A method is proposed for identifying the carbon source for methane generated by carbonate-reduction microbial methanogenesis using the isotopic and compositional mass balance of carbon in cogenerated carbon dioxide (CO2) and methane. Microbial methanogenesis from bitumen or petroleum generates more methane than CO2, so the carbon isotopic compositions of methane and CO2 are relatively heavy and similar to the carbon isotopic composition of thermogenic methane. Microbial methanogenesis that uses kerogen or recent organic matter as a carbon source generates relatively more CO2 than methane; therefore, methane and CO2 have the light carbon typical of shallow microbial methane. This concept was quantified and tested against three literature examples to determine if the relative amount of CO2 generation could be estimated with sufficient resolution to predict source carbon type and generation temperature in gas shales and coalbed methane. Antrim Shale (Michigan) gases are demonstrated to be derived mainly from immature bitumen in the shale at modem reservoir temperatures and cooler temperatures. Fruitland coal gases in the western San Juan Basin are derived mainly from bitumen in oil-window maturity coals at modern temperature. Southwest Indiana coal gases are derived mainly from thermally immature kerogen at modern and warmer temperatures. Identification of the methane carbon source and temperature helps delineate microbial methane resource fairways, where fairways are limited by microbial gas generation. Temperatures may help determine if microbial methane is actively generated today or a relict from earlier microbial generation.