Microbial oil transformation processes accompanied by methane and hydrogen‐sulfide formation

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Abstract
The pattern of distribution and the proportions of microorganisms belonging to several physiological groups present in the formation waters of water-flooded petroleum-bearing rocks of the Apsheron Peninsula [USSR] were investigated. Hydrocarbon-oxidizing bacteria occur predominantly in the near-bottom zones of injection wells, accessible to dissolved O2. Anaerobic microorganisms (sulfate-reducing, fermenting, and methanogenic bacteria) were present in all zones affected by water flooding. The variety of bacterial species present in oil-bearing rocks was investigated. Using a radioisotope method, the existence of a contemporaneous process of CH4 generation in the formation waters of water-flooded oil-bearing rocks was revealed. Laboratory experiments showed that H2, CH4, and H2S generation from petroleum was possible in an aerobic-anaerobic microbial succession. A sequence of processes of petroleum hydrocarbon modification, resulting in the accumulation of CH4 and H2S, was revealed. Aerobic oxidation of petroleum by hydrocarbon-oxidizing bacteria resulted in the production of acetate and higher fatty acids. The products of the aerobic oxidation of petroleum were fermented with H2 and CO2 release. Acetate, H2, and CO2 were direct substrates for methanogenesis and sulfate reduction. In the latter process, higher fatty acids were also directly utilized. The possibility of syntrophic utilization of intermediate fatty acids and alcohols yielding CH4 and H2S is discussed.