Logging apparatus and method for determining absolute elemental concentrations of subsurface formations

摘要

Logging apparatus and methods for detecting first signals indicative of the absolute concentrations of the first category of elements in the rock matrix, for irradiating the formation with a pulse of high energy neutrons and for detecting a second signal indicative of the relative concentrations of a second category of elements in a rock matrix. The absolute and relative concentrations of a plurality of elements are determined, and this information is used to transform at least one of the relative concentrations into an absolute concentration. One of the measured absolute concentrations is for aluminum, from which correlations are used to determine the absolute concentrations of other, non-measured elements. A tool system for measuring the absolute aluminum concentration includes a californium-252 source and a gamma ray detector having a plurality of windows from which the aluminum count rate can be compensated for interference by manganese activation. A further correction to the aluminum count rate has a functional dependence on the neutron slowing down length in the formation, the formation macroscopic cross section, the borehole microscopic cross section, the bulk density of the formation, and the borehole diameter. When used in conjunction with a density tool sensitive to the effects of photoelectric absorption of gamma rays in the formation, the techniques of the invention allow the magnesium concentration to be determined.