Porosity, cation-exchange capacity (CEC), and electrical conductivity have been measured on 28 doleritic minicores from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Hole 504B (Legs 137, 140, and 148). Conductivity experiments were performed under five different saturating-fluid salinities, at room temperature and pressure, and with a two-electrode conducting cell. At 4 kHz, the porosity is related to the "intrinsic" formation factor by an inverse power law similar to Archie's formula F = a phi–m with m = 0.94 and a = 16 for the fine-grained specimens. Such a low m value suggests a current conduction in cracks and microcracks present at mineral scale throughout the rock. The presence of clays as alteration products is reflected by the high values of CEC. As expected, QV/F (where QV is the CEC per unit pore volume) and the surface conductivity are well correlated. This point leads to an estimate of the surface tortuosity. The temperature dependence of the surface conductivity is extracted using data on DC electrical conductivity at different temperature. For temperature correction purposes, a linear temperature dependence appears to be a good estimate for this parameter.
On the basis of these results, an equation was given that allows estimation of the porosity from resistivity measurements in Hole 504B. This equation was tested by comparing the predicted conductivity against the measured conductivity of a large set of samples.
Date of initial receipt: 22 August 1994
Date of acceptance: 14 March 1995
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