Petrophysics studies of sedimentary rocks from a cross-hole seismic test site

Abstract

In order to provide a site for studying in detail seismic wave propagation over a large range of frequencies through layered sedimentary rocks, four vertical boreholes of approximately 260 m depth and maximum distance apart of 95 m have been drilled through five cyclical sequences of shallow-dipping sandstone, mudrock and limestone formations in the Carboniferous Limestone series in northern England. Two of the boreholes were fully cored and a number of core samples from these were carefully preserved, at 1 m intervals in the first and at selected intervals in the second. Bulk density, porosity and P- and S-wave velocities and attenuation in the vertical and horizontal directions have been made on a total of 180 specimens from the preserved core samples. Permeability and electrical properties have been measured for the sandstone samples. Multiple linear regression equations have been derived relating the P- and S-wave velocities to porosity and clay content. These equations indicate the relative importance of increases in porosity and in clay content on decreases in all velocities, and the relative insensitivity to changes in clay content of PH- and SH-wave velocities. For the samples tested, the average anisotropy observed under in situ stress conditions is approximately 0.5% for limestones (porosities to 12%) and 5% for sandstones (porosities in the range 2 to 18%). For mudrocks (porosities in the range 2 to 15%), the average anisotropy is 21% for P waves and 27% for S waves. The results have been used to confirm the magnitudes of anisotropy observed in cross-hole seismic tomography at the site, in modelling the seismic response of sedimentary rocks with an appreciable clay content, and in modelling velocity dispersion and attenuation for rocks at the site over a wide range of frequencies.