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 Guine a and analysed to characterise the petroleum sourcing system in the ba sin. The oil samples are extensively biodegraded as is typical of seep s. They are derived from a dominantly marine source, although in some samples there is evidence of higher land plant input. The thermal matu rity of the samples is variable, from low to moderately high. Some of the gas samples are clearly biogenic, some are thermogenic and some co ntain a mixture of both source types. delta D analysis on selected sam ples helped identify two biogenic groups which are characterised by: ( a) high carbon dioxide content and light delta D, and (b) lower carbon dioxide content and heavier delta D. These characteristics reflect th e microbial pathway of biogenic methane production: the first group is predominantly a product of acetate fermentation and is associated wit h low salinity samples (recent meteoric water); the second group is a product of CO2 reduction and is associated with higher salinity water samples. The delta D 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 sug gests the presence of biogenic accumulations in the subsurface. The th ermogenic gases are associated with water samples of intermediate sali nity and demonstrate exceptionally heavy delta(13)C for the carbon dio xide component (up to +28 parts per thousand PDB).