Quantifying the Effects of Heavy Minerals on Thermal Neutron Porosity in Permo-Carboniferous Sandstone

Abstract

Recent studies document the presence of rare earth elements and heavy minerals in Permo-Carboniferous sandstones of the Arabian Peninsula. The studies establish that the measure of the gadolinium concentration and the formation capture cross-section (sigma) are both good proxies for the rare earth elements and heavy minerals. The objective of this study is to assess, quantify and then correct for the effect of these rare earth elements on the thermal neutron porosity measurement. Thermal and epithermal neutron porosity datasets were acquired in wells intersecting the Permo-Carboniferous sandstone. Relevant environmental corrections were applied to the data except for the formation water salinity correction. The hydrogen index values are obtained from the thermal neutron porosity measurement using corrections incorporating the measured sigma from the gamma ray spectroscopy tool and the formation bulk density. The measured sigma provides a measure of the water salinity in the flushed zone that is used to account for salinity effects on the thermal neutron porosity. The measured sigma also senses the effect of the heavy matrix minerals whose neutron capture cross-section is very high. The bulk density provides the required input to perform a density correction on the thermal neutron porosity. The hydrogen index that results from the measured sigma and formation density corrections to the thermal neutron porosity is referenced to the epithermal neutron porosity for these Permo-Carboniferous sandstones. When shale is present, the computed hydrogen index shows small discrepancies from the epithermal neutron porosity. These discrepancies are due to minor remaining density effects on the epithermal neutron porosity. In clay free reservoir intervals and in zones with moderate gadolinium content, the matrix-corrected hydrogen index is essentially identical to the matrix-corrected epithermal neutron porosity. The corrections from the thermal neutron porosity to the hydrogen index amount to 1.5-2.0 pu in the clay free sands. Applying this process to another well, additionally shows a good agreement between the matrix-corrected hydrogen index and the hydrogen index from the nuclear magnetic resonance. A third and final well application illustrates the extreme effect that the presence of rare earth elements can have on the conventional thermal neutron porosity, with negative corrections up to 5 pu to obtain the hydrogen index. Implementing a practical correction process to the thermal neutron porosity for the rare earth element bearing Permo-Carboniferous sandstones; this work quantifies the effects that rare earth elements have on the thermal neutron porosity measurements for the first time.