Deep geothermal energy is widely recognised as a source of low carbon heat. However, to date there have been no specific assessment of the carbon intensity of low-enthalpy deep geothermal; previous studies focussed on geothermal power or higher enthalpy heat. As such, there is no established method for assessing the CO2 emissions from implementing a deep geothermal heating scheme. Here we address these gaps. We perform a life cycle assessment of greenhouse gas emissions relating to a deep geothermal heat system to (i) calculate the carbon intensity of geothermal heat; (ii) identify key factors affecting these values; (iii) consider the carbon abated if geothermal heat substitutes conventional heating; and (iv) present information that future projects can apply to assess the carbon emissions reduction offered by geothermal heat development. Our work is informed by parameters from a feasibility study for a proposed geothermal heat system in Banchory, Scotland. The project planned a 2.5 MWth geothermal plant extracting heat from the Hill of Fare granite via two boreholes, one injection and one production. We find that the majority of the emissions are associated with site construction, and sensitive to site and materials specific factors, for example the depth of the drilled boreholes and type and quantities of steel and cement used to seal them, or soils disturbed for laying pipelines and constructing access roads. During operation the carbon intensity of the electricity grid used to power hydraulic pumps largely determines the carbon intensity of the produced heat. We calculate that the carbon intensity of the heat produced is 9.7-14.0 kg(CO2e) MWhth which is 4.9-7.3% of the emissions from heat from natural gas. These values are compatible with Scotland's plans for long term decarbonisation of heat in line with national emission reduction obligations and would likely be compatible with any country's decarbonisation goals.