[en] A method is described for logging a borehole which penetrates a number of subsurface formations and which comprises: 1) bombarding a formation penetrated by the borehole with high energy neutrons to obtain inelastic gamma rays and neutron capture gamma rays, 2) measuring the relative amounts of inelastic gamma rays at an energy level characteristic of a first element contained in fluid within the formation, 3) measuring the relative amount of inelastic gamma rays at an energy level characteristic of a second element contained in the formation fluid, 4) obtaining the ratio between the relative amounts of inelastic gamma rays for the first and second elements, 5) measuring the relative amount of neutron capture gamma rays at an energy level which is characteristic of a third element in the formation matrix, 6) measuring the relative amount of neutron capture gamma rays at an energy level which is characteristic of a fourth element in the formation matrix, 7) determining the ratio between the relative amounts of neutron capture gamma rays of the third and fourth elements, and 8) comparing the inelastic gamma ray ratio to the neutron capture gamma ray ratio to determine the type of fluids contained within the formation and to determine the lithology of the formations. (author)