OS2B-6:MICROBIAL MODEL FOR BIOGENIC GAS HYDRATES IN INDIAN DEEP WATERS

Dr. A.K. JAIN
Domain Expert, Unconventional Oil & Gas, ONGC, INDIA

    Gas hydrate as potential source of energy is reported from Permafrost and deep oceanic areas. To establish the presence and its resource potential in India, 21 test wells were drilled in Indian deep waters in bathymetry range of 900-2700 m. in Western, Eastern, Mahanadi and Andaman offshore under the aegis of National Gas Hydrate Program (NGHP) of India, with special emphasis on understanding the geological, geochemical and microbial controls on the occurrence, formation and dissociation of gas hydrates Biogenic methane constitutes the major source of methane hydrates encountered so far world wide, thus study of microbial communities, including the rate at which they produce methane, their probable natural substrates, their interrelationship, or even their distribution with respect to the gas hydrate deposits is needed. Therefore microbial profiling of these wells in respect of methanotrophs, SRB and methanogens has been carried out to understand the interplay between microbial processes of methanotrophy and methanogenesis involved in the formation and dissociation of gas hydrates.  Sediment samples were collected from all the wells at every 10 meter of the cored depth under aseptic and anaerobic conditions on board ’JOIDES Resolution’. The cold spot IR anomalies were observed and were taken as the basis for presence of physical, dispersed and dissociated gas hydrates in the well. Microbial analysis was performed by following standard protocols.

    The sediments from a well from the western continental margin in the bathymetry of 2663 m, did not show any evidence of methanogens or any other proxies for gas hydrate.  Methanotrophs and SRB count vary 450 cfu/g and 900 cfu/g respectively.  BSR is also of low amplitude.

    All other sites of eastern margins (Mahanadi and KG Basin) and Andaman depicted well defined BSR. However, Gas hydrates were physically recovered from only two wells of KG eastern offshore. It is worth mentioning that one of the well contained massive gas hydrate from the depth of 30 to 160 meters. Hydrates were visible occurring as pore filling or as inclusions. In some intervals the gas hydrate formed the sediment matrix with gas hydrate saturation values exceeded 80% of the pore volume. The sediments with intact gas hydrates contained 6500 cfu/g and 1900 cfu/g of methanotrophs and sulfate reducers up to 30 m bsf and methanotrophy and sulfate reduction are predominant processes. Methanogens are encountered below 30 m indicating sulfate methane interface at about 30-40 m and corroborates with profiles of methane and sulphate.

    Methanogens have been confirmed by detecting diagnostic mcrA gene by quantitative PCR and agarose endo-electrophoresis and cultured successfully. In vitro methanogenesis experiments by deploying isolated methanogens from the gas hydrate associated sediments resulted in production of very low quantity (100 ppm) of methane gas in 30 days. Low methanogenic activity rules out continuing of in-situ methanogenesis. CO2 reduction pathway of methanogenesis predominate, however acetate fermentation pathway is also observed.  H2S production coupled with methanogenesis and δ13C value of produced methane and CO2 as –55 and -19 per mil indicates anaerobic methanotrophy. The compositional and carbon isotopic analysis of hydrate dissociated gas comprise of only methane with carbon isotopic value (δ13C) as –61.4 per mil, indicating bacterial origin of methane in gas hydrate. TOC percentage is good (average 1.7%). However, the concentration is comparatively less especially in gas hydrate embedded sediments.

    The sediment from Mahanadi and Andaman offshore, wherein very good quality of BSR was detected, no gas hydrate has been encountered. Only free gases of mixed origin (biogenic mixed with thermogenic) is encountered. These sediments show methanotrophs up to 30 m in the range of 4000-7000 cfu/g, while the SRB and Methanogens were detected up to 100 m in varying abundance. Methanogenesis and anaerobic methanotrophy prevailed along the entire cored depths. The IR cold spot anomalies inferred it as dispersed gas hydrate but it seems possible that inter play of predominant anaerobic methanotrophy coupled with methanogenesis is responsible for absence of gas hydrate. 

    Microbial studies suggests that bacterial methane generated at shallower depths by utilization of simple organic substrate got buried down under rapid sedimentation and formed gas hydrate at gas hydrate stability zones and only pockets of methane hydrates are expected despite large BSR depicted areas