MICROBIAL GENERATION OF ECONOMIC ACCUMULATIONS OF METHANE WITHIN A SHALLOW ORGANIC-RICH SHALE

ALTHOUGH methane of bacterial origin is ubiquitous in marine and feshw ater sediments, economic accumulations of bacterial gases occur mainly at depths of several kilometres in Tertiary basins that had high sedi mentation rates(1'2). Here we present an integration of geochemical an d isotopic data from gas and water extracted from the Upper Devonian A ntrim shale, along the northern margin of the Michigan basin, which de monstrates that significant volumes of bacterial gas have been generat ed in organic-rich shales at depths of less than 600 metres. The Antri m shale is mainly a self-sourced reservoir, in contrast to conventiona l gas deposits that have migrated from a source to a reservoir, and ha s become one of the most actively exploited gas reservoirs(3) in the U nited States. The gas-forming processes operating at shallow depths in the Antrim shale are noe unique(4) and an understanding of these proc esses should lead to the identification acid development of other econ omic, non-conventional gas deposits around the world.