The nanometer-scaled pore systems of gas shale reservoirs were investigated from the Barnett, Marcellus, Woodford, and Haynesville gas shales in the United States and the Doig Formation of northeastern British Columbia, Canada. The purpose of this article is to provide awareness of the nature and variability in pore structures within gas shales and not to provide a representative evaluation on the previously mentioned North American reservoirs. To understand the pore system of these rocks, the total porosity, pore-size distribution, surface area, organic geochemistry, mineralogy, and image analyses by electron microscopy were performed. Total porosity from helium pycnometry ranges between 2.5 and 6.6%. Total organic carbon content ranges between 0.7 and 6.8 wt. %, and vitrinite reflectance measured between 1.45 and 2.37%. The gas shales in the United States are clay and quartz rich, with the Doig Formation samples being quartz and carbonate rich and clay poor. Higher porosity samples have higher values because of a greater abundance of mesopores compared with lower porosity samples. With decreasing total porosity, micropore volumes relatively increase whereas the sum of mesopores and macropore volumes decrease. Focused ion beam milling, field emission scanning electron microscopy, and transmission electron microscopy provide high-resolution (∼5 nm) images of pore distribution and geometries. Image analysis provides a visual appreciation of pore systems in gas shale reservoirs but is not a statistically valid method to evaluate gas shale reservoirs. Macropores and mesopores are observed as either intergranular porosity or are confined to kerogen-rich aggregates and show no preferred orientation or align parallel with the laminae of the shale. Networks of mesopores are observed to connect with the larger macropores within the kerogen-rich aggregates.

The geological controls on methane sorption capacity for the Lower Cretaceous Buckinghorse Formation and equivalent strata in northeastern British Columbia, Canada have been investigated. The methane sorption capacity ranges between 0.04 to 1.89 cm³/g at 6 MPa (3.2 to 60.4 scf/ton at 870 PSIA) and the corresponding total organic carbon (TOC) content is between 0.5 and 17 wt%. Equilibrium moisture content is between 1.5 and 11 wt% and the organic maturity measured by T_max ranges between 416°C (immature) and 476°C (overmature). TOC content is the most significant control on methane sorption capacity, however, other important factors include the kerogen type, maturity and clay content, in particular the abundance of illite. A positive correlation exists between the TOC content and methane capacity. Samples with higher surface area have higher methane sorption capacities. The micro- and mesoporous surface area increase with TOC and illite content. On a per unit TOC volume basis, type II/III and III kerogens have higher methane sorption capacity compared to types I and II because of their higher micropore volumes. Micropore volume on a per unit TOC volume basis increases with maturity for all kerogen types. Across the study area there is a decrease in TOC concentration with increasing maturity which, in part, is attributed to hydrocarbon generation but also coincidently reflects the different depositional environments. The amount of illite also increases with maturity through the process of illitization. No correlation exists between moisture content and methane capacity. Samples with high moisture content can have high methane capacities which indicate water and methane molecules occupy different sorption sites.