Subsurface microbial (biogenic) methane production is an important part of the global carbon cycle and has resulted in natural gas accumulations in many coal beds worldwide. Laboratory experiments indicate coal beds can act as natural geobioreactors and produce additional low carbon renewable natural gas with algal or yeast compounds, yet the effectiveness of these nutrients in situ are unknown. This study uses down-well monitoring methods in combination with deuterated water (99.99% D2O) and a 200-liter injection of 0.1% yeast extract to stimulate and isotopically label newly generated methane. A total dissolved gas pressure sensor was placed down-well into the Flowers-Goodale coal bed at the USGS Birney Test Site in the Powder River Basin, USA, to facilitate long-term real-time gas measurements 641 days pre-injection and for 478 days post-injection. Calculations from isotopic labeling from D2O indicates the methane concentration increased 132% above pre-injection levels 266 days post-injection. This study demonstrates the ability to both isotopically label newly generated methane in situ and monitor increases which has immediate implications for investigating methane generation in other environments.