Shales are characterized by unimodal pore-size distributions. The relationship between shale permeability (k) and the mode (dm) of these distributions has been investigated, for 22 tight shale samples from Canadian offshore and interior regions, to determine the degree of accuracy and reliability of using dm to estimate shale permeability. Such an attempt is made, using an indirect method, because tight shales represent some of the lowest permeabilities (10-22 m2 to 10-19 m2) and are extremely difficult to measure. Results indicate that, although a considerable scatter exists for k plotted against dm, a relationship of significance exists. Estimates of k using dm are within one order of magnitude of the measured values for 90% of the data, and within a factor of 3 for 65% of the data, using the equation: k = 10-21exp(0.13dm - 0.68) m2 Improved estimates of k may result from a better understanding of the role of other pore-structure parameters.

Fifty-seven shale samples from several Cretaceous stratigraphic units (Viking Formation of Colorado Group to Belly River Group) in the southern section of the Western Canada Sedimentary Basin were obtained for porosity analysis by helium and mercury porosimetry. This is part of a joint study between PanCanadian Petroleum Limited and the Geological Survey of Canada to investigate the source of subhydrostatic pressures within the basin. Results indicate that these samples have petrophysical characteristics of extremely tight and well compacted shales, suggesting that most samples have experienced maximum paleoburial depths of more than 2.5 -- 3.0 km. Given current sample depths of 0.5 -- 2.8 km this implies up to 2 km of erosion over the study area. These shales are generally at an early stage of diagenetic alteration. There is a general east to west decrease in pore-surface area, suggesting either an increase in degree of cementation or grain size or both, with some exceptions in the deeper formations.

Ten shale samples from several Cretaceous stratigraphic units (Colorado Group) of the Western Canada Sedimentary Basin were obtained for determination of effective porosity (øE) and apparent formation-factor (Fa) as a function of confining pressure (Pc). These samples are part of a study to investigate the possibility that shale petrophysical property changes, in response to unloading by erosion of overburden, have contributed to the development of subhydrostatic pressures within the Cretaceous succession of the basin. Results indicate that øE (0.9-13%) and Fa values (200-3800) at different confining pressures (Pc: 0-65 MPa) show trends that are typical of shale, with øE decreasing and Fa increasing with increased Pc; however, one sample from a siderite concretion unexpectedly displays a reverse Fa-Pc relationship. There are subtle indications that increased pore compressibility may be associated with abnormally pressured shale. These data form the basis for detailed analysis and interpretation in forthcoming studies.

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