Changes

The resistivity of most soils and rocks (including virtually all of the rocks of interest to hydrocarbon exploration) at the frequencies utilized by electrical methods is controlled by the fluids contained within the rock<ref name=pt07r46>Parkhomenko, E. I., 1967, Electrical properties of rocks: New York, Plenum Press, 314 p.</ref> (see [[Determination of water resistivity]]). This is because the dry soil or rock matrix is a virtual insulator at DC and near DC frequencies. The pore fluid is in most cases water, with dissolved salts. The salinity is the primary factor in determining the resistivity of the pore fluid, with pore configuration also playing a part. Of lesser importance at oil reservoir depths is the temperature of the formation. Oil and/or gas, when present, occur over such limited formation thicknesses that their effects on bulk average resistivity is, in most cases, undetectable.

The resistivity of most soils and rocks (including virtually all of the rocks of interest to hydrocarbon exploration) at the frequencies utilized by electrical methods is controlled by the fluids contained within the rock<ref name=pt07r46>Parkhomenko, E. I., 1967, Electrical properties of rocks: New York, Plenum Press, 314 p.</ref> (see [[Determination of water resistivity]]). This is because the dry soil or rock matrix is a virtual insulator at DC and near DC frequencies. The pore fluid is in most cases water, with dissolved salts. The salinity is the primary factor in determining the resistivity of the pore fluid, with pore configuration also playing a part. Of lesser importance at oil reservoir depths is the temperature of the formation. Oil and/or gas, when present, occur over such limited formation thicknesses that their effects on bulk average resistivity is, in most cases, undetectable.

Faulting or [[Fracture|fracturing]] of porous sedimentary formations in most instances has little effect on the bulk average resistivity since the additional fracture [[porosity]] changes the already high porosity by only a small percentage. However, in very tight rocks, such as igneous, metamorphic, and nonporous carbonate rocks, where intrinsic porosity is very low, the fluids in joints, cracks, and faulted zones may become the primary conducting paths (see [[Porosity]]).

Faulting or [[Fracture|fracturing]] of porous sedimentary formations in most instances has little effect on the bulk average resistivity since the additional fracture [[porosity]] changes the already high porosity by only a small percentage. However, in very tight rocks, such as [[igneous]], metamorphic, and nonporous carbonate rocks, where intrinsic porosity is very low, the fluids in joints, cracks, and faulted zones may become the primary conducting paths (see [[Porosity]]).

In summary, the factors affecting ''in situ'' average resistivity are the total porosity, including fault and fracture porosity, and the resistivity of the fluids present within the rock. The average resistivity can be considered constant over the frequency range of interest to most of the methods under consideration here.

In summary, the factors affecting ''in situ'' average resistivity are the total porosity, including fault and fracture porosity, and the resistivity of the fluids present within the rock. The average resistivity can be considered constant over the frequency range of interest to most of the methods under consideration here.