oxide-closure model

1. n. [Formation Evaluation]
A model for converting relative elemental yields from a pulsed neutron spectroscopy log to absolute weight concentrations using the assumption that the sum of all oxides in the rock matrix is 1. The model is based on the observation that, with few exceptions, sedimentary minerals are oxides, so that the sum of the dry weight percent of all oxides must be 100%. The weight percent of an oxide can be calculated from the dry weight percent of the cation by knowing the chemical formula (for example, SiO2 from Si). The absolute dry weight percent, W, of element i is given by Wi = F * Yi / Si where F is the unknown normalization factor, Yi is the measured spectral gamma ray yield and Si is the tool sensitivity to that element, measured in the laboratory. The dry weight percent of the oxide is then Oi = F* Xi * Yi / Si where Xi is the oxide association factor, given by the chemical formula. Since the sum of all Oi equals 1, it is possible to calculate F and determine each Wi .