Abstract

Paleo-oil-water contacts (paleo-OWCs) defined in eight wells from the Griffin, Chinook, and Scindian fields, Barrow Subbasin, Australian North West Shelf, are 47–147 m (154.2–482.3 ft) deeper than the current OWCs. The paleo-OWCs are deeper than the spillpoints separating the fields and are consistent with the existence of a single large paleo-oil accumulation. A plane of best fit projected through these paleo-OWCs describes an inclined surface with a northwesterly dip of approximately 1.5°.

Assuming that this paleo-OWC was horizontal at the time of charge, three-dimensional structural restoration was used to assess the trap geometry at that time and the impact of subsequent post-charge structuring. The paleo-OWC tilt is reduced to 0.6° by restoration to the middle Miocene, and by the base Tertiary, the paleo-OWC became over-rotated by 0.1°. A horizontal paleo-OWC plane occurred some time between these periods and is broadly consistent with oil-charge estimates derived from fluid-inclusion paleotemperature data combined with one-dimensional basin modeling.

The restored paleostructure and the approximate position of the paleo-OWC are consistent with the combined trap having initially been filled to spill. The bulk rock volume of the paleo-accumulation is more than 10 times larger than the current accumulation, indicating the loss of significant oil volumes as post-charge westerly tilting progressively reduced the trap capacity. Based on the current oil volumes (about 200 million bbl), nearly 2 billion bbl of oil was spilled from the trap by the relative shallowing of the eastern spillpoint. The region to the east and north of the Griffin area fields is lightly drilled, and remigration of such large volumes of oil to this area provides great incentive for future exploration. New targets are also inferred on the hanging wall of the main Griffin fault, which could represent bypassed pay.

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