The origin of unusual gas seeps from onshore Papua New Guinea

In September 1989, gas seepage samples and associated oils and waters were collected from onshore in the Aure Thrust Belt of Papua New Guinea and analysed to characterise the petroleum sourcing system in the basin. The oil samples are extensively biodegraded as is typical of seeps. They are derived from a dominantly marine source, although in some samples there is evidence of higher land plant input. The thermal maturity of the samples is variable, from low to moderately high. Some of the gas samples are clearly biogenic, some are thermogenic and some contain a mixture of both source types. δD analysis on selected samples helped identify two biogenic groups which are characterised by: (a) high carbon dioxide content and light δD, and (b) lower carbon dioxide content and heavier δD. These characteristics reflect the microbial pathway of biogenic methane production: the first group is predominantly a product of acetate fermentation and is associated with low salinity samples (recent meteoric water); the second group is a product of CO 2 reduction and is associated with higher salinity water samples. The δD values for the second group are characteristic of biogenic gas produced in marine environments although the salinity of the associated waters has a halite dissolution character rather than that of mixing with sea water. The unique character of these gases suggests the presence of biogenic accumulations in the subsurface. The thermogenic gases are associated with water samples of intermediate salinity and demonstrate exceptionally heavy δ 13C for the carbon dioxide component (up to +28%. PDB).