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Permeability and diffusivity are critical parameters of tight reservoir rocks that determine their viability for commercial development. Current methods for measuring permeability and ⁄ or diffusivity may lead to erroneous results when applied to very tight rocks including gas shales, coal, and tight gas sands, as well as rocks considered as seals for nuclear waste repositories and strata for geological sequestration of CO 2 . The use of He as routinely applied to measure porosity, permeability, and diffusivity may result in non-systematic errors because of the molecular sieving effect of the fine pore structure to larger molecules such as reservoir gases. Utilizing gases with larger adsorption potentials than He, such as N 2 , and including all reservoir gases to measure porosity or permeability of rocks with high surface area is a viable alternative, but requires correcting for adsorption in the analyses. This study expands several approaches to measure permeability and diffusivity with considerations for gas adsorption, which has not been explicitly considered in previous studies. We present new models that explicitly correct for adsorption during pulse-decay measurements of core under reservoir conditions, as well as on crushed samples used to approximate permeability or diffusivity. We also present a method to determine permeability or diffusivity from on-site drill-core desorption test data as carried out to determine gas in place in coals or gas shales. Our new approach utilizes late-time data from experimental pressure-decay tests, which we show to be more reliable and theoretically (and practically) accurate than the early-time approach commonly used to estimate gastransport properties.

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.

S U M M A R YThe Queen Charlotte Fault zone is the transpressive boundary between the North America and Pacific Plates along the northwestern margin of British Columbia. Two models have been suggested for the accommodation of the ∼20 mm yr −1 of convergence along the fault boundary: (1) underthrusting; (2) internal crustal deformation. Strong evidence supporting an underthrusting model is provided by a detailed teleseismic receiver function analysis that defines the underthrusting slab. Forward and inverse modelling techniques were applied to receiver function data calculated at two permanent and four temporary seismic stations within the Queen Charlotte Islands. The modelling reveals a ∼10 km thick low-velocity zone dipping eastward at 28 • interpreted to be underthrusting oceanic crust. The oceanic crust is located beneath a thin (28 km) eastward thickening (10 • ) continental crust.