Interpretation of lithology from the formation resistivity of unconsolidated argillaceous/arenaceous sediments

A simple physical model for ionic conduction in shaly sands is modified and adapted to unconsolidated clayey sands. A brief review of the effects of porosity and cation exchange in clays on formation resistivity is made, assuming a linear relationship between clay content and porosity. The formation factor can be expressed in terms of the modified porosity and used to calculate the formation resistivity. The formation resistivity is adjusted for the effects of clay conductance under different saline conditions. Characteristic curves are constructed for particular environments, showing the effects of variations in clay content and porosity on formation resistivity. The use of characteristic curves and equations derived from the model is recommended as a semiquantitative aid to interpreting lithology from apparent resistivity data.