Using both nuclear capture and inelastic spectroscopy in deriving elemental concentrations in a formation

摘要

Methods and apparatus are provided for determining the content of a first chemical element in a geological formation traversed by a borehole which cannot otherwise be determined via a nuclear capture spectrum measurement. The methods and apparatus utilize the inelastic spectrum measurement of the first element and the inelastic and nuclear capture spectrum measurements of at least a second element. The methods and apparatus have particular application to determining the carbon content of a formation although they are not limited thereto. The inelastic and capture spectrum measurements of silicon are useful in making such determinations, although other chemical elements may be used as the second element. The spectroscopy tool comprises at least one neutron source and at least one gamma ray sensor. The method may employ a closure model to generate a weight fraction for the elements. The method may be further employed to estimate the amount of organic carbon in the formation.