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ABSTRACT: Wrench Interpretation of the Jabiru Oil Field, Australia

Andrew Nelson

The Jabiru Field in the Timor Sea, offshore northwestern Australia, was developed on the basis of 12 million bbl of proven recoverable oil but by October 1989, after three years of production, has produced over 32 million bbl of oil with little water.

The field was originally thought to be a tilted horst block in an extensional tectonic environment, but production and appraisal drilling data have shown that the early fault interpretations were substantially misleading in that most oil has subsequently been found in areas originally thought to be outside the field boundary.

Consideration of section restoration and balancing techniques suggests that the extensional model is incorrect and that the structure can be explained in terms of a wrench-induced tulip structure and subsidiary tulips or pop-up blocks. A field bounding fault, which was reversed during the Jurassic and Cretaceous, experienced normal movement subsequently, probably because of a modification of stress fields and a reversal of the wrench sense. The throw on the faults in the shallow section is thus completely misleading when used as a guide to the throw at the reservoir level.

The discovery well was accidentally drilling on the upthrown side of the bounding fault (at reservoir level) because of poor seismic resolution of the fault. Three subsequent appraisal wells which were thought to have been placed in safer locations with respect to the fault were on its downthrown side at the reservoir level. The success ratio for appraisal wells (now numbering 10, with three sidetracks) is about 50%.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990