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ABSTRACT: Oil Play Concepts Offshore Tonga

BARCLAY, WILLIAM, and JON RODD, South Pacific Applied Geoscience Commission (SOPAC), Suva, Fiji, and J. C. PFLUEGER and K. R. HAVARD, Consultants, Calgary, Alberta, Canada

The Tonga Ridge is the forearc component of a classic island arc system of the Java type. Within the ridge are several basins covering up to 70,000 sq km with sediment thicknesses of up to 4000 m. Some of these areas in less than 400 m water depth occur along the axis of the Tonga Ridge making them attractive from the point of view of production economics and technology.

In 20 yr of exploration by oil companies and by SOPAC-coordinated institutional surveys conducted on behalf of the Kingdom of Tonga, 150 lines covering over 14,000 km of multifold seismic, gravity, and magnetic data have been acquired, half of them within the last decade. However, drilling activity has been confined to the onshore areas, where five wells were drilled on the basis of gravity/magnetic data or poor-quality seismic data.

Hydrocarbon charge is proven by several oil seeps generated from thermally mature source rocks probably located west of the Tonga ridge where sediments are thickest. Burial analysis indicates that undrilled upper Eocene sediments are within the oil window.

A reinterpretation of well and seismic data in the offshore areas has given rise to several structural and stratigraphic play concepts; reefal mounds sealed by overlying pelagic shales channel and submarine fan deposits sealed by basinal shales, and tilted fault blocks. Some of the prospects are associated with seismic flat spots.

Play concepts based on interpretation of the new seismic include shallow basin margin reefal mounds sealed by pelagic shales, and submarine fan lobes and channel deposits sealed by basinal shales, and/or volcaniclastics, and tilted fault blocks.

 

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)