A Spectrum of Fluid-Escape Pseudo-Volcanic Features Revealed by a Combination of Seismic and Multibeam Data, Indonesia

Peter Baillie¹, Philip A. Teas², Dan Orange², and John Decker^2
1TGS-NOPEC Geophysical Company, Perth, WA, Australia
²Black Gold Energy, Jakarta, Indonesia

A spectrum of non-igneous pseudo-volcanic features, generated by fluid-escape have been imaged by a combination of seismic and multibeam bathymetry data acquired as part of a non-exclusive regional prospectivity analysis of the deepwater frontier basins of Indonesia.

“Extrudite” is the term we apply to these deposits. Extrudites display features analogous to lava flows (generally “lumpy” on the surface but may be ropy) and volcanic cones (= mud volcanoes, may be greater than 1 km in diameter) with calderas, rim vents, and flank cones. Core samples from extrudites are consistently comprise mud or sand, pebbles, and cobbles in a mud matrix. Larger clasts are also expected to occur, but could not be collected with a 3-inch core barrel. Cored extrudites are lithologically similar to rocks described elsewhere as pebbly mudstone, olistolith, debrite or mélange.

Extrudites are recognized in a variety of tectonic settings, but most common in transtensional or pull-apart basins and in accretionary complexes. They also occur above known or suspected offshore oil and gas fields. Our premise is that these features form where fluid pressures exceed lithostatic pressures, but the cause of the overpressure may vary with tectonic setting.

Of particular interest to the petroleum community are extrudites which contain thermogenic hydrocarbon geochemical signatures. Hydrocarbon seepage, does not always produce extrudites and extrudites do not always contain hydrocarbons, but in the many places where they coexist, they are of special exploration interest.

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