Electrical properties of basalts from DSDP Hole 504B: a key to the evaluation of pore space morphology

Abstract

Abstract Archie’s Law is a well known empirical relationship between electrical formation factor and porosity: FF = a φ − m . As such it is widely applied in the analysis and interpretation of downhole measurements. Despite its frequent application, however, the mechanics underlying the relationship that it embodies remain largely a mystery. A brief review of some previously published, but often neglected, data is used here to describe recent results for laboratory and downhole electrical measurements in basaltic ocean crust. The results disagree with many previously published studies of crystalline rocks in that a ≠ 1.0 and m is not always close to 2.0. Instead a increases with increasing fracturing, while m remains low and constant, though it increases slightly in the altered rock, possibly implying increased ‘tortuosity’. The value of a allows discrimination between fractured pillows and massive flows and may be inversely related to the degree of crack connectivity. Cation exchange capacity measurements on core samples show there is potential for surface conduction to occur, but probably only at higher temperatures. The precise nature of this temperature dependence is unclear, as is the interplay of the surface conduction mechanism with that of electrolytic (pore fluid) conduction.