C. FLAUM and G. PIRIE
Schlumberger-Doll Research
Ridgefield, CT
ABSTRACT
The Schlumberger Induced Gamma-Ray Spectroscopy Tool (GST) measures the relative yields of gamma rays resulting from interactions of neutrons with the different elements (isotopes) present in the formation. In particular, in the continuous (or capture-tau) mode, the tool is sensitive to gamma rays resulting from thermal neutron capture by isotopes of silicon, calcium, hydrogen, chlorine, iron, and sulfur.
The data are interpreted by associating the elemental yields with sedimentary minerals dominated by the corresponding elements (e.g. silicon = quartz). The yields are proportional to the volume fractions of the minerals, as well as to the values of effective neutron flux, microscopic thermal neutron capture cross-sections, gamma-ray production and detection efficiency and the elemental concentration in the mineral.
By judicious use of core or laboratory data to calibrate relative sensitivities, the effects of some of the unknowns can be minimized and a quantitative mineral analysis can be obtained. A pictorial computerized presentation shows volume fractions of sandstone, limestone, clay, and total porosity. With some additional information (SARABAND, NGT) the porosity can be subdivided into effective and clay porosities, while clay minerals may be subdivided into chlorite and illite. This presentation becomes an invaluable aid in formation analysis.
In a field study of tight shaly sandstones in the Cotton Valley Group, the analysis not only described the Iithology but also indicated geological features important in designing a good hydraulic fracturing program, e.g. shale bedding, shale hydration, pore-filling calcite cement-- which help in identifying fracture containment boundaries. The shale bedding description may also have some effect on permeability or producibility of the zone. Several other cases are also discussed.