Structural Style and Petroleum Prospects of Tonga Ridge

John Pflueger, Ken Havard, Robert Gatliff, Baimone Helu

Recent reprocessing of seismic data from the Tonga Ridge has provided a clearer picture of the tectonic style of a forearc basin. Sediments along the ridge achieve a maximum thickness of more than 4,000 m. Many faults have been observed, almost all of which are normal. The presence of scarps on the sea floor show that a significant percentage of the faults are active today. Although seismic transverses are not spaced closely enough to allow unambiguous definition of fault orientations, we believe that most of the tensional faults are parallel to the ridge axis. A second set of near-vertical faults with apparent transcurrent movement exists at a high angle to the ridge axis. Bathymetry has played a major role in defining the strikes of the second set of faults.

The tensional failures are due to the flexing of the ridge as the leading edge of the Indo-Australian plate is pulled down and under by frictional attachment to the subducting Pacific plate. The development of major fault blocks is related to the subduction of Pacific plate seamounts.

Fault-bounded closures and Eocene to Miocene reefs would form excellent traps for hydrocarbons. Oil seeps of biodegraded but thermally mature crude oil on the island of Tongatanu encouraged the drilling of five exploratory test wells, but the source rock feeding the seeps was not discovered. Burial history and geothermal modeling indicate that mature source rocks could occur deeper than depths penetrated by the test wells.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.