ABSTRACT: Structure, Seismic Stratigraphy and Petroleum Potential of the Tongatapu-'Eua Area of the Kingdom of Tonga

Robert W. Gatliff, John Pfleuger, Kenneth Havard, Saimone P. Helu

The Tonga Ridge is an elongate forearc, crossed by several fault-controlled bathymetric lows that divide it into a series of major blocks. Bathymetric, seismic, and magnetic data suggest that the Tongatapu block has been rotated through about 30° in an anticlockwise direction prior to the opening of the Lau Basin. A large basement high, outcropping on 'Eua, forms the eastern margin of the block.

Oil seeps discovered on Tongatapu encouraged petroleum exploration and the results of five wells drilled on Tongatapu and three commercial seismic surveys revealed a substantial sedimentary basin. Recent reprocessing of the seismic has clarified the structure, and six seismostratigraphic units, ranging from Eocene to Recent, can be mapped. Most units thin towards 'Eua, which appears to have had an effect on sedimentation throughout the Tertiary. Intermittantly it has acted as a source of sediment, contrasting with the more widely reported westerly sources from arc volcanoes. Along the margin of the basin to the west of 'Eua a NNW-SSE-trending series of highs has been interpreted as a series of reefs of Eocene or possibly early Oligocene age.

During the early Tertiary, Tonga lay close to the New Caledonia Ridge. A source for the oil seeps may have been deposited during this time and preserved in an undrilled sequence.

The reefs along the basin margin are buried to about 1500 m and generally lie beneath water depths of 200-300 m, and form an interesting and untested play.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990