The Activity of sulphate reducers, saprophytes and liquid paraffin oxidizers in water samples from water sources and watered oil horizon L of Laojunmiao oil field was examined. Their major activity,especially that of sulphate reducers, takes place in the bottom hole zones of injected wells. One ml of the stratum water was found to contain hundreds thausands to millions of sulphate reducers in some cases. The max. content of H2S is about 120mg/L. The active development zones of bacteria were calculated to be...