Applications of cross section data for nuclear geochemical well logging

Abstract

Geochemical analyses of oil bearing formations, based on nuclear techniques, are often limited by the current level of uncertainties on neutron cross section data. The problems created for nuclear geochemistry are most acute in the areas of high energy neutron scattering, inelastic neutron scattering gamma-ray production, and production of prompt gamma rays and delayed activations generated by (n,p) and (n,ar) reactions for neutron energies up to 14 MeV. We will present examples illustrating the ways in which spectroscopic measurements are used for geochemical applications and how the uncertainties in the nuclear cross section data affect the usefulness of the results. Particular elements that cause significant problems in the analysis are emphasized. INTRODUCTIONThe discovery of hydrocarbon bearing geological formations and the probability of successful fluid recovery is dependent on the identification of fluids and a knowledge of the mineralogy of the surrounding rock.' An important technique for providing information on these questions is provided by continuous subsurface measurements in a borehole of the elemental concentrations of the' surrounding rock and fluid? With the

Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt

“…By adding a gamma -gamma density log to measure electron density and photoelectric capture cross section (PEF), Mg concentration can be determined from the difference between the measured and calculated PEF based on the directly measured major elements concentrations. Once all relative concentrations have been measured or calculated, this information can be incorporated into an oxide-closure model to determine abundances of major and trace elements, including K, Ca, Mg, Na, Fe, S, Ti, Si, Al, U, Th, and Gd (Herron and Herron, 1990;Hertzog et al,1986;Hertzog et al, 1987). Schlumberger's proprietary method was originally developed for use in sedimentary rocks for petroleum exploration purposes, and the original oxide-closure models were based on elemental abundances representative of sedimentary environments.…”

Nuclear Techniques for Subsurface Geology

“…Most of these applications have used isotopic chemical sources, as have the more general logging uses (32,55). Future efforts for more complete elemental analysis will undoubtedly use delayed activity from both isotopic sources and l4-MeV accelerator-produced sources (21,66,67).…”

“…The element k can also be contained in the spec trometer or the borehole, which requires an independent correction. Finally, the neutron fl ux <Dn depends on the cross sections fo r individual elements (79), and uncertainties in the cross sections themselves are also refl ected in the accuracy with which Yi can be related to 11k (66,67). Naturally, fo r delayed activity the Yi also have a time dependence governed by the half-life of the parent nucleus.…”

“…But each new environmental situation requires a new approach to be developed from experimental and/or calculational studies. Neutron-induced cross sections are generally not well enough deter mined to perform realistic, detailed calculations to relate gamma-ray yields to elemental concentrations (21,66,67). While thermal neutron cross sections are sufficiently well determined, high-energy neutron-induced reaction cross sections and gamma-ray production cross sections (60) are still inadequate.…”

Nuclear modeling techniques and nuclear data

“…The q u a l i t y and t h e demands o f t h e n u c l e a r data have been discussed i n t h e l i t e r at u r e [44-491. References [47] and [49] …”