Habitat of Natural Gases in Papua New Guinea

M. Schoell, M. A. Beeunas, D. K. Baskin, F. Monnier, L. I. Eisenberg, and G. L. Valenti

Thermogenic natural gases in Papua New Guinea occur in hanging wall anticlines and related structures along a 160 mile section of the Papuan fold and thrust belt between S.E. Hedinia in the SE and P'nyang in the NW. Isotopic compositions of the oil associated gases in the SE between Hedinia and Mananda varies little (^dgr¹³C_CH4=-44±2^pmil and ^dgrD_CH4=-200±20^pmil). However, subtle isotopic and compositional patterns in these gases are structurally controlled and indicate primary differences in the filling history of the individual structures. In addition, secondary redistribution of the gases between the Agogo and Iagifu structure can be traced through isotopic similarities. In S.E. Mananda, however, gas isotope patterns are aff cted by bacterial degradation of the gas.

The concentration of CO₂ in the oil associated gases in the SE is low (0.6-3.0%) and the carbon isotope values (^dgr¹³C_CO2=-10 to -19^pmil) suggest an organic origin with minor inorganic contributions. Gas in the Juha structure (^dgr¹³C_CH4=-36.8^pmil) is likely from a more mature source and has a CO₂ concentration of 9.6% with a ^dgr¹³C_CO2=-5.9^pmil, indicating additional CO₂ generating processes in this area, likely related to magmatic activity in the vicinity of the Juha structure. The P'nyang structure in the NW of the area holds a gas (^dgr¹³C_CH4=-40.5^pmil) which is isotopically intermediate between the Juha gas and the oil associated gases in the SE. The low C ₂ concentration of 0.2% suggests that P'nyang is sourced from a gas kitchen similar to, but more mature than, the kitchen for the oil associated gases in the SE of the province. This is consistent with the high GOR in this structure and the association of the gas with a high API gravity condensate.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California