In the recently released report entitled, "Emissions Associated with Electric Vehicle Charging: Impact of Electricity Generation Mix, Charging Infrastructure Availability, and Vehicle Type" the US DOE, NREL illustrates the carbon neutral status of renewable fuels. Then goes on to compare the overall emissions of compared vehicles.
While the report is focused on showing the emissions benefits of BEVs and PHEVs from clean and dirty electric grids around North America when compared to a conventional gasoline burning vehicle, they also use data that shows the carbon neutrality of electricity produced by bioCNG (biomethane), the renewable, chemically identical substitute of fossil CNG (fossil methane aka shale gas or generically "natural gas", though technically, it is all natural gas).
According to the DOE's AFDC website, BioCNG can be a carbon gain of as much as -117% depending on the feedstock source. Most BioCNG is at a minimum carbon neutral by definition, when best practices are used meaning the production is a closed system and has no fugitive emissions.