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Characterising the pore structure of gas shales is of critical importance to establish the original gas in place and flow characteristics of the rock matrix. Methods of measuring pore volume, pore size distribution, and sorptive capacity of shales, inherited from the coalbed methane and conventional reservoir rock analyses, although widely applied, are of limited value in characterising many shales Helium which is routinely used to measure shale skeletal and grain density, permeability and diffusivity, has greater access to the fine pore structure of shale than larger molecules such as methane. Utilizing gases other than He to measure porosity or flux requires corrections for sorption to be incorporated in the analyses. Since the permeability of shales vary by several orders of magnitude with effective stress, methods that do not consider effective stress such as crushed permeability, permeability from Hg porosimetry, and from desorption are of limited utility and may be at best instructional. For shales investigated to date, clay-rich rocks have higher porosity and permeability than biogenic silica-rich shales or carbonate-rich shales. Shales rich in detrital quartz have higher porosity and permeability than shales rich in biogenic quartz and hence simply knowing the mineralogy of a shale may not be diagnostic. The porosity of most shales is mainly dependent on the degree of pore volume development in pores less than 10 um. Quantifying total gas in place in shales by much of the industry using coal desorption methods and porosity and water saturation determinations, developed for conventional reservoir rocks, may lead to substantial errors. Canister 'desorption' methods applied to gas shales routinely captures free and solution gas as well as sorbed gas which, if considered as only sorbed gas, results in a significant overestimation of gas in place. A proprietary method of analyses, referred to as MARIO, results in rigorous total gas in place determinations that avoids errors including those associated with molecular sieving and provides a maximum value of the sorbed gas contribution to total gas.

The Lower Jurassic Gordondale Member is an organic-rich mudrock and is widely considered to have potential as a shale gas reservoir. Influences of Gordondale mudrock composition on total gas capacities (sorbed and free gas) have been determined to assess the shale gas resource potential of strata in the Peace River district, northeastern British Columbia. Sorbed gas capacities of moisture-equilibrated samples increase over a range of 0.5 to 12 weight percent total organic carbon content (TOC). Methane adsorption capacities range from 0.05 cc/g to over 2 cc/g in organic-rich zones (at 6 MPa and 30ºC). Sorption capacities of mudrocks under dry conditions are greater than moisture equilibrated conditions due to water occupation of potential sorption sites. However, there is no consistent decrease of sorption capacity with increasing moisture as the relationship is masked by both the amount of organic matter and thermal maturation level. Clays also affect total gas capacities in as much as clay-rich mudrocks have high porosity which may be available for free gas. Gordondale samples enriched with carbonate (calcite and dolomite) typically have lower total porosities than carbonate-poor rocks and hence have lower potential free gas contents. On a regional reservoir scale, a large proportion of the Gordondale total gas capacity is free gas storage (intergranular porosity), ranging from 0.1-22 Bcf/section (0.003-0.66 m 3 /section). Total gas-in-place capacity ranges from 1-31.4 Bcf/ section (0.03-0.94 m 3 /section). The greatest potential for gas production is in the south of the study area (93-P) due to higher thermal maturity, TOC enrichment, higher reservoir pressure, greater unit thickness and improved fracture-potential. RÉSUMÉLe Membre de Gordondale du Jurassique inférieur est un mudstone organique riche, et est généralement considéré comme présentant un potentiel de réservoir de gaz de schiste. Les influences de la composition du mudstone du Membre de Gordondale sur ses capacités en gaz total (gaz sorbé et gaz libre) ont été déterminées afin d'évaluer le potentiel de ressource en gaz de schiste des couches dans le district de Peace River, au nord-est de la Colombie Britannique. Les capacités en gaz sorbé d'échantillons, en teneur d'humidité équilibrée, augmentent sur une gamme allant de 0.5 à 12 pour cent du poids du contenu en carbone organique total (TCO). Les capacités d'adsorption du méthane vont de 0.05 cc/g à plus de 2 cc/g dans les zones organiques riches (à 6 MPa et 30ºC). Les capacités de sorption des mudstones, dans des conditions sèches, sont meilleures que dans des conditions d'humidité équilibrée, en raison de la présence d'eau dans les sites de sorption potentiels. Toutefois, il n'y a pas de diminution consistante de la capacité de sorption avec une augmentation de l'humidité, car cette relation se trouve masquée à la fois par la quantité de substance organique et le niveau de maturation thermale. La présence d'argiles affecte également les capacités de gaz total dans la mesure où les mudstones, rich...