The equation to compute the volume of clay from a neutron-density cross-plot is known as the "neutron-density clay volume equation." This equation is derived by integrating the density and neutron porosity logs to calculate the total porosity, and then using a clay model to estimate the amount of clay in the formation.

The equation is as follows:

Vclay = (PHIe - PHIh) / (PHIe - PHIsh)

Where:

Vclay = volume of clay

PHIe = effective porosity

PHIh = total porosity

PHIsh = porosity of shale

Note that this equation assumes that the formation contains only clay and shale. If other minerals are present, they may need to be accounted for separately.

Source: "Well Logging for Earth Scientists" by Darwin V. Ellis and Julian M. Singer.