Microbial Enhanced Oil Recovery (MEOR) represents one of the promising methods to extract the remaining immovable
hydrocarbons from different reservoirs. Selection of bacteria is the important criteria for recovering the immovable
residual hydrocarbons. The investigation on Clostridium tyrobutyricum 90F and DSMZ 663 strain in North Sea – been
studied in laboratory for different salinity, pressure and temperature conditions in carbonate formation based on different
physical and chemical conditions of the reservoir. The fundamental understanding of structure and function of Clostridium
tyrobutyricum and its linkage to biogeochemistry in reservoirs and understanding of mechanisms in MEOR environmental,
microbial and physiological process is a prerequisite to developing microbial methods to enhance the quantity and quality
of the hydrocarbon (HC) recovery. The average porosity increase value was about 0.04 representing an increase of about
11% per sample. Also density of the sample is increased due to the bulk volume of the samples that generally reduced due
the dissolution of the rocks after microbial treatment.