Anaerobic microbial degradation of crude oil is the main reason of the formation of heavy oil reservoir and associated gas reservoir. In order to study the formation process of heavy oil and associated gas by anaerobic microbial degradation of crude oil，shallow gas and heavy oil in Linfanjia area were studied. The heavy oil generated from the same source rock was selected for biodegradation experiment using oil-degrading bacterial consortium of the anaerobic microbe. It was found that the thin oil could be biodegraded and transformed to heavy oil，and at the same time 3 mmol methane and 0.5 mmol carbon dioxide could be produced per gram of crude oil on average after 248 days of degradation. The analysis on the produced gas carbon isotopes indicates that the δ13C1 of CH4 ranges from -46.36‰ to -45.27‰ and the δ13C1 of CO2 ranges from 4.24‰ to 8.5‰. Biodegradation gas in shallow gas of Linfanjia area accounts for 69% based on calculation result of carbon isotope data. The content of saturated hydrocarbon in crude oil was greatly reduced from 72.77% to 44.0%，which is the main reason of crude oil densification. Biomarker 25-norhopane/hopance significantly rose from 0.009 to 0.056，which indicates that the oil has suffered serious biodegradation. The process of crude oil biodegradation can be simulated in laboratory with shorter time.