NOVELTY - The method involves testing a microbial community sample by incubating the sample in a liquid substrate including oil in a reservoir simulator. A targeted regime for a subterranean light-crude oil reservoir is determined based on positive growth of methane-producing microbes from the sample in the liquid substrate. A substrate for providing positive growth of methane-producing microbes is injected into the reservoir to enable conversion of the oil into methane gas. The reservoir is monitored. The methane gas is extracted from the reservoir.    USE - Method for converting oil trapped in a subterranean light-crude oil reservoir into methane gas and extracting the methane gas from the reservoir. Can also be used for repressurizing the reservoir to extract the oil and reducing oil viscosity in the reservoir to extract additional oil, for carbon dioxide or water-after-gas carbon dioxide floods, for producing acid in a carbonate reservoir to dissolve reservoir rock, for co-producing solvents and surfactants and production of carbon dioxide to repressurize reservoir, for heavy oil deposits, for biodegrading large, heavy-oil molecules, for in-situ bio-upgrading of heavy oil and oil sands deposits, and for bio-converting trapped or uneconomical hydrocarbon deposits into methane.    ADVANTAGE - The method enables growing multiple reservoir microorganisms under a set of anaerobic conditions and a set of reservoir conditions of high temperature and high pressure. The method enables enhancing recovery of oil, and increasing an amount of oil based on an increased displacement of oil and/or improved volumetric efficiency of flooding process. The method enables reducing potential low costs for methane gas extraction such that multiple mechanisms are simultaneously worked to enhance effectiveness of the methane gas extraction in an eco-friendly manner. The method enables exploiting indigenous reservoir microorganisms by using only nutrient infusions similar to existing waterfloods. The method enables complementing and enhancing selected secondary and tertiary recovery process so as to assure a set of predictable and reliable microbial enhanced oil recovery (MEOR) results by characterization of reservoir response.    DETAILED DESCRIPTION - The substrate is selected from a group of molasses, fertilizer, paper/pulp waste, sugar beet waste, brewery waste or feedlot waste. The substrate has a targeted nutrient for producing a targeted product or metabolic byproduct or gas selected from a group of carbon dioxide, methane, hydrogen and hydrogen sulfide.