The present invention is generally directed to a method for releasing trapped oil in reservoirs. The method hereof includes identifying a reservoir, obtaining a microbial community sample of the reservoir, maintaining the sample under high temperature and high pressure conditions that mimic natural conditions of the reservoir, growing the sample on at least one substrate, determining a targeted treatment regime for the reservoir based on the positive growth of the sample on the substrate, injecting the reservoir with the targeted substrate to release trapped oil in the reservoir, monitoring the reservoir, and extracting the oil from the reservoir.