Methods and apparatus for determining formation lithology by gamma ray spectroscopy

摘要

Inelastic scattering gamma ray spectra taken in a borehole are analyzed by a least squares spectral fitting process to determine the relative elemental contributions thereto of chemical elements postulated to be present in unknown earth formations and contributing to the measured spectra from the formations. The relative inelastic yields for silicon, calcium and magnesium are calibrated to provide straightforward estimates of the respective elemental concentrations for those elements and of the volumetric fractions of the elements or associated rock types, such as sandstone, limestone and dolomite, in the formations. The ratio of the relative inelastic yields for magnesium and calcium provides an indication of the degree of dolomitization of a formation. A technique for determining formation iron content from the relative inelastic iron yield in open or cased holes is also disclosed. Based on the calibrated inelastic yields for silicon and/or calcium, calibrated estimates of the elemental yields from measured thermal neutron capture gamma ray spectra may also be determined, from which further information concerning formation lithology may be derived. In addition, the calibrated inelastic yields and the calibrated capture yields may be combined to provide an alternative technique for deriving volumetric fractions.