Fracture Partitioning and its Controls on the Distribution of Transmissivity in Sedimentary-Rock Aquifers, Chateauguay River Basin, Province of Quebec

The Chateauguay River Basin in the Province of Quebec is underlain by sedimentary rocks of Lower Paleozoic age. These gently dipping sandstones and dolostones are interspersed with orthogonal sets of subhorizontal bedding-plane partings and subvertical joints; it is through this network of fractures that groundwater flow is primarily conducted. Geophysical logs obtained in 13 boreholes were used to identify and characterize the three primary rock units: (1) relatively soft arkosic sandstones [Covey Hill], (2) well-cemented quartz-rich sandstones [Cairnside], and (3) dolostones with dolomitic sandstones [Beekmantown]. Acoustic televiewer images of the borehole walls identify a fracture partitioning scheme unique to each formation, where joint spacing (S) and bed thickness (H) are related to rock elastic properties as determined from the full-waveform sonic logs. Results of hydraulic tests in these boreholes using both flowmeter and straddlepacker techniques confirm that groundwater flow is predominantly controlled by distinct bedding-plane partings that have transmissivities several orders of magnitude greater than those of the competent rock matrix. Moreover, there appears to be a correlation between the spatial distribution of these permeable features and the rock unit. In the softer sandstones, beds are more massive (H=49 cm) and joints are sparse. On average, only one permeable zone is identified for every 80 m depth, but it may extend laterally for several km based on curve matching of gamma logs and results of pumping tests. The hard sandstones have thinner beds (H=36 cm) and more closely spaced joints. Permeable zones are intercepted more frequently (approx. 5 per 80 m depth), but hydraulic communication appears to be more limited laterally. Finally, dolostones have even thinner beds (H=27 cm) and more closely spaced joints, but the frequency of permeable zones appears to be intermediate between the other two rock types (approx. 2 per 80 m depth). Moreover, no extensive horizontal connection between boreholes has been observed. These results provide information on the vertical distribution of transmissivity with respect to rock type and its particular fracturing scheme, and permit us to make general observations regarding horizontal to vertical transmissivity anisotropy.