Field Test Results of a New Neutron-Induced Gamma-Ray Spectroscopy Geochemical Logging Tool

Abstract

A new geochemical logging tool has been developed to provide measurements of elements important to the mineralogical analysis of rock formations and geochemical stratigraphy. The measurement system uses a chemical americium-beryllium neutron source to introduce moderate energy neutrons into the surrounding environment. These neutrons lose energy through scattering reactions and are eventually absorbed by materials in the borehole, the formation and the tool itself. The scattering and absorption reactions cause various elements among the atoms in the surrounding materials to emit gamma rays which have characteristic energies. Emitted gamma ray spectra are detected and recorded using a large bismuth germanate scintillation detector coupled to a high-efficiency photomultiplier and pulse-height analyzer.The recorded 256-channel spectra are analyzed to derive individual contributions, or elemental yields, arising from neutron capture reactions involving hydrogen, silicon, calcium, iron, sulfur, potassium, titanium, gadolinium, magnesium, aluminum and chlorine. The spectral analysis is performed with a constrained, weighted least-squares solver to ensure physically meaningful solutions are obtained for the elemental yields. Additional signals from inelastic neutron reactions with oxygen, carbon, silicon, calcium, and iron are treated separately in the spectral fitting process.The paper discusses several subjects including basic operation of the tool, derivation of characteristic elemental spectral responses, processing of the recorded spectra to obtain elemental yields and the conversion to elemental weight fractions for downstream formation mineralogy evaluations. Example logs from initial field tests are presented to demonstrate tool performance under various borehole conditions and lithologic environments including sandstone and carbonate reservoirs as well as organic shales.

Sweet spot areas for shale oil and shale gas plays in the Upper Cretaceous rocks of the Middle Magdalena Valley, Colombia: insights from basin modeling

“…The multimineral model and the TOC estimation are necessary to calculate effective porosity, and organic porosity. We applied Galford et al (2013) and Alfred & Vernik (2013) methods for organic porosity estimation, constrained with pyrolysis datasets, which measure the pore spaces created by thermal maturation and conversion of the organic matter into hydrocarbons within sourcerock reservoirs.…”

Using a New Spectroscopy Tool to Quantify Elemental Concentrations and TOC in an Unconventional Shale Gas Reservoir: Case Studies from Saudi Arabia

“…One of the first applications of spectroscopy logging was in the Middle East carbonates, and the initial motivation was carbon/oxygen (C/O) logging for saturation monitoring in fresh, variable, or unknown formation water salinity environments. Other tools are more suitable for formation evaluation, featuring larger detectors to measure accurate and precise elemental concentrations for mineralogy analysis (Herron and Herron 1996;Pemper et al 2006;Galford et al 2009). Since the early devices tested in the 1980s, many more tools have been developed taking advantage of new detector materials, improvements in the electronics, and improvements in processing and interpretation techniques.…”

A Workflow to Evaluate Mineralogy, Porosity, TOC, and Hydrocarbon Volume in the Eagle Ford Shale

“…The geochemical measurements used in this paper were obtained with a new logging tool (Galford et al 2009a;Galford et al 2009b). The tool provides measurements of elemental weight fractions of the elements magnesium, aluminum, silicon, potassium, calcium, titanium, manganese, iron, and gadolinium, which are important for mineralogical analysis of rock formations and geochemical stratigraphy.…”