Where did all the coal gas go?

Isotopic models indicate that reservoired gases in mid-Norway were generated predominantly from marine source rocks, despite the presence of abundant mature coal and terrestrial carbonaceous shales in the Åre Formation. There are at least three different possible explanations as to why this may be the case: (i) source models for gas from coals are not valid; (ii) modification of the carbon isotope signature of gas occurs post-expulsion; (iii) loss of gas due to poor expulsion efficiencies and complex migration pathways. Each of these possibilities is discussed in turn. The most likely explanation is that loss of gas due to poor expulsion and/or complex migration routes in the Åre Formation results in most of the gas not reaching the reservoir. However, it should also be noted that current isotopic models for gas from coal are less rigorous than those for gas from marine source rocks.