With increased demand for fossil fuels of more than 50% in the next 25 years, several methods of either enhancing oil and gas production from existing fields or finding new fields and tackling unconventional sources, such as shale gas reservoirs, are currently underway. Microbially-generated methane gas is a significant portion of commercial gas production around the world. At least 20% of the world's methane originates from methanogens that reside within organic-matter-rich shales and coals. The metabolic processes and chemical reactions carried out by microorganisms in shale gas reservoirs are currently unknown making it difficult to predict or enhance gas generation rates within a given reservoir. Field production data reveal that gas production from these reservoirs declines initially and then stabilizes after a specified time. The stabilized rate is controlled by contributions from biogenic gas generation, desorption of gas from kerogen, and diffusion and transport of gas through nanometer to potentially even micron scale pore systems. It still remains unclear which one of microbial gas generation or gas transport is the key limitation on production of biogenic gas from shale gas reservoirs. This paper presents a modified gas material balance on production data for shale gas wells to account for biogenic gas generation. Although the gas material balance approach is well established, it has not been used to estimate biogenic gas generation rates. By using actual gas production data from a field where biogenic gas production is known to be the main source of gas generation, the amount of biogenic gas that was produced within the reservoir during the resource lifetime of the well could be determined. The results of the theory presented here were compared to gas production data from Nexen's Bigstick Field and Husky's Abbey Field. The results reveal that a significant fraction, up to about one-third, of gas production is sourced from constant biogenic gas generation. The implication is that biogenic shale gas productivity can be potentially enhanced if microbes are stimulated. (c) 2012 Elsevier Ltd. All rights reserved.