1. n. [Formation Evaluation]
A log of elemental concentrations from which the geochemistry of the formation may be derived. Several logs provide information on elemental weight concentrations: natural gamma ray spectroscopy, elemental capture spectroscopy or pulsed neutron spectroscopy and aluminum activation. The combination of some or all of their outputs is known as a geochemical log, since it provides information on most of the principal elements found in sedimentary rocks. Pulsed neutron spectroscopy provides relative elemental yields, whereas absolute concentrations are needed for quantitative results. Absolute concentrations can be derived by calibration to core or by using a model such as the oxide-closure model. The absolute elemental concentrations can then be converted into mineral concentrations using a model that defines what minerals are present. The first complete geochemical logs were run in the mid 1980s.