar oil is deduced as follows: (a) API gravity = 35-37° ; (b) (S) = 0.05 0.10%; (c) ^dgr¹³C_aliph = -27.8^pmil to -27.6^pmil; (d) 24-ethylcholestanes are the dominant steranes; and (e) 20S-24-ethyldiacholestane, various bicadinanes and oleanane are present. The ratio of 18alpha(H)-22,29,30-trisnomeohopane to 17alpha(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 infer a deep Paleogene source for these oils. Geochemical arguments reinforce this inference in the following two respects: (a) the occurrence of saturated and unsaturated C₁₅ and C₃₀ cadinane monomers and dimers in pyrolyzates of an Eocene resin and the kerogen from a coeval 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 t is model, near-surface oil and surface seepage could result from subsurface traps being filled to spill point.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994