Nuclear log interpretation is simply the practice of solving tool-response mixing-law equations with the judicious application of some assumptions and constraints. As more factors are taken into account, the interpretation usually improves, but the model becomes more complicated. For the neutron-porosity log, the simplest interpretation model is to naively accept the raw log reading. If the reservoir is shaly, or if the fluid density is not the same as water, a hydrogen-index linear-mixing law will generally do. This equation can be solved for ϕe given the neutron-tool response to each of the various formation components.