James A. Grau

Response of the Carbon/Oxygen Measurement for an Inelastic Gamma Ray Spectroscopy Tool

In the inelastic mode of operation, a gamma ray spectroscopy tool (GST) performs a measurement of the carbon and oxygen in the formation and borehole on the basis of a spectral analysis of gamma radiation induced by fast neutrons. The carbon-to-oxygen ratio contains valuable information concerning the location of oil- and/or gas-bearing rock. Hundreds of laboratory formation measurements have been performed to determine the partitioning between formation and borehole response of this tool. An interpretation model has been developed for the carbon-to-oxygen ratio that determines the borehole effects as a function of the following parameters: borehole size, borehole fluid composition, borehole fluid displacer size, casing size, casing weight, and formation porosity.

Absolute elemental concentrations estimated from geochemical well logging using neutron-induced gamma-ray spectrometry and a geological model

Elemental concentrations of several inorganic elements were determined on a continuous basis with depth using the Schlumberger gamma-ray spectrometry (GST) log in a Santa Fe Energy Co. well in the Kern Front field, Bakersfield, California, Relative gamma-ray yields of Si, Ca, Fe, S, Cl, H, Ti, Gd + Sm, and K were determined using a weighted least-squares fitting of standard elemental spectra, determined from laboratory measurements, to the measured spectra. The relative yields were then placed on an absolute basis using measured sensitivity coefficients and the assumption that, excluding Cl and H, the abundances of these elements plus Al, considered as oxides and, for Ca, carbonates, total 100% of the rock. This assumption removes variations in porosity and salinity from impacting the denormalization procedure. The output is estimated absolute concentrations in weight percent of the rock that are compared with elemental analyses made on over 60 core plug and sidewall samples by x-ray fluorescence and neutron activation analysis.