Elemental concentrations in igneous rocks determined by high-resolution borehole gamma-ray spectrometry and applications to petrochemical problems

A high-resolution gamma-ray spectrometric method using a 252Cf neutron source is employed to obtain a partial elemental analysis of an igneous rock down a borehole without taking a core sample. The concentrations of the elements Si, Al, Fe, Ca, Na, K, Ti, Mn, Cr, V, Ni, and B are measured in terms of their capture gamma-ray response, using silicon as a calibration standard. Magnesium, which is not measured directly, is calculated as a residual. The method was tested experimentally in the Palisade Diabase, but is applicable to any acid or basic igneous rock. The data are presented as elemental borehole logs which can be determined in real time. Computer programs use the elemental composition data to compute petrochemical information, which is displayed in real time in the form of borehole logs. In this way it is possible to make borehole logs showing changes in alteration, mafic and felsic indices, and normative mineral composition. Such borehole logs can be a powerful tool for the classification and comparison of rocks. (author). 32 refs, 8 figs, 4 tabs