ABSTRACT
Shales are generally considered to be mixtures of one or more clay minerals and silts, sands, and/or carbonates. Time, weight, and temperature modify the shales, squeezing out the water and other (organic) fluids. Thermal conductivities of clays and shales are lower than for sands and carbonates. Hard-rock environments usually are characterized by high resistivities, short acoustic travel times, moderate to high densities (hard rock types often show densities above 2.5 g/cc.), and low values of neutron capture cross section. Compaction of the shales will force some of the contained water and other fluids out of the shale. If the sands are hydraulically sealed by the shales, the excess fluids will begin to support the overburden, instead of the sand grains supporting it. The authors will discuss temperature logging later but it is routine to measure the borehole temperature for resistivity logging purposes.
