The potential of biogenic gas exploration

– a paradigm shift for exploration in Japan?

Abstract

Biogenic methane gas is generated by methanogen in sediments. The generated gas can be concentrated as water dissolved gas and accumulated as methane hydrate or free gas. The sea areas around Japan have optimal conditions for biogenic gas generation comparable with other biogenic gas generating basins in the world. Gas bubbles generated from oversaturated dissolved gas cause free gas accumulation effectively, and the mechanism to produce oversaturation differs by basin. The main mechanism for oversaturation is the continuation of gas generation and decompression caused by vertical crustal movement or upward migration of formation water.

Biogenic free gas accumulation often occurs at shallow depths, and direct hydrocarbon indicators（ DHIs） on the seismic section provide a strong clue of their existence. The combined implementation of marine electro-magnetic survey with seismic survey is however recommended to reduce exploration risk, as low saturation gas also causes DHIs.

In near feature, it is expected that the understanding of the underground carbon cycle and gas accumulation mechanism will advance, and biogenic gas exploration in the seas around Japan will become more common.