Advances in Nuclear Geophysical Methods in Oil Geology and Rock Analysis

Nuclear techniques have been developed for the analysis of rocks in the laboratory and in the earth surrounding boreholes drilled to search for oil and other useful minerals. A combination of neutron methods offers the possibility of giving analysis for most of the common elements found in igneous and sedimentary rocks. A summary is presented of two-year extensive study of the combination neutron method for analysing surface rocks of the moon and planets. The combination method uses a pulsed accelerator source of 14- MeV neutrons and measurement of prompt gamma-rays, capture gamma-rays, activation gamma-rays, and thermal and epithermal neutron die-away. From this work has come the new technique of cyclic neutron activation, which is applicable to lunar surface analysis, borehole logging and on-stream analysis of rocks and minerals. Rock analysis studies have been made with a high resolution lithium-drifted germanium detector and a pulsed accelerator source of neutrons. The detector resolution of 0.5% at 1.3 MeV is about an order of magnitude better than for a good sodium iodide detector. Gamma-rays arising from inelastic scattering of fast neutrons, capture of fast and slow neutrons, and activation were studied. From the results with the high resolution detector some interesting conclusions and speculations are made concerning the future of nuclear elemental analysis in the field. The present activity in the United States of America in commercial nuclear logging is summarized including neutron logging (steady-state and pulsed methods), scattered gamma-ray logging, and combinations of logs for identifying lithology and non-metallic minerals. A new small-diameter pulsed accelerator neutron tool for logging through 2½ -in. tubing is described, and a pulsed neutron source using a plurality of alpha sources and beryllium targets with a rotatable shutter between them. (author)