ABSTRACT: Gas migration in the southeast Barrow Sub-basin, Carnarvon Basin, Australia

Understanding the migration history for the oil and gas fields trapped against the Long Island and Flinders Fault Systems, which lie on the southeast margin of the Barrow Sub-basin, is key to identifying the remaining hydrocarbon potential in this area. The discoveries vary from oil fields with no gas caps (Saladin, Chervil, and North Herald) to gas fields with thin oil rims (Australind, Roller, and Skate).

The gas columns in these fields contain both hydrocarbon and inert gas. Carbon-13 isotope analysis of carbon dioxide has been undertaken on several gas columns in the region. For example, Australind produces 1.02 mole% carbon dioxide with an isotope value of -13.1o/oo, which suggests thermal degradation of organic material is the source of the inert gas. Skate 3 produces 4.03 mole% carbon dioxide with an isotope value of +12.7 o/oo, which suggests the source of the inert gas is due to methanogenesis or biodegradation of the underlying oil column. The carbon dioxide in these two fields is produced from two different sources, while the charge mechanism for both is via the bounding fault. Consequently, a new charge model has been suggested in which the bounding faults play an important role in the migration of gas into the structures.

Acknowledging the presence and determining the source of inerts in the gas columns has increased our understanding of the sealing and conduit potential of individual faults within the fault systems. Exploration geochemistry, utilising gas composition analysis, can greatly impact the evaluation of petroleum prospectivity of an area through the rigorous assessment of a key risk parameter, the timing and migration of hydrocarbons.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia