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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 925

Last Page: 925

Title: Induced Spectroscopy: A Mineralogy-Lithology Well Log: ABSTRACT

Author(s): Charles Flaum, Gordon Pirie

Article Type: Meeting abstract


Nuclear spectroscopy logging, specifically with the induced gamma-ray tool (GST), measures the relative yields of gamma rays resulting from interactions of neutrons with different elements (isotopes) present in downhole formations.

The data are interpreted by associating the spectra of certain elements with sedimentary rocks which are typically dominated by the element (e.g., silicon = sandstone). The measured elemental yields are proportional to the volume fractions present in the rock formations. The yields are also proportional to some unknown values: effective neutron flux, concentration of selected elements in other sedimentary rocks, microscopic thermal neutron capture cross sections, and gamma-ray production and detection efficiencies. The effects of these unknowns can be minimized by laboratory calibration of yields in known formations or by comparison with whole or sidewall core data.

With the above and some additional information (SARA-BAND, NGT), the well-site analysis or a more sophisticated computerized presentation yield volume fractions of effective porosity, sandstone, limestone, feldspar, and the total clay ("shale") volume which may be further divided into volumes of clay porosity, illite, and chlorite. The resulting visual representation of these data becomes an invaluable aid to stratigraphic analysis.

In a field study of tight gas shaly sandstones in the Cotton Valley Group (Upper Jurassic) in east Texas, the analysis not only described the clay mineralogy and lithology but also highlighted several important geologic features which are important in designing an efficient hydraulic fracturing program: genetic units of sedimentation, laminated and thick-bedded shales, hydrated shale beds, and zones of well-developed intergranular pore-filling (quartz or calcite) cements--all of which may be fracture-containment boundaries.

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Copyright 1997 American Association of Petroleum Geologists