Abstract: Numerous studies have focused on factors influencing biogenic enhancement of coalbed methane (CBM). However, less attention has been given to the surrounding rock formations associated with coal, which may also impact coalbed methane enhancement. Simulated experiments with coal were conducted under conditions with various surrounding rock types to investigate their effects on biogenic gas production. Results indicate that the addition of mudstone or sandy mudstone increased biogenic gas production by 8.7%–34.7% and raised methane concentration by 2.56%–3.37%. This enhancement is mainly attributed to the addition of sandy mudstone, which increased trace element Fe in the fermentation system, thereby optimizing the microbial community structure. Specifically, the abundance of Synergistota (electrochemical bacteria) and Euryarchaeota (archaea) significantly increased, along with genes related to methane metabolic pathways. Furthermore, the gene abundance of Fe receptor fepA increased, and there was enhanced enrichment of transport gene for Co and Ni (cbiO, cbiM, cbiQ, and cbiN). These findings reveal the influence of mudstone and sandy mudstone on coalbed biogenic gas formation, providing experimental insights for enhancing biogenic gas production in coal-related systems.