Since almost 20 years it is known from stable isotope studies that large amounts of biogenic methane are formed in oil and coal reservoirs. The investigation of this degradation process and of the underlying biogeochemical controls are of great economical and social importance due to several reasons: (1) The understanding of reservoir biodegradation is of great use for the exploration industry, (2) a biotechnological stimulation of the methane formation in reservoirs could provide new economical perspectives, (3) the use of microorganisms to enhance oil recovery (MEOR) has proven to be successful. Even under optimal conditions, today not more than 30-40 % of the total oil in a reservoir are actually recovered. The majority remains in a polyphasic mixture with exploration water in the reservoir. The conversion to methane or recovery of at least parts of this non-recoverable oil or coal via an appropriate biotechnological treatment would provide an extensive and ecologically sound energy resource.