Factors contributing to high gamma-ray levels in Upper Jurassic sandstone reservoirs of the Claymore Oilfield, North Sea

Abstract

Sandstones of the Claymore Sandstone Member, which comprise the main reservoir in the Claymore oilfield, have gamma-ray intensities of 70–90 API units and locally reach 150 API units. A combination of techniques, including petrography, scanning electron microscopy, autoradiography and fission-track radiography, has been used to identify the sources of radioactivity. The generally high background is considered to be due to potassium feldspar concentrations of up to 27% and this is enhanced by the presence of radioactive heavy minerals, particularly zircon and monazite. Contributions to radioactivity from organic matter, mica, clays and lithic grains are minor. A failure to recognize the source of radioactivity in these sandstones can result in the assignment of high V shale values and low porosity during log analysis and productive zones may be underestimated.