Two significant factors can be used to make a comparison. One is the carbon intensity (the amount of carbon dioxide emitted per kWh of generated heat or electricity) and the other is the energy returned on the energy invested (EROI) in getting the gas to the consumer.
One study in 2011, of shale gas from a single geological basin in the USA, made a direct comparison of greenhouse gas emissions from the two sources. It concluded that emissions over the life cycle up to the well completion stage (excluding eventual combustion) are 11% higher for shale gas than for average conventional gas but are only 3% higher when the larger contribution from the subsequent production-to-combustion stage is included. A study carried out for DECC in 2013 indicated that in the worst case UK shale gas would have a carbon footprint 27% greater than UK conventional gas. At best however the difference in emissions could be just a few percent.
EROI determines how efficient it is, in energy terms, to extract a fuel source. Obviously if more energy is required to extract a fuel than the energy it will provide then the exercise is pointless. In practice, given the complexity and overheads of societies in developed countries, the EROI ratio probably needs to be at least 3. EROI data for shale and conventional gas are rather incomplete and differences in methodology from study to study make any direct comparison difficult. The two most complete studies of shale gas fields in the USA arrived at EROIs of 10 and 12. Prior to 2006 the EROI of conventional oil and gas had been at least 18.
Limited data therefore suggest that shale gas has a slightly higher carbon footprint and a significantly lower EROI than conventional gas but this conclusion is rather tentative given differences in methodology used and the fact that there is a general trend over time for EROIs to decrease as fossil fuels become more difficult to exploit.