Large volumes of greenhouse gases such as CH4 and CO2 form by contact
metamorphism of organic-rich sediments in aureoles around sill
intrusions in sedimentary basins. Thermogenic gas generation and
dehydration reactions in shale are treated numerically in order to
quantify basin-scale devolatilization. We show that aureole thicknesses,
defined as the zone of elevated metamorphism relative to the background
level, vary within 30-250% of the sill thickness, depending on the
temperature of the host-rock and intrusion, besides the sill thickness.
In shales with total organic carbon content of >5 wt.%, CH4 is the
dominant volatile (85-135 kg/m(3)) generated through organic cracking,
relative to H2O-generation from dehydration reactions (30-110 kg/m(3)).
Even using conservative estimates of melt volumes, extrapolation of our
results to the scale of sill complexes in a sedimentary basin indicates
that devolatilization can have generated similar to 2700-16200 Gt CH4 in
the Karoo Basin (South Africa), and similar to 600-3500 Gt CH4 in the
Wring and More basins (offshore Norway). The generation of volatiles is
occurring on a time-scale of 10-1000 years within an aureole of a single
sill, which makes the rate of sill emplacement the time-constraining
factor on a basin-scale. This study demonstrates that thousands of
gigatons of potent greenhouse gases like methane can be generated during
emplacement of Large Igneous Provinces in sedimentary basins. (C) 2010
Elsevier Ltd. All rights reserved.