Thermogenic oil seeps to the surface of the Tonga Ridge at Tongatapu, an elevated Pliocene-Pleistocene reef complex. The summit region of the Tonga Ridge is a relatively shallow (<1000 m) submerged plateau or platform underlain by a little-deformed sedimentary sequence of Eocene and younger volcaniclastic and carbonate beds. The platform sequence is thickest, 4 to 5 km, where lower Tertiary deposits fill structural basins in an underlying basement of arc-igneous rocks, most likely of early middle or early Eocene age.

From insular exposures and seismic reflection data four regional stratigraphic units that roughly comprise beds of mostly late Eocene, early to middle Miocene, late Miocene and early Pliocene, and late Pliocene and younger age can be distinguished. Their separating unconformities are associated with carbonate buildups, including reefal masses. The unconformities are thought to mainly record uplift and subsequent submergence of the summit platform linked to tectonic events of plate-boundary arc rifting and crustal collision.

The most likely reservoirs for petroleum accumulations are buried reefs, in particular of Eocene and early Neogene age. The lower part of the platform sequence has been sufficiently warmed to generate petroleum, but exploratory drilling at Tongatapu found neither thermogenic hydrocarbons nor source beds. In addition, organic-rich deposits have not been found at either insular or submarine outcrops. Although seeps of mature crude are known, the limited exploratory drilling and sampling of subaerial and submerged outcrops to date have not located the source beds.

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The lack of encouraging information about suitably positioned source beds dampens a sanguine assessment of the resource potential of the otherwise prospective platform sequence.