Abstract: The problems of preservation and conversion of organic substances in sedimentary regions of present water spheres, oil pools as well as coal mines were discussed with respect to energy flows, food chains of microbial ecosystems and microbiological processes of C.N.S. cycles at the same time changes of C/N in present sediments were also described in detail. It has been pointed out that in aerobic conditions, organic substances can not be preserved because microogranisms mineralize them into CO₂. In anaerobic conditions, when microorganisms consume organic substances, in addition to CO₂, small molecular organic and reduced inorganic products are produced. Energy is stored in them, which results in a more complex process of substance and energy conversion than that under the former condition. But both aerobic and anaerobic are present in today's sediments. ( 1 ) The range of energy source substance available for microorganisms is reduced with the decrease of oxidative abilities of electron accepters O₂, NO₃, SO₄ and C0₂, thus the range and the amount of organic substance to be consumed by microorganisms become limited. It follows that food chain ( or food net ) turns more complex, which helps preserve a large amount of organic substance. ( 2 ) Energy produced by metabolates in anaerobic microorganisms, is partially transferred to sediments so as to absorb O₂ and to create reduced phase, which is favourable for the preservation of organic substance. The mechanisms as mentioned above have been dealt with in detail and some instances were taken to explain the relationships and the reaction levels between microorganisms and energy conversions. In short, the types, the enriched levels of organic substances and O₂ volume are the key factors for the preservation of organic substance. In addition, some microbial genus and species in oil pools and coal mines, and their roles in substance and energy conversions as well as necessary ecological conditions have also been described.