Porosity structure and distribution in the Antrim shale: implications for biogenic gas generation, storage and transport

Biogenic gas shales, predominantly microbial in origin, form an important class of organic-rich shale reservoirs with a significant economic potential. Yet large gaps remain in the understanding of their gas generation, storage, and transport mechanisms, as previous studies have been largely focused on mature thermogenic shale reservoirs. In this work we analyze porosity structure, pore sizes and pore types of the Antrim Shale from Michigan Basin, one of the major biogenic and mixed-origin gas plays in the US, using a combination of SEM, MICP, XRD, helium porosimetery, low-pressure gas sorption, and organic petrology studies. These observations are then used to explore the implication for desorption and non-Darcy flow processes, as well as microbial colonization and gas generation in low-maturity shales. Unlike thermogenic gas shales, minerals host most of the porosity in the Antrim shale, especially larger meso- and macro- pores, while organic matter pores play a minor role and are generally small and isolated. Mineral-related pores contribute most to the total pore structures. Since up to 80% of gas is stored in adsorbed form, understanding desorption and diffusion processes in the mineral matrix are central for more accurate prediction of gas production in such reservoirs with progressive depletion. The micropore size distribution (from CO2 adsorption) and the meso-/macro- pore size distribution (from N2 adsorption) show a multimodal pattern and differ between various Antrim members. The majority of pores, however, and especially the pore throats, appear too small to support movement of methanogens through the rock, which emphases the importance of natural fractures for microbial colonization of the formation.