The Tonga platform is roughly outlined by the 2,000-m isobath with an average width of about 60 to 75 mi (100 to 120 km) surrounding the Tongan Islands of Vava'u, Ha'apal, and Tongatapu. Single-channel seismic profiles across the platform were acquired between 1977 and 1979 by scientific expeditions sponsored by CCOP/SOPAC (Committee for Cooperation of Joint Prospecting for Mineral Resources in South Pacific Offshore Areas) and ORSTOM (Office de la Recherche Scientific et Technique Outre Mer). This survey and a five-well drilling program on Tongatapu were inspired by the discovery of seeps of weathered crude oil from vuggy coralline limestone. Reef buildups and normal fault structures are interpreted from seismic data, and the wells penetrated Miocene and Eocene limestone , volcanogenic clastics, and some reached volcanic basement. Few oil and gas fields are found in fore-arc basins, of which the Tonga platform is typical, but the reefs offer prospects and the seeps suggest that the requirements of source rock and maturation may be satisfied.

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