Evolution characteristics of biogas in seabed sediments and their influencing factors on gas sources

In order to seek for the influencing factors on the biogenic gas sources in marine natural gas hydrate deposits and its genetic model, three marine sediment samples were collected for microbial evolution experiment. Various biogas production conditions, such as temperatures, pH, carbon source, carbon source concentration, nitrogen source, and formation salinity are changed to look for their effects on the biogas production by methanogenic bacteria in seabed sediments. The experimental results suggest that the methanogenic bacteria reach the highest methane production rate when temperature is at 55 ℃. The production remains high when pH changes between 6～8, and there is still methane gas produced until pH increased to 10. Both the nitrogen or carbon have certain degrees of promoting effect. However, carbon source concentration will inhibit methane gas production if it is too high. Change of formation salinity have no significant effect on methane gas production as the experiment indicates. According to the microbial evolution and gas production rate at different temperatures, biogas evolution can be divided into three stages: early stage, peak stage and late stage. When pH is between 6～8, and lactose (disaccharide) in concentration of 2.0 ml/L is selected as carbon source, and protein selected as the main nitrogen source, the biogas production rate of methanogens is significantly enhanced. Based on the geothermal gradient and environmental conditions of the sampling area, it is inferred that the major biogenic gas source rock is buried in a depth of about 200～500 m, and the area with weak alkalinity and weak runoff is the most favorable exploration target.